Supine

The most common position for surgery is the supine or dorsal decubitus position ( Fig. 34.1 ). Classically, the head, neck, and spine all retain neutrality. Because the entire body is close to the level of the heart, hemodynamic reserve is well maintained. Tissues overlying all bony prominences, such as the heels and sacrum, must be padded to prevent soft tissue ischemia as a result of pressure, especially during prolonged surgery.

Supine position.

The base of the table is asymmetrical. When positioned in the usual direction, the patient’s center of gravity is over the base. Weight limits decrease when in reverse orientation to the base.

The arms can be abducted, adducted, or one arm abducted and one arm adducted. In any variation, the arms should be placed in as neutral a position as possible, minimizing stretch and over extension. When the arms are adducted, they must remain securely placed next to the body. For abducted arm position, abduction should be limited to less than 90 degrees to minimize the likelihood of brachial plexus injury. Hands and forearms can be supinated or kept in a neutral position with the palm toward the body. This also reduces external pressure on the spiral groove of the humerus and the ulnar nerve ( Fig. 34.2 ). Particular attention should be paid to pad bony prominences, like the elbows, and any protruding objects, such as intravenous fluid lines, monitoring equipment, and poles ( Fig. 34.3 ).

Arm position using the arm board.

Abduction of the arm is limited to less than 90 degrees whenever possible. The arm is supinated, and the elbow is padded.

Arm tucked at patient side.

The arm is in the neutral position with the palm to the hip. The elbow is padded, and the arm is well supported by the mattress.

Variations of the Supine Position

Several variations of the supine position are frequently used. These include the lawn (or beach) chair position, frog-leg position, and Trendelenburg and reverse Trendelenburg positions. The lawn chair position ( Fig. 34.4 ) reduces stress on the back, hips, and knees by placing the patient’s hips and knees in mild flexion. This position is often better tolerated by patients who are awake or undergoing monitored anesthesia care than the full supine position. The lawn chair position also facilitates lower extremity venous drainage because the legs are placed slightly above the level of the heart. Abdominal wall tension is also reduced because the xiphoid to pubic distance is decreased. Proper positioning involves positioning the patient’s hips at the break of the surgical table and avoiding venous pooling in the legs.

Lawn or beach chair position.

Flexion of the hips and knees decreases tension on the back.

The frog-leg position allows procedural access to the perineum, medial thighs, genitalia, and rectum. The patient is positioned supine and then the hips and knees are flexed and the hips are externally rotated with the soles of the feet facing each other. Support of the patient’s knees to minimize stress and postoperative pain in the hips is required.

The Trendelenburg position, achieved by tilting a supine patient head down ( Fig. 34.5 ), is linked by name to a 19th-century German surgeon, Friedrich Trendelenburg, who described its use for abdominal surgery. Walter Cannon, a Harvard physiologist, is credited with popularizing the use of Trendelenburg positioning to improve hemodynamics for patients in shock during World War I. Today the Trendelenburg position is frequently utilized to improve exposure during abdominal and laparoscopic surgery, during central line placement to prevent air embolism and distention of the central vein, and to offset hypotension by temporarily increasing venous return. A steep (30-45 degrees) head-down position is now frequently used for robotic prostate and gynecologic surgeries.

Head-down tilt (Trendelenburg) position and head-up tilt (reverse Trendelenburg) position. Shoulder braces should be avoided to prevent brachial plexus compression injuries.

For all positions in which the head is at a different level than the heart, the effect of the hydrostatic gradient on cerebral arterial and venous pressures should be considered when estimating cerebral perfusion pressure. Careful documentation of any potential arterial pressure gradient is especially prudent.

The Trendelenburg position does produce hemodynamic and respiratory changes; however, the hemodynamic changes are not as long-lasting as often thought. Initial placement of the patient in head-down supine position will increase cardiac output approximately 9% in less than 1 minute via an autotransfusion from the lower extremities. This effect is not sustained and within approximately 10 minutes the cardiac output begins to return to baseline. Nevertheless, the Trendelenburg position is still considered an essential part of initial resuscitation efforts to treat hypotension and acute hypovolemia. Functional residual capacity is decreased due to gravitational pull of the diaphragm cephalad. Pulmonary compliance is increased by decreased functional residual capacity and is often further decreased in the Trendelenburg position, due to patient-positioning straps across the chest. In a spontaneously breathing patient, the work of breathing increases. In patients under general anesthesia, these pulmonary changes result in higher airway pressures. Changes to the mechanical ventilator settings can compensate for some of the respiratory changes. However, with patient body habitus and variations in positioning, the higher airway pressures, and changes to minute ventilation are too great to safely continue in the steep Trendelenburg position. Testing the position for patient tolerance after anesthetic induction and completed positioning, prior to the initiation of the surgical procedure, is recommended.

Intracranial and intraocular pressures (IOCs) also increase in Trendelenburg position. Trendelenburg is contraindicated in patients with increased intracranial pressures. In fact, for some patients with severe intracranial hypertension, even supine position is not tolerated. Consideration of the impact of positioning on intracranial pressure is important, as it may not only affect intraoperative positioning but also may have consequences on site selection for central line placement. Frequently, femoral vein site selection is preferred in patients with severely elevated intracranial pressure in order to avoid exacerbating intracranial hypertension with patient position changes during line placement.

Prolonged head-down positioning can also lead to swelling of the face, conjunctiva, larynx, and tongue, with an increased potential for postoperative upper airway obstruction. The Trendelenburg position increases intraabdominal pressure and displaces the stomach placing the patient at a higher risk for aspiration. Endotracheal intubation is often preferred in order to prevent aspiration of gastric contents.

Care must be taken to prevent patients in steep head-down positions from slipping cephalad on the surgical instruments. Techniques to restrain the patient include antiskid bedding, knee flexion, shoulder braces, beanbag cradling, and padded cross-torso straps. Shoulder braces are specifically not recommended because of the risk of compression injury to the brachial plexus. Beanbag pads become rigid when suction is applied to set the shape, and their use in the Trendelenburg position has been associated with brachial plexus injuries. If either shoulder braces or beanbag shoulder immobilization is used to prevent sliding, additional caution is recommended regarding abducting the arm; brachial plexus injuries on the side of the abducted arm have been reported in conjunction with beanbag shoulder immobilization and steep Trendelenburg positioning. These injuries may be due to stretch of the upper and middle trunks of the brachial plexus, as they course around the head of the humerus ( Fig. 34.6 ).

