Chapter 20 Arterial puncture is the most accurate blood sampling technique for true arterial blood gas (ABG) and acid-base determination. The absence of arterial blood pressure defines cardiac arrest and serves as a definitive end point for resuscitative efforts. Intraarterial cannulation with continuous blood pressure measurement remains an accepted standard in critically ill patients. Intraarterial monitoring of blood pressure better reflects the force of systemic perfusion and is one of the most important determinants of cardiac work. In recent years, noninvasive technologies have achieved an accuracy that is nearly equal to that of invasive monitoring, but these techniques also have limitations.1,2 Invasive modalities require specific expertise and support to perform.3 Hippocrates suggested that the blood and arteries deliver life-giving energy to the body. Building on these ideas, the ancient Greek physician Galen first noted that arteries carry blood from the heart although he wrongly asserted that the heart constantly produced new blood. The concept of circulating blood volume generating pulse and pressure was not recognized until the Age of Enlightenment. The Spanish physician Severetus was among the first to accurately describe the circulation in 1580, but most of his writings were destroyed when he was executed for his teachings. Soon thereafter in 1616, William Harvey described a circulatory system with a finite amount of blood and the heart at its center. Stephen Hales first recorded blood pressure measurement in 1733 using a brass pipe. He inserted a pipe into a horse’s artery and connected it to a glass conduit in which he observed blood rising and falling.1,4 The first measurement of human blood pressure was accomplished in 1847 with the kymograph. It was developed by the German physiologist Carl Ludwig and involved the insertion of catheters directly into the artery. In 1949, Peterson and colleagues illustrated continuous recording of pulse waves and blood pressure with a catheter percutaneously inserted into the brachial artery. Soon thereafter, percutaneous femoral artery cannulation with a polyethylene catheter through a large-bore needle was described by Peirce. Barr described radial artery cannulation with a Teflon catheter in 1961.5 An important accomplishment in vascular access occurred in 1953 when Seldinger presented a technique in which percutaneous catheterization could be achieved over a guidewire.5,6 In the 1960s, the introduction of electrical monitoring of arterial pressure with transducers and recorders allowed mathematical waveform analysis in addition to visual analysis. More recent advances have included continuous, invasive monitoring of ABG values.7 Currently, there are a growing number of noninvasive devices and methods for accurate monitoring of arterial blood pressure; however, none match the proven accuracy of intraarterial monitoring. Indications for and contraindications to arterial puncture and cannulation are listed in Review Box 20-1. The use of arterial lines for continuous monitoring is generally reserved for the intensive care setting; however, arterial cannulation may be initiated in the emergency department (ED). The indications for placement of an arterial catheter fall into three major categories8,9: 1. Repetitive and direct arterial blood sampling. Catheter access removes the need for multiple arterial punctures and allows either repeated sampling or placement of sensors for continuous monitoring of blood gas and other chemistry values. 2. Continuous real-time monitoring of blood pressure. Catheter access allows superior monitoring and moment-to-moment detection of changes. Intraoperative and intensive care unit (ICU) management is often facilitated by placement of an arterial line. 3. Failure or inability to use indirect blood pressure monitoring. Some patients such as those with severe burns, dialysis grafts or shunts, or morbid obesity may need ongoing monitoring of perfusion, which can best be accomplished by arterial catheterization. Although acute respiratory decompensation and metabolic emergencies are the most common reasons for ABG sampling, all blood tests performed on venous blood are also possible on an arterial sample. Cultures performed on blood obtained from an indwelling arterial line have a sensitivity and specificity similar to that of cultures performed on blood obtained from a venipuncture site.10,11 Patients with moderate respiratory decompensation may be managed without arterial puncture by using continuous, noninvasive pulse oximetry, end-tidal or transcutaneous carbon dioxide monitoring, carboxyhemoglobin and methemoglobin monitoring, or any combination thereof.12 Nonetheless, a role still exists for arterial blood sampling. The initial correlation between noninvasive values and acid-base status via arterial sampling is often important in critical illness to set a baseline or verify a trend. Some authors use ABG sampling in the initial