The brachial plexus, shown in yellow , is vulnerable to stretch and compression due to its long course. Arm abduction is limited to less than 90 degrees when supine because when the arm is raised the head of the humerus rotates caudad and stretches the plexus. Shoulder braces should be avoided; they may cause direct compression of the plexus medially between the clavicle and first rib or laterally below the head of the humerus. Excessive head rotation should be avoided, especially away from an abducted arm. Abduction of the arm should be avoided when in a steep head-down position if shoulder braces or a beanbag holds the shoulders.

The reverse Trendelenburg position (head-up tilt; see Fig. 34.2 ) is often used to facilitate upper abdominal surgery by shifting the abdominal contents caudad. This position is increasingly popular because of the growing number of laparoscopic surgeries requiring this position. Again, caution is advised to prevent patients from slipping on the table. As mentioned earlier, any position where the head is above the heart reduces cerebral perfusion pressure and may also cause systemic hypotension. If invasive arterial pressure monitoring is used then the arterial pressure transducer should be zeroed at the level of the Circle of Willis.

Lithotomy

The classic lithotomy position ( Figs. 34.7–34.9 ) is frequently used during gynecologic, rectal, and urologic surgeries. The patient’s hips are flexed 80 to 100 degrees from the trunk, and the legs are abducted 30 to 45 degrees from the midline. The knees are flexed until the lower legs are parallel to the torso. The legs are then placed in supports or stirrups. The foot section of the surgical table is lowered and sometimes removed from the end of the table.

Lithotomy position.

Hips are flexed 80 to 100 degrees with the lower leg parallel to the body. Pressure near the fibular head is absent. Arms are on armrests away from the hinge point of the foot section.

Lithotomy position with “candy cane” stirrup leg holders.

Lithotomy position.

The correct position of “candy cane” supports is well away from the lateral fibular head.

Positioning a patient into and out of lithotomy requires a coordinated team. The legs should be raised together; simultaneously, the knees and hips are flexed. This prevents torsion and injury to the lumbar spine. Padding of the lower extremities is critical, particularly over bony prominences, to prevent compression against the leg supports. The peroneal nerve is particularly prone to injury as it lies between the fibular head and compression from the leg support (see the peripheral nerve injury section of this chapter).

If the arms are tucked or placed alongside the patient, then the patient’s hands and fingers are at risk of injury if they lie near the open edge of the lowered section of the table. When the foot of the table is raised at the end of the procedure the fingers near the open edge can get crushed. Strict attention must be paid to the position of the hands to avoid a potentially disastrous crush injury to the fingers ( Fig. 34.10 ). For this reason, the recommended position of the arms is on armrests far from the table hinge point. If the arms must be tucked at the patient’s side, then the hands need to be visualized and confirmed to be safe whenever the leg section of the surgical table is manipulated.

Improper position of arms in lithotomy position.

The fingers are at risk for compression when the lower section of the bed is raised.

The lithotomy position may also cause significant physiologic changes. When the legs are elevated, venous return increases, causing a transient increase in cardiac output and, to a lesser extent, cerebral venous and intracranial pressure in otherwise healthy patients. In addition, the lithotomy position increases intraabdominal pressure and causes the abdominal viscera to displace the diaphragm cephalad, reducing lung compliance and potentially resulting in a decreased tidal volume. In obese patients, or when large abdominal mass is present (e.g., tumor, gravid uterus), abdominal pressure may increase enough to obstruct venous return to the heart. As with the supine position, the curvature of the lumbar spine is lost in lithotomy and can put the patient at risk of back pain.

Lower extremity compartment syndrome is a rare but potentially devastating complication of the lithotomy position. Compartment syndrome is caused by increased tissue pressure within a fascial compartment due to tissue ischemia, edema, and rhabdomyolysis. Inadequate arterial inflow (from lower extremity elevation) and decreased venous outflow (due to direct compression or excessive hip flexion) elevates the risk of compartment syndrome for patients in lithotomy. Local arterial pressure decreases 0.78 mm Hg for each centimeter the leg is raised above the right atrium. Reperfusion after ischemic injury further increases edema, exacerbating the problem. In a large retrospective review of 572,498 surgeries, the incidence of compartment syndromes was higher in the lithotomy (1 in 8720) and lateral decubitus (1 in 9711) positions, as compared with the supine (1 in 92,441) position. Long procedure time was the only distinguishing characteristic of the surgeries during which patients developed lower extremity compartment syndromes. A survey of urologists in the United Kingdom suggested that compartment syndrome after surgery in the lithotomy position is underreported and more common than appreciated. Affected patients in this study all had surgical durations greater than 3.5 hours. In a retrospective multicenter review of 185 urologic patients who were placed in high lithotomy position, two patients suffered from compartment syndrome. For both of these patients, operative times exceeded 5 hours. If surgical time extends beyond 2 to 3 hours, periodically lowering the legs is recommended. Additional risk includes factors known to compromise tissue oxygenation, such as blood loss, peripheral vascular disease, hypotension, and reduced cardiac output. Elevated body mass index is also a risk factor for compartment syndrome. Intermittent leg compression devices remain controversial.

Lateral Decubitus

The lateral decubitus position ( Fig. 34.11 ) is most frequently used for surgery involving the thorax, retroperitoneal structures, and hip. Positioning a patient in the lateral decubitus position requires the cooperation of the entire surgical staff. The nonoperative side is dependent and the dependent leg is flexed to minimize stretch of lower extremity nerves. Padding is placed between the knees to minimize excessive pressure on bony prominences. The torso must be balanced and supported both anteriorly and posteriorly. When a kidney rest is used for this purpose, it must be properly placed under the dependent iliac crest to prevent inadvertent compression of the inferior vena cava.

Lateral decubitus position.

The lower leg is flexed with padding between the legs, and both arms are supported and padded.