evaluation of critically ill trauma patients.13 Vasoactive drugs (e.g., nitroprusside and norepinephrine) are best administered with continuous monitoring of arterial pressure to guide titration. The response of trauma and post-cardiac arrest patients to acute resuscitative efforts may also be more easily monitored with the use of arterial catheterization. There are reports of patients with bleeding complications who require transfusion. Some patients have suffered compression neuropathies secondary to hematomas at the puncture site.14 Repeated arterial sampling in such patients should be accomplished by insertion of an indwelling cannula to minimize trauma to the arterial wall. The presence of severe arteriosclerosis, with or without diminution in flow, is a relative contraindication to arterial puncture. In hemodynamically unstable patients with advanced cardiovascular disease, the benefits of invasive monitoring may nonetheless outweigh its risks.9 Consider an alternative site if an isolated, decreased palpable pulse or bruit is felt over the site selected. Consider an alternative site if there is evidence of decreased or absent collateral flow in areas where flow normally exists, such as in Raynaud’s syndrome or an abnormal result on the modified Allen test (discussed later in the section “Techniques”). Avoid puncturing a specific arterial site when infection, burn, or other damage to cutaneous defenses exists in the overlying skin. In addition, avoid performing an arterial puncture through or distal to a surgical shunt. Arterial sampling has been the traditional approach to evaluating acid-base abnormalities in critically ill patients, especially those being maintained on a ventilator. In most ED settings, however, venous blood gas analysis may suffice. Studies have demonstrated that analysis of venous blood (especially central venous blood) for pH, bicarbonate, lactate, base excess, and carbon dioxide pressure (Pco2) are within 95% limits of agreement with arterial sampling and can safely supplant it.15–17 On the other hand, arterial blood sampling is still required for accurate analysis of oxygen pressure (Po2).18–20 Arterial Puncture with a Needle/Syringe Precoated blood gas plastic syringes (with dry lithium heparin) are commonly used and allow a longer shelf life and ready use (Fig. 20-1). Such devices are designed to minimize sampling error as a result of heparin.21 If necessary, prepare a regular syringe with 1 or 2 mL of a heparinized saline solution (1000 IU/mL) drawn into the syringe to coat the barrel and needle. Fully eject the heparin through the needle immediately before skin puncture to minimize heparin-related errors. Although the syringe may appear devoid of heparin, enough heparin remains in the needle and syringe to provide anticoagulation. Even dry heparin may produce abnormalities in ABG results because of a heparin-induced dilutional effect. Stored heparin solution has higher Po2 and lower Pco2 values than blood does.22 A dilutional effect from heparin would mean that the addition of 0.4 mL of heparin solution to a 2-mL sample of blood (dilution of 20%) will lower Pco2 by 16%.21 Proper technique with dry lithium heparin-prefilled syringes or full ejection of excess heparin will prevent such problems if more than 2 mL of blood is collected. A falsely low Pco2 is the most clinically significant change caused by excess heparin.21,22 Neither Po2 nor pH levels are significantly altered by the addition of heparin in most instances, although a slight increase in Po2 and a minimal decrease in pH may occur if high concentrations of heparin (25,000 IU/mL) are used.23 If 2 to 3 mL of blood is collected, heparin-related effects are likely to be clinically inconsequential. The fluid-filled recording systems used with arterial cannulation have a great influence on the accuracy of pressure measurements. The frequency responses of tubing, transducers, and other components of the monitoring system influence the accuracy of systolic and diastolic pressure measurement. Failure to recognize recording system artifacts will lead to errors in interpretation of the pressure.3 Various catheter types have demonstrated similar frequency-response characteristics, but some studies have found different complication rates. Teflon catheters may carry an increased rate of thrombosis.24,25 Another contributing element leading to thrombosis is catheter diameter; the incidence of thrombosis is inversely related to the ratio of vessel lumen to catheter diameter.26,27 Thus, the risk for thrombosis increases as the diameter of the catheter decreases. The incidence of thrombosis also increases with increased duration of catheter placement. In contrast, a higher risk for thrombosis was seen in the femoral artery than in the radial artery in a study involving a pediatric population.28 Catheters coated with a combination of chlorhexidine and silver sulfadiazine have produced lower infection rates.29 Box 20-1 lists the usual equipment