Patients may be laterally flexed while in the lateral position in order to gain better access to the thoracic cavity or retroperitoneum during renal surgeries. The point of flexion and the kidney rest should lie under the iliac crest rather than the flank or ribcage to minimize compression of the dependent lung ( Fig. 34.12 ). The dependent arm should be placed on a padded arm board perpendicular to the torso. The nondependent arm needs to be carefully supported ( Fig. 34.13 ). Neither arm should be abducted more than 90 degrees. For some high thoracotomies, the nondependent arm may need to be elevated above the shoulder plane for exposure; however, vigilance is warranted to prevent neurovascular compromise. The patient’s head must be kept in a neutral position to prevent excessive lateral rotation of the neck and to avoid stretch injuries to the brachial plexus. Often additional head support is required ( Fig. 34.14 ). The dependent ear and eye may be at risk of injury and should be checked regularly.

Flexed lateral decubitus position.

The point of flexion should lie under the iliac crest, rather than under the flank or lower ribs to optimize ventilation of the dependent lung.

Lateral decubitus position showing placement of arms and head.

Additional padding is under the headrest to ensure the alignment of the head with the spine. The headrest is kept away from the dependent eye.

Use of chest roll in the lateral decubitus position.

The roll, in this case, is a bag of intravenous fluid and is placed well away from the axilla to prevent compression of the axillary artery and brachial plexus.

The dependent brachial plexus and axillary vascular structures are at particular risk of pressure injury in the lateral decubitus position. In order to avoid compression, an axillary roll is frequently placed between the chest wall and the table just caudal to the dependent axilla (see Fig. 34.13 ). The purpose of the axillary roll is to protect the dependent shoulder and the axillary contents from the weight of the thorax. The axillary roll should never be placed in the axilla. Sometimes a beanbag is used for positioning without an axillary roll. In this scenario, the axilla should be checked to ensure that it is free from compression. Regardless of the technique, the pulse should be monitored in the dependent arm for early detection of compression to axillary neurovascular structures. Vascular compression and venous outflow obstruction in the dependent arm are risks of the lateral decubitus position. Low pulse oximeter readings can be an early sign of compromised circulation. Similarly, hypotension measured in the dependent arm may be due to axillary arterial compression.

Pulmonary mechanics change in the lateral decubitus position. In a patient who is mechanically ventilated, the combination of the lateral weight of the mediastinum and the disproportionate cephalad pressure of abdominal contents on the dependent lung favors overventilation of the nondependent lung. At the same time, the effect of gravity causes the pulmonary blood flow to the underventilated, dependent lung to increase. Consequently, ventilation-perfusion matching worsens, potentially affecting gas exchange and ventilation.

The lateral decubitus position is preferred during pulmonary surgery and one-lung ventilation. When the nondependent lung is collapsed, the minute ventilation is allocated to the dependent lung. This, combined with decreased compliance as a result of positioning, may further exacerbate the airway pressure required to achieve adequate ventilation. Head-down tilt in the lateral position worsens pulmonary function yet further, increasing shunt fraction.

Prone

The prone or ventral decubitus position ( Fig. 34.15 ) is primarily used for surgical access to the posterior fossa of the skull, the posterior spine, the buttocks and perirectal area, and the posterior lower extremities. The patient may receive either monitored anesthesia care or general anesthesia depending on the type of surgery and the patient’s body habitus and comorbidities. When general anesthesia is planned, the airway is usually secured via an endotracheal tube while the patient is still supine. Special attention should be paid to securing and taping the endotracheal tube to prevent dislodgement while the patient is prone or during changes in position. Placing an anesthetized patient in the prone position requires the coordination of the entire surgical staff. The anesthesiologist is primarily responsible for coordinating the move while maintaining inline stabilization of the cervical spine and monitoring the endotracheal tube. An exception might be the patient in whom rigid pin fixation is used when the surgeon often holds the pin frame. The endotracheal tube should be disconnected from the circuit during the move from supine to prone in order to prevent dislodgement. Which, and how many, monitors and lines are disconnected during the move is up to the clinical judgement of the anesthesiologist for an individual patient. Lines and monitors connected to the inside arm (the arm moving the least during the move) can often be easily maintained without disconnecting. Ventilation and monitoring should be reestablished as rapidly as possible.

Prone position with Wilson frame.

Arms are abducted less than 90 degrees whenever possible, although greater abduction may be better tolerated while prone. Pressure points are padded, and the chest and abdomen are supported away from the bed to minimize abdominal pressure and to preserve pulmonary compliance. Soft head pillow has cutouts for eyes and nose and a slot to permit endotracheal tube exit. Eyes must be checked frequently.

Prone head position is critical. For patients under sedation, the head may be turned to the side if neck mobility is adequate. During general anesthesia, the head is usually kept neutral using a surgical pillow, horseshoe headrest, or head pins. Weight should be on the bony facial prominences and not soft tissue and especially not on the eyes. The face is not always visible. Mirror systems are available to facilitate intermittent visual confirmation that the eyes are not compressed, although direct visualization or tactile confirmation is prudent ( Fig. 34.16 ). Several commercially available pillows are specially designed for the prone position. Most pillows support the forehead, malar regions, and chin, with a cutout for the eyes, nose, and mouth (see Fig. 34.15 ). The forehead and malar regions are supported by the horseshoe headrest and allow for reasonable access to the airway ( Figs. 34.17 and 34.18 ). Pin fixation, which is most used in cranial and cervical surgery, is advantageous because there is no direct pressure on the face ( Fig. 34.19 ). Patient movement must be prevented when the head is held in pins; movement in pins can result in scalp lacerations or a cervical spine injury. Both horseshoe and pin headrests attach to the operating room table with adjustable articulating supports. All articulating supports must be fully locked as failure of this bracketing device may lead to complications if the head suddenly drops.

Mirror system for prone position.

Bony structures of head and face are supported, and monitoring of the eyes and airway is facilitated with a plastic mirror. Although not illustrated, the eyes should be taped closed.

Prone position with horseshoe adapter.

Head height is adjusted to position the neck in a natural position without undue extension or flexion.

Prone position with horseshoe adapter.