for arterial cannulation, although the majority of prepackaged kits contain the supplies most needed (see Review Box 20-1). Shorter catheters are ideal for peripheral artery cannulation, whereas use of a longer catheter and the Seldinger technique is preferable for the femoral artery. For arterial cannulation in adults, use a 16- to 18-gauge catheter for the femoral artery and a 20-gauge catheter for the radial artery (Fig. 20-2). Small children and infants require a 22- to 24-gauge catheter, which may need to be inserted percutaneously via the Seldinger technique or through a femoral cutdown. Based on patient size, older pediatric patients usually require 20- to 22-gauge catheters. The tubing that connects the catheter to the pressure transducer has a significant effect on accuracy of the monitoring system. The higher the frequency response of the entire system, the more accurate the determination of systolic and diastolic pressure; however, artifact also becomes more of a problem.8,30 Use stiff, low-capacitance plastic tubing for arterial catheterization and monitoring. Place the electronic pressure transducer connection as close as possible to the patient and zero it appropriately because the frequency response of a tube is inversely related to its length.30–32 The pressure wave produced with each contraction is transmitted from the artery through the catheter and connecting tubing to a measuring device. The arterial fluid wave is received by an electromechanical transducer that changes the mechanical pressure wave into an electrical signal that can be displayed on the monitor. The most basic technique for obtaining blood pressure values involves the use of a simple manometer.33 This system can be assembled quickly if the material is available. A continuous method of flushing the pressure tubing is required to maintain patency of the catheter lumen during intraarterial pressure monitoring. A three-way stopcock through which the tubing is intermittently flushed (a minimum of every 15 to 30 minutes) with saline is a simple, effective method. Continuous flush devices push a set amount of fluid (usually 2 to 3 mL/hr) through the line.9 A typical monitoring system that includes this device is shown in Figure 20-3. The pressure transducer must be mounted at the level of the patient’s heart. Current pressure-monitoring setups include not only built-in stopcocks but also in-line flushing plungers to facilitate clearance of blood after sampling. Intravascular transducers were initially seen as an improvement over the external electromechanical transducers in use since the mid-1970s. Many of the numerous brands are fragile, temperature sensitive, of variable quality, and much more difficult to place in vessels than catheters are. Despite anecdotal reports of fibrin deposition on these devices, no increased incidence of thrombus formation has been noted. The most important advantages of intravascular transducers are the ability to continuously monitor ABG values and elimination of the potential error induced by catheters, stopcocks, and connecting tubing.34,35 Palpate the arterial pulse to ascertain the location of the vessel and prepare the overlying skin with an antiseptic solution (Fig. 20-4, step 1). Anesthetize the patient’s skin with a wheal of local anesthetic (e.g., 1% lidocaine without epinephrine) through a small needle (25 or 27 gauge) (see Fig. 20-4, step 2). If local anesthesia is to be performed, take care to use only a small amount of local anesthetic because a large wheal may obscure the pulse. One study found no significant alterations in Pco2 or pH from the pain or anxiety of an unanesthetized arterial puncture (Table 20-1).36 If the patient is in extremis or unresponsive to pain in the area to be punctured, the anesthetic infiltration step may be omitted. TABLE 20-1 Parameters That Affect Interpretation of Arterial Blood Gases *Use only a 1000-IU/mL concentration. Fill the dead space of the needle and syringe only and collect 3 mL of blood. †Changes unpredictable at 20 minutes regardless of the storage method. ‡There are reports of slight increases in Po2 with excessive heparin. §The falsely lowered Pco2 that occurs with added heparin is the most clinically significant change noted. pH may be decreased if a large volume of concentrated heparin (25,000 IU/mL) is used. Isolate the arterial pulsation with the index and middle fingers of the gloved, nondominant hand and identify the course of the vessel. Puncture the skin through the anesthetic wheal, immediately distal to the palpated pulse under the index finger (Fig. 20-4, step 3). The older technique of placing the needle between the index and middle finger risks self-puncture and is no longer advised. Hold the syringe like a dart with the bevel up and the syringe kept in view so that blood flow can be seen immediately. Advance the needle slowly toward the pulsating vessel at an approximately 30-degree angle. A larger angle is required to puncture the