The face is seen from below. The horseshoe adapter permits superior access to the airway and visualization of eyes. The width may be adjusted to ensure proper support by facial bones.

Prone position with Mayfield head pins.

Rigid fixation is provided for the cervical spine and posterior intracranial surgeries. The head position may include neck torsion or flexion that affects the depth of the endotracheal tube, and extreme head positions may increase the risk of cervical cord injury.

Regardless of the type of head-support technique, proper positioning must be frequently verified during the surgery, checking that there is no pressure on the eyes, that the airway is secure, and that the head weight lies on the bony facial prominences only. The prone position is a risk factor for perioperative visual loss (POVL), which is discussed separately in this chapter. If motor-evoked potentials are used during spine or neurosurgery, then the position of the tongue and placement of bite blocks must be frequently checked; bite injuries can be severe.

A prone patient’s legs should be padded and flexed slightly at the knees and hips. The arms may be positioned to the patient’s sides or placed outstretched above the head. If the arms are at the patient’s sides then they should be tucked in the neutral position. If the arms are outstretched above the head, the arms should be placed on arm boards with slight flexion at the elbows to prevent undue stretch on the peripheral nerves. Extra padding under the elbow may be needed to prevent compression of the ulnar nerve. The arms should not be abducted greater than 90 degrees.

If the legs are in plane with the torso, then hemodynamic reserve is relatively maintained; however, if any significant lowering of the legs or tilting of the entire table occurs, then venous return may increase or decrease accordingly. The prone position does not alter the ability of pulse pressure variation to predict fluid responsiveness. However, the variation has been shown to be augmented at baseline; therefore, fluid responsiveness is observed at a slightly greater level of variation than when supine.

When patients are in the prone position, weight should be distributed to the thoracic cage and bony pelvis, the abdomen should hang freely in order to prevent increases in intraabdominal and intrathoracic pressure. This is accomplished with specific types of prone beds or with gel or foam bolsters. The prone beds and bolsters all place support along each side of the patient from the clavicles to the iliac crests. Placement beyond the iliac crests can cause compression on the femoral vessels and femoral nerve. Some prone beds include the Wilson frame (see Fig. 34.15 ), Jackson table, Relton frame, and Mouradian/Simmons modification of the Relton frame. Breasts should be placed medially to the prone torso supports (or bolsters), and genitalia should be clear of compression. During posterior spinal surgery, relatively low venous pressure is desirable to minimize bleeding and to facilitate surgical exposure. Elevated abdominal pressure can transmit elevated venous pressures to the abdominal and spine vessels, including the epidural veins, which lack valves. Increased abdominal pressure may also impede venous return through compression of the inferior vena cava, decreasing cardiac output.

Pulmonary function is usually better in the prone position than in the supine position. The prone position has been used to improve respiratory function in patients with adult respiratory distress syndrome. Under anesthesia, the prone position has advantages over the supine position with regard to lung volumes and oxygenation without adverse effects on lung mechanics, including patients who are obese (see also Chapter 58 ) and pediatric patients (see also Chapter 77 ). Newer investigations using high-resolution imaging have shown the prone position to provide superior ventilation-perfusion matching in the posterior segments of the lung near the diaphragm when compared with the supine position. The aeration and ventilation of these posterior segments are better, while blood flow is maintained, despite their nondependent position.

Other complications of the prone position include airway edema, eye injury, pressure injury, and inadvertent loss of the endotracheal tube, monitoring, and intravenous lines. For long cases, or cases with large intravascular volume shifts, consider checking and documenting an endotracheal cuff leak at the start and end of the case. Lines and tubes need to be placed and should be well secured prior to turning the patient prone.

Sitting

The sitting position offers excellent surgical exposure to the upper posterior cervical spine and posterior fossa. Gravitational venous drainage of blood in the sitting position does decrease blood in the operative field and therefore possibly reduces surgical blood loss. The lawn or beach chair position is a variation of the sitting position and is commonly used for shoulder surgeries ( Fig. 34.20 ). The lawn chair position is really a semi-sitting position, with the head of the patient more reclined than in the traditional sitting position. For the surgeon, its advantages versus the lateral decubitus position are superior access to the shoulder from both the anterior and posterior aspect and the potential for great mobility of the arm at the shoulder joint. Access to the airway is generally excellent in the sitting position, facial swelling is minimized, and pulmonary mechanics are reasonably preserved ( Chapter 57 ).

Sitting position adapted for shoulder surgery, often called the lawn or beach chair position.

The arms must be supported to prevent stretching of the brachial plexus without pressure on the ulnar area of the elbow. As with all head-up positions, blood pressure should be regulated with the height of the brain in mind.

There are unique risks to the sitting position that require much vigilance. One of the most concerning risks is for venous air embolism (VAE). The veins lie above the level of the heart in this position; therefore, air entrainment through the veins to the heart is a real danger. Furthermore, dural veins are valveless and tented open by the cranium ( Fig. 34.21 ; see also Chapter 57 ). Other complications from the sitting position include quadriplegia, spinal cord infarction, hemodynamic instability pneumocephalus, macroglossia, and peripheral nerve injuries.

Sitting position with Mayfield head pins.

The patient is typically semi-recumbent rather than sitting; the legs are kept as high as possible to promote venous return. Arms must be supported to prevent shoulder traction and stretching of the brachial plexus. In a commonly used variation, the arms are placed on the abdomen and supported. The head support is preferably attached to the back section of the table to allow the back to be adjusted or lowered emergently without first detaching the head holder.

Placing an anesthetized patient in the sitting position requires flexion at the torso. Hip flexion should be less than 90 degrees in order to minimize stretch on lower extremity nerves (including the sciatic nerve). Arms are supported so that the shoulders are slightly elevated in order to ensure avoidance of traction on the shoulder muscles and potential stretching of upper extremity neurovascular structures. The knees are also usually slightly flexed for balance and to reduce stretching of the sciatic nerve, and the feet are supported and padded. The patient’s head must be specially fixed in the sitting position with either rigid pins or taped into a special headrest.

The head and neck position while in the sitting position has been associated with complications. Surgery in the sitting position was found to be a risk factor for cervical spinal cord injury in a review of the ASA Closed Claims Project database from 1970 to 2007. Although the exact mechanism for cervical spinal cord injury is unknown, cervical hyperextension, cervical hyperflexion, or excessive cervical rotation have been implicated as risk factors. Extreme neck positions can impede both arterial and venous blood flow, causing hypoperfusion or venous congestion of the brain. The patient’s cervical range of motion should be examined in the preoperative assessment, and adequate distance should be maintained between the mandible and the sternum when the cervical spine is flexed in order to provide for adequate arterial and venous blood flow.

VAE is a constant concern in the sitting position due to the position of the surgical field above the level of the heart and the tented open dural venous sinuses. The reported incidence of VAE varies greatly in the literature due to a lack of standardization of measurement and grading scale for VAE. VAE can cause arrhythmias, oxygen (O ₂ ) desaturation, acute pulmonary hypertension, circulatory compromise, and cardiac arrest. If there is a patent foramen ovale (PFO), then the patient is at risk for a paradoxical arterial embolism causing stroke or myocardial infarction. Traditionally, preoperative contrast echocardiography is recommended to evaluate for a PFO. However, failure to detect a PFO on echocardiography does not ensure that the intraatrial septum is intact. The presence of a PFO has generally been considered a contraindication to the sitting position. A recent review suggested that paradoxical VAE is so rare that the presence of a PFO should not necessarily preclude placing the patient in the sitting position. This study found that a VAE was detected in 40% of patients with a known PFO and that no paradoxical embolism was detected. The decision to proceed should be made with patient informed consent and with a discussion between the surgeon and anesthesiologist.

Continuous monitoring for VAE during surgery in the sitting position is essential. There is no standard for type of VAE monitor. Clinical severity of VAE depends on the amount of air and the speed of entrainment. Extrapolation from animal studies suggests that 3 to 5 mL/kg is a lethal amount of air for an adult human, but in reality much less could be required. Transesophageal echocardiography (TEE) is the most sensitive monitor, able to detect as little as 0.02 mL/kg of air. In fact, TEE is so sensitive that some degree of entrained air can be demonstrated in a large majority of patients during neurosurgery in the sitting position. Transthoracic Doppler (TTD) is the most sensitive noninvasive monitor with detection rates of approximately 0.05 mL/kg of air. The TTD probe is placed on either the left or right sternal border between the second to fourth intercostal spaces. Transcranial Doppler (TCD) is a monitor of the middle cerebral artery and is nearly as sensitive as TEE. Pulmonary artery catheters, esophageal stethoscopes, and end-tidal carbon dioxide monitors are all much less sensitive monitors. Electrocardiographic and pulse oximeter changes are later findings.

Treatment for VAE includes first stopping further air entrainment. The surgeon is asked to stop operating, to flood the field with normal saline, and possibly apply bone wax. The inspired percent of O ₂ is changed to 100%. This will aid in treatment during hypoxemia or hypotension and may help reduce the volume of the air embolism via denitrogenation. Hemodynamic compromise is treated with intravenous fluids and vasoactive agents. Consideration is given to placing the patient in left side down and Trendelenburg in order to move an air lock in the right ventricular outflow track (although this can be difficult or impossible in some surgeries). A central venous catheter is often placed preoperatively in order to aspirate entrained air. An ex-vivo study examining different central venous catheter types and positions found that a multiorifice catheter and single orifice catheter both aspirated 50% to 60% of experimentally introduced air.

Pneumocephalus, quadriplegia, spinal cord infarction, cerebral ischemia, and peripheral nerve injuries are all risks of sitting positions. Pneumocephalus is almost universally found on postoperative imaging from cervical or posterior fossa surgery performed in the sitting position. Tension pneumocephalus, which is accumulation of air in the subdural or ventricular space causing pressure on intracranial structures, is very rare but reported after neurosurgery in the sitting position. Prompt diagnosis and surgical evacuation of air is the treatment. Positioning complications causing quadriplegia or spinal cord infarction are thought to be caused by impaired arterial perfusion with hyperextension, hyperflexion, or excessive rotation of the neck. Theories relating the sitting positions to cerebral ischemia include reduced cerebral perfusion caused by reduced cardiac output, deliberate or permissive hypotension, loss of compensatory mechanisms caused by anesthesia, failure to compensate for the height of the head in the regulation of the blood pressure, dynamic vertebral artery narrowing or occlusion with the rotation of the head, and air emboli. Investigators have demonstrated positional effects on cerebral oxygen saturation, as well as transient reductions in cerebral oxygen saturation associated with hypotensive periods during shoulder surgeries in the sitting position that reversed after use of ephedrine and phenylephrine to restore cerebral perfusion pressure. One observational study of 124 patients undergoing shoulder arthroscopy demonstrated cerebral desaturation by oximetry in 80% of those who were in the lawn or beach chair position and none in the lateral decubitus position. Cerebral oxygen saturation monitoring may be helpful; however, no gold standard limits exist, and values may change along with alterations in patient position and carbon dioxide concentration. Therefore, if measured, trends in cerebral oxygen saturation are best interpreted during periods of constant ventilation and patient position. Reasonable recommendations for patients undergoing surgery in the sitting position are to monitor blood pressure carefully in reference to the level of the brain, avoid and rapidly treat any hypotension or bradycardia, and position the head carefully to avoid extreme positions that may compromise cerebral vessels.

Hypotension is a known and very common problem for anesthetized patients in the sitting position. Pooling of blood in the lower body places anesthetized patients in the sitting position at particular risk to hypotensive episodes. Studies reveal that mean arterial pressure, systolic blood pressure, and cardiac index all decrease in the sitting position. Therefore, placement of the patient into the sitting position should be incremental in order to adjust for hemodynamic changes. Intravenous fluids and vasopressors should be in-line and ready.

Robotic Surgery

Since its introduction approximately 30 years ago, the use and scope of robotic surgery has expanded greatly. Robotic surgery is now the norm for many types of urologic and gynecologic operations, and is extending to other abdominal operations, thoracic surgery, and head and neck operations. Robotic surgery offers technical advantages for surgeons regarding range of motion and accuracy of laparoscopic instrumentation. Once the robot is docked, direct access to the patient is limited. It is therefore imperative that all monitors, lines, and invasive lines are placed prior to docking the robot, and that proper padding and positioning are completed.