deeper femoral artery. Once the needle enters the arterial lumen, allow the syringe plunger to rise with the arterial pressure on its own to discriminate between arterial and venous sampling (see Fig. 20-4, step 4). As soon as blood flows, stop advancing the needle and allow the syringe to fill. If no blood flow is obtained or if bone has been hit, withdraw the needle slowly because both walls of the vessel may have been punctured and the lumen may be entered as the needle is withdrawn. Redirect the needle only when the needle has been retracted to a location just deep to the dermis. After at least 1 to 2 mL of blood has been obtained, remove the needle from the artery. Apply firm pressure at the puncture site for a minimum of 3 to 5 minutes (see Fig. 20-4, step 5). If the patient is anticoagulated or has a coagulopathy, 10 to 15 minutes of pressure is required. Use of ultrasound is rapidly becoming a standard in procedures involving central vascular access. The same ultrasound-guided techniques are also being applied to arterial cannulation (see Ultrasound Box.) Arterial Punctureby Christine Butts, MD The target vessel should first be identified with ultrasound. A high-frequency transducer (10 to 12 mHz) should be used to ensure proper resolution. The target vessel is typically easiest to identify in the transverse plane (with the indicator pointing toward the patient’s right side) (Fig. 20-US1). Arteries usually appear as rounded structures with thick walls, as opposed to veins. Depending on the size of the vessel, pulsations may be noted. Application of color flow Doppler to a suspected artery will frequently demonstrate a pattern of pulsatile flow (Fig. 20-US2). Although arteries are usually thought to resist collapse from outside pressure (such as from applying pressure to the overlying transducer), smaller arteries in the periphery may show a degree of collapse. When seeking to identify the target vessel, multiple means of identification should be used before an attempt at cannulation. Figure 20-US1 Placement of the transducer over the distal end of the arm in the transverse plane to localize the radial artery. Figure 20-US2 Image of the radial artery with color flow. Applying color flow will enable the operator to correctly identify the artery. Once the vessel has been identified, ultrasound can also be used to directly guide cannulation. Sterile technique should be maintained, particularly when accessing central arteries such as the femoral artery. A full description of the technique of ultrasound-guided venous access can be found in both the general ultrasound chapter (Chapter 66) and the central venous access chapter (Chapter 22) in this textbook. The principles of these procedures also apply to arterial cannulation, although a few points should be emphasized. When guiding access to the radial artery, a transverse approach is typically used because the vessel is small and difficult to visualize in the longitudinal approach. Additionally, it may be difficult to keep the image of the longitudinal vessel centered on the screen, especially when one physician is directing both the ultrasound and the needle. In their review of the current literature, Shiloh and colleagues reported 71% improvement in the likelihood of success at the first attempt when using ultrasound guidance.37 Vascular Doppler can also be used to improve success rates. Hold the probe over the artery just proximal to the puncture site. An important indication of vessel identification is the loss of audible pulsations with compression. Proper handling of the sample and rapid analysis are very important. When the needle is withdrawn, expel any air bubbles present in the syringe to avoid a false elevation in Po2.38 Remove the air neatly and easily by tapping the inverted syringe (needle pointing upward) to force any air to the top; then carefully and slowly depress the syringe plunger to push out the remaining air (see Fig. 20-4, step 6). A gauze pad or alcohol wipe may be used to collect any excess blood expelled with the syringe held upright and the plunger side down. Remove the needle and cap the syringe to ensure anaerobic conditions. Alternatively, many of the commercially available arterial blood gas kits come with an air bubble removal device (Filter-Pro) that allows the clinician to expel air bubbles from the sample and reduce potential exposure from the blood product. Air in the sample will significantly increase Po2 (mean increase, 11 mm Hg) after 20 minutes of storage, even if kept at 4°C. pH and Pco2 are not significantly altered by air bubbles if the blood is stored at 4°C for 20 minutes and no significant deterioration has occurred.23,38 If blood is stored at room temperature for longer than 20 minutes, Pco2 will increase and the pH will decrease, probably as a result of leukocyte metabolism. In a stored sample, Po2 varies to such an extent that the change is unpredictable for chemical interpretation at 30 minutes, regardless of the