Most of the literature about robotic positioning involves urologic and gynecologic operations, which are generally performed with the patient in steep Trendelenburg (30-45 degrees) and lithotomy with arms tucked in neutral position to the sides. The patient must be very well secured in order to avoid slipping in steep Trendelenburg. Non-slip mattresses, chest straps, and shoulder braces may be useful, but shoulder braces are also reported to cause brachial plexus injuries due to stretch between the shoulder and neck. If shoulder braces are employed, monitoring for excessive stretch at the patient’s neck is essential. The endotracheal tube should be well secured to avoid migration. Often a tray or table is placed above the patient’s face in order to provide protection from laparoscopic equipment. It may be prudent to trial the steep Trendelenburg prior to docking the robot to ensure that the patient is properly positioned, does not slip, and can tolerate steep Trendelenburg from a physiologic standpoint.

Physiologic changes during robotic surgery are due to both laparoscopic insufflation as well as positioning. Hemodynamic changes are largely due to laparoscopic insufflation, whereas changes in respiratory mechanics are also affected by positioning. Functional residual capacity is decreased with both laparoscopy and further decreased with the addition of steep Trendelenburg. This is due to a combination of pressure of abdominal contents from laparoscopy and Trendelenburg pushing up on the diaphragm, and can also be worsened by chest fixation that is applied to prevent the patient from slipping off the table. Peak and plateau airway pressures have been shown to rise as much as 50%. Between changes in pulmonary compliance, decreased functional residual capacity, and the need for increased minute ventilation with carbon dioxide insufflation, intraoperative mechanical ventilation can be quite challenging during these cases.

The incidence of positioning complications from robotic surgery vary substantially from 0.8% to 6.6%; most studies indicate an incidence of less than 1%. One study found longer operative times, higher ASA physical status, and increased intravenous fluid administration to be risk factors for intraoperative positioning complications. A history of prior abdominal surgery was the only associated risk factor found in a study of gynecologic robotic surgeries. Eye injury, peripheral nerve injuries, rhabdomyolysis, and compartment syndrome were the most frequent positioning complications found in patients with robotic assisted radical prostatectomies. Incidence of injury in this study was not different between robotic versus traditional open prostatectomy. In a recent survey of ASA members, 21.7% of responders answered “yes” that they have experienced a “complication related to Trendelenburg positioning (during robotic surgery).” Airway and facial edema, as well as brachial plexus injuries, were the most common recalled complications in this survey. When steep Trendelenburg is used, consideration should be given to documenting the endotracheal tube airway leak at the start and at the end of the surgical procedure prior to extubation.

Ulnar Nerve Injury

The ulnar nerve lies in a superficial position at the elbow. Morbidity associated with ulnar neuropathy can be severe. In a prospective study among 1502 patients undergoing noncardiac surgery, 7 patients developed perioperative ulnar neuropathy, of which 3 patients had residual symptoms after 2 years. In a study on the effect of arm position on the ulnar nerve somatosensory-evoked potentials (SSEPs) in 15 healthy awake male volunteers, the supinated position was associated with the least pressure on the ulnar nerve, and the neutral position was the next most favorable. When in the neutral position on a surgical armrest, pressure decreased as the arm was abducted between 30 and 90 degrees. Interestingly, not all patients had symptoms of nerve compression when the SSEP was abnormal. Current consensus is that the cause of ulnar nerve palsy is multifactorial and not always preventable. In a large retrospective review of perioperative ulnar neuropathy lasting longer than 3 months, the onset of symptoms occurred more than 24 hours postoperatively in 57% of patients; 70% were men and 9% experienced bilateral symptoms. Very thin or obese patients were at increased risk, as were those with prolonged postoperative bed rest. No association with intraoperative patient position or anesthetic technique was confirmed. The ASA Closed Claims Project also demonstrated that perioperative ulnar neuropathy occurred predominately in men, in an older population, and with a delayed onset (median of 3 days). The large predominance of ulnar injury in men may possibly be explained by anatomic differences. Men have a more developed and thickened flexor retinaculum with less protective adipose tissue and a larger tubercle of the coronoid process that can predispose them to nerve compression in the cubital tunnel. In the published ASA Closed Claims Project data, only 9% of ulnar injury claims had an explicit mechanism of injury, and in 27% of claims, the padding of the elbows was explicitly documented. Postoperative ulnar nerve palsy can occur without any apparent cause, even when padding and positioning of the patient’s arm was carefully managed and documented in the anesthetic record.

Brachial Plexus Injury

The brachial plexus is susceptible to stretching because of its long superficial course from the neck to the arm via the axilla with two points of fixation—the cervical vertebrae and the axillary fascia. The nerves are vulnerable to compression as they pass between the clavicle and the first rib because of the proximity and mobility of both the clavicle and the humerus (see Fig. 34.6 ). Among patients undergoing noncardiac surgeries, the incidence of brachial plexus injury is reported to be 0.02%. After brachial plexus injury, the patient often complains of sensory deficit in the distribution of the ulnar nerve. This symptom is most commonly associated with intraoperative arm abduction greater than 90 degrees, lateral rotation of the head away from the side of the injury, asymmetric retraction of the sternum for internal mammary artery dissection during cardiac surgery, or direct trauma or compression. To avoid injury, patients should ideally be positioned with the head midline, arms kept at the sides, the elbows mildly flexed, and the forearms supinated, without pressure on the shoulders or the axilla.

Brachial plexus injury is particularly associated with the use of shoulder braces in patients undergoing surgery in the Trendelenburg position. Medial placement of the braces can compress the proximal roots, and lateral placement of the braces can stretch the plexus by displacing the shoulders against the thorax (see Fig. 34.6 ). The patient with injury often complains of painless motor deficit in the distribution of the radial and median nerves; however, pain may also be present. A report of three cases of upper- and middle-trunk brachial plexopathy after robotic prostatectomy highlights the potential risk of a combination of compression of the shoulder girdle against the thorax in the steep Trendelenburg position with abduction of an arm. Signs of vascular compromise to the upper extremities, such as difficulty obtaining consistent blood pressure or a poor pulse oximetry signal, may be indications of compromise to the neurovascular bundle as reported in a case of bilateral injury related to shoulder braces with abduction of the arms in the Trendelenburg position. Studies of the brachial plexus tension test in human volunteers and nerve strain in cadavers have demonstrated deleterious positional elements including arm abduction, rotation, or flexion of the head away from the affected arm, elbow and wrist extension, and depression of the shoulder girdle. For transaxillary robotic thyroidectomy, a recently developed approach has the arm abducted to 180 degrees. An incidence of brachial plexus injury has been reported to be 0.3%. When an extreme position is used, neurophysiologic monitoring, such as motor-evoked potentials and SSEPs, has been shown to detect an evolving injury and allow for repositioning to prevent permanent damage. Nerve function monitoring may therefore become increasingly common with newer surgical techniques that carry increased risk related to patient positioning.

In patients undergoing cardiac surgery requiring median sternotomy, brachial plexus injury has been specifically associated with the C8 to T1 nerve roots. In a prospective study in which the incidence of injury was 4.9%, 73% of the injuries occurred on the same side as the internal jugular vein cannulation; however, this study antedated the widespread use of ultrasound to guide cannulation. Unilateral sternal retraction to harvest the mammary artery is associated with brachial plexus dysfunction, presumably caused by stretching the nerves. SSEP monitoring of the brachial plexus during sternal retraction has been shown to predict injury.

In the 1999 ASA Closed Claims Project report, 10% of brachial plexus injuries were directly attributed to patient positioning. Of those, one half involved the use of shoulder braces in patients in the Trendelenburg position. Consequently, nonsliding mattresses should be used, along with a concerted effort to avoid compression of the shoulders as much as possible. Of the 311 brachial plexus injuries in the ASA Closed Claims Project, 59 (19%) occurred after a regional block without general anesthesia, including axillary and interscalene blocks. In those cases, the role of patient positioning cannot be determined.

Other Upper Extremity Nerve Injury

In a retrospective study of 1000 consecutive spine surgeries that used SSEP monitoring, five arm positions were compared regarding SSEP changes in the upper extremities. A modification of the arm position reversed 92% of upper extremity SSEP changes. The incidence of position-related upper extremity SSEP changes was significantly higher in the prone “superman” (7%) and lateral decubitus (7.5%) positions, compared with the supine arms out, supine arms tucked, and prone arms tucked positions (1.8%-3.2%). Reversible SSEP changes were not associated with postoperative deficits ( Chapter 39 ).

Although quite rare, the radial nerve can be injured from direct pressure as it traverses the spiral groove of the humerus in the lower one third of the arm. The injury often exhibits a wrist drop with an inability to abduct the thumb or extend the metacarpophalangeal joints. An isolated median nerve injury most often occurs during the insertion of an intravenous needle into the antecubital fossa in a patient who has been anesthetized where the nerve is adjacent to the medial cubital and basilic veins. Patients with this injury are unable to oppose the first and fifth digits and have decreased sensation over the palmar surface of the lateral three and a half fingers. Surprisingly, in an evaluation of the ASA Closed Claims Project database from 1970 to 2007, peripheral intravenous and arterial line insertion accounted for 2.1% of all claims filed, particularly among patients who underwent cardiac surgery where the arms were tucked and the lines were not visible for inspection. Nerve injury accounted for 17% of intravenous line complications, second only to skin slough or necrosis (28%) and swelling, inflammation, and infection (17%).

Lower Extremity Nerve Injury

Injuries to the sciatic and common peroneal nerves occur most often in the lithotomy position. Because of its fixation between the sciatic notch and the neck of the fibula, the sciatic nerve can be stretched with external rotation of the leg. The sciatic nerve and its branches cross the hip and knee joints and are stretched by hyperflexion of the hips or extension of the knees. The common peroneal nerve, a branch of the sciatic, is most often damaged from the compression between the head of the fibula and an external structure, such as the frame of a leg support. Most often, patients who suffer a peroneal nerve injury will complain of a foot drop and the inability to extend the toes in a dorsal direction or evert the foot. In a prospective study of 991 patients undergoing surgery under general anesthesia in the lithotomy position, the incidence of lower extremity neuropathies was 1.5%, with injuries to the sciatic and peroneal nerves representing 40% of the cases. Interestingly, symptoms were predominantly paresthesia, with onset within 4 hours of surgery and resolution generally within 6 months. No motor deficits were noted, but in a previous retrospective study, the same authors found the incidence of severe motor disability in patients who underwent surgery in the lithotomy position to be 1 in 3608.

Injury to the femoral or obturator nerves generally occurs from lower abdominal surgical procedures with excessive retraction. The obturator nerve can also be injured during a difficult forceps delivery or by excessive flexion of the thigh to the groin. A femoral neuropathy will exhibit decreased flexion of the hip, decreased extension of the knee, or a loss of sensation over the superior aspect of the thigh and medial or anteromedial side of the leg. An obturator neuropathy will exhibit an inability to adduct the leg with decreased sensation over the medial side of the thigh.

In a retrospective review of 198,461 patients undergoing surgery in the lithotomy position from 1957 to 1991, injury to the common peroneal nerve was the most common lower extremity motor neuropathy, representing 78% of nerve injuries. A potential cause of the injury was the compression of the nerve between the lateral head of the fibula and the bar holding the legs. When the “candy cane” stirrups are used, special attention must be paid to avoid compression (see Fig. 34.9 ). The injury was more common with patients who had low body mass index, recent cigarette smoking, or prolonged duration of surgery. Perhaps as a result of an increased awareness of potential injuries, no lower extremity motor neuropathies were reported in a prospective review of 991 patients undergoing surgery in the lithotomy position from 1997 to 1998. Paresthesias in the distribution of the obturator, lateral femoral cutaneous, sciatic, and peroneal nerves were reported in 1.5% of patients, and nearly all recovered. Surgical times longer than 2 hours were significantly associated with this complication.

Evaluation and Treatment of Perioperative Neuropathies

When a nerve injury becomes apparent postoperatively, it is essential to perform and document a directed physical examination to correlate the extent of sensory or motor deficits with the preoperative examination as well as any intraoperative events. It is prudent to seek neurologic consultation to help define the neurogenic basis, localize the site of the lesion, and determine the severity of injury for guiding prognostication. With proper diagnosis and management, most injuries resolve, but months to years may be required. In addition, perioperative neuropathies associated with pain must be differentiated from surgically induced neuropathic pain, which is receiving increasing attention by surgeons because it affects an estimated 10% to 40% of surgical patients postoperatively.

If a new sensory or motor deficit is found postoperatively, then electrophysiologic evaluation by a neurologist within the first week may provide useful information concerning the characteristic and temporal pattern of the injury. Another examination after 4 weeks, when enough time has elapsed for the electrophysiologic changes to evolve, will provide more definitive information about the site and severity of the nerve injury. Regardless, electrophysiologic testing must be interpreted within the clinical context. No single test can define the cause of injury. Nerve conduction studies may be useful to evaluate potential peripheral nerve injuries, as they permit the assessment of both motor and sensory nerves. To evaluate motor integrity, the nerve is supramaximally stimulated at two points along its course, and a recording is made of the electrical response of one of the muscles that it innervates. The size of the muscle action potential provides an estimate of the number of motor axons and muscle fibers that are activated by the stimulus. For sensory conduction studies, the nerve fiber is supramaximally stimulated at one point and the sensory nerve action potential is recorded from another point. The latency of the response can be interpreted as a reflection of the number of functioning sensory axons. Nerve conduction studies are useful for several reasons; they may reveal the presence of a subclinical polyneuropathy that made the individual nerves more susceptible to injury and help distinguish between axon loss and demyelination, which has significant implications regarding course and overall prognosis.

For motor neuropathy, an electromyogram can be performed to characterize the injury. An electromyographic examination involves recording the electrical activity of a muscle from a needle electrode inserted within it. If present, abnormalities may point to the affected component in the motor unit, which consists of the anterior horn cell, its axon and neuromuscular junctions, and the muscle fibers that it innervates. Certain findings are suggestive of denervation, including the presence of abnormal spontaneous activity in the resting muscle (fibrillation potentials and positive sharp waves, which results from muscle irritability) and increased insertion activity. Insertion activity increases within a few days of muscle denervation, whereas abnormal spontaneous activity takes 1 to 4 weeks to develop, depending on the distance from the nerve lesion to the muscle. Depending on the pattern of abnormalities, an electromyographic study may distinguish between radiculopathies, plexopathies, and neuropathies.

Most sensory neuropathies are generally transient and require only reassurance to the patient with follow-up visits, whereas most motor neuropathies include demyelination of peripheral fibers of a nerve trunk (neurapraxia) and generally take 4 to 6 weeks for recovery. Injury to the axon within an intact nerve sheath (axonotmesis) or complete nerve disruption (neurotmesis) can cause severe pain and disability. When reversible, recovery often takes 3 to 12 months. Interim physical therapy is recommended to prevent contractures and muscle atrophy.

Retinal Ischemia: Branch and Central Retinal Artery Occlusion

Central retinal artery occlusion (CRAO) decreases blood supply to the entire retina, whereas branch retinal artery occlusion (BRAO) is a localized injury; these are generally unilateral. There are four mechanisms: (1) external compression of the eye, (2) decreased arterial supply (embolism to retinal arterial circulation or decreased systemic blood flow), (3) impaired venous drainage, and (4) thrombosis from a coagulation disorder. The most common in the perioperative period is improper positioning with external compression producing sufficiently high IOP to stop blood flow in the central retinal artery. It most often occurs during spine surgery performed with the patient in the prone position. Pressure within the orbit also can be increased after retrobulbar hemorrhage, associated usually with vascular injury from sinus or nasal surgery.

Although rare in most surgical procedures, emboli can directly impair blood flow in the central retinal artery or produce BRAO. Paradoxical embolism from the operative site reaching the arterial circulation through a patent foramen ovale has been reported in perioperative retinal vascular occlusion. Venous drainage can be impaired after radical neck surgery by jugular vein ligation. Retinal microemboli, however, are common during open heart surgery.

Clinical Findings

There is painless visual loss, abnormal pupil reactivity, opacification or whitening of the ischemic retina, and narrowing of retinal arterioles. BRAO is characterized by cholesterol, and calcific or pale platelet fibrin emboli. A cherry-red macula with a white ground-glass appearance of the retina and attenuated arterioles is a “classic” sign. Pallor in the ischemic, overlying retina makes visible the red color of the intact, underlying choroidal circulation, but its absence does not rule out RAO. Differential diagnosis from other causes of visual loss is presented in Table 34.2 .

TABLE 34.2

Differential Diagnosis: Eye Examination in Retinal, Optic Nerve, or Visual Cortex Injury ^∗

	AION	PION	Cortical Blindness	CRAO	BRAO
Optic disk	Pale swelling, peripapillary flame-shaped hemorrhages, edema of optic nerve head Late: Optic atrophy	Initially normal Late: Optic atrophy	Normal	Normal Late: Optic atrophy	Normal Late: Optic atrophy
Retina	Normal; may have attenuated arterioles	Normal; may have attenuated arterioles	Normal	Cherry-red macula † ; pallor and edema, narrowed retinal arteries	Emboli may be present ‡ ; partial retinal whitening and edema
Light reflex	Absent or RAPD	Absent or RAPD	Normal	Absent or RAPD	Normal or RAPD
Fixation and accommodation	Normal	Normal	Impaired	May be impaired with external compression	May be impaired with external compression
Opticokinetic nystagmus	Normal	Normal	Absent	Normal	Normal
Response to visual threat	Yes, if some vision remains	Yes, if some vision remains	No	Yes	Yes
Object tracking	Normal, if some vision remains	Normal, if some vision remains	Absent	Normal	Normal
Ocular muscle function	Normal	Normal	Normal	May be impaired if results from external compression	May be impaired if results from external compression
Perimetry	Altitudinal defect; scotoma	Altitudinal defect; blind; scotoma; Often no light perception	Hemianopia (depending on lesion location); periphery affected usually	Usually blind	Scotoma; usually normal periphery

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