storage method. High leukocyte or platelet counts, such as those seen in leukemic patients, may shorten acceptable storage intervals.39,40 Direct Over-the-Needle Catheter Cannulation Take time to ensure proper alignment of the desired site. Delays, complications, and inability to successfully cannulate an artery often occur as a result of failure to properly prepare the desired site and involved limb. An important preparatory step is to ensure that the target limb is secured flat and not rotated; any rotation could result in the desired artery being shifted from the expected anatomic position and make it more difficult to cannulate. For example, to adequately prepare the radial artery, immobilize the wrist and hand in mild dorsiflexion with some padding for support underneath the wrist (Fig. 20-5, step 1). Prepare the skin with sterile technique. Inject local anesthetic with a 25-gauge or smaller needle and achieve sufficient infiltration to ensure a painless procedure. Subcutaneous infiltration of lidocaine or a similar anesthetic may also reduce vessel spasm at the time of arterial puncture. Check the catheter assembly for proper movement and function. Alternatively, a 3-mL syringe with the plunger removed can be used as a blood reservoir. Advance the catheter toward the palpated artery at a comfortable angle for the operator, generally 30 to 45 degrees from the skin (see Fig. 20-5, step 2). Make a small incision with a No. 11 scalpel blade or a larger-bore needle to eliminate the problem of damage to the catheter from kinking on the skin. The tip of the needle is often perceived to pierce the artery, but successful puncture is confirmed by identifying a “flash” of arterial blood flow into the needle hub and reservoir. As the needle-catheter assembly advances through the skin toward the artery, the initial flash of arterial blood is obtained by the needle alone, which protrudes beyond the catheter. For this reason, the needle-catheter assembly should be lowered and advanced 2 mm forward to ensure that the tip of the catheter has cannulated the vessel, along with the needle (see Fig. 20-5, step 3). The position of the catheter within the vessel lumen is confirmed by continuous return of arterial blood. The catheter alone can now be advanced with care over the needle into the artery (see Fig. 20-5, step 4). If the catheter fails to thread, it has not properly entered the vessel lumen and should not be forced to advance without confirmation of placement by active blood return. When blood flow into the needle-catheter assembly has ceased, it may have pierced the backside of the arterial wall. This double-puncture method is useful for cannulating small vessels, yet it is not recommended as a routine procedure for inexperienced clinicians.5 If double puncture has occurred and blood has ceased to flow into the collection reservoir, do not remove the entire needle-catheter assembly. Instead, simply retract the needle slightly to determine whether blood flow into the catheter can be reestablished. If blood flow occurs, gently advance the catheter. If not, slowly withdraw the catheter until pulsatile blood flow reappears and then advance the catheter into the artery. It is important for the clinician to be aware of whether the tip of the needle or the catheter is the leading edge within the vessel.8 Once the catheter is fully advanced into the vessel lumen, maintain occlusive pressure on the proximal end of the artery to limit blood loss, and then remove the needle (see Fig. 20-5, step 5). Next attach narrow-bore, low-compliance pressure tubing to the catheter (see Fig. 20-5, step 6). Apply an appropriate sterile dressing after the apparatus has been securely sutured to the wrist. Occasionally, one will encounter difficulty advancing the catheter into the lumen. The “liquid stylet” method may aid further passage of the catheter.41 Fill a 10-mL syringe with about 5-mL of sterile normal saline. Attach the syringe to the catheter hub, and aspirate 1 to 2 mL of blood to confirm intraluminal position. Then slowly inject the fluid from the syringe and advance the catheter behind the fluid wave. Ultrasound guidance is now routinely used for both peripheral and central venous access and can also assist in arterial cannulation (see Ultrasound Box). When compared with the palpation technique, ultrasound-guided arterial cannulation significantly improves the likelihood of success on the first attempt.42 With B-mode ultrasound, the targeted artery can be visualized in real-time by using a 7.5- to 10-MHz linear-array transducer. Differentiate the target artery from the adjacent vessel by its pulsatility and noncompressibility with mild pressure by the transducer. Use color Doppler to further assist in distinguishing between arteries and veins. Either transverse or longitudinal views can be applied during arterial cannulation, but the transverse view is often more useful in smaller arteries. Once the catheter has entered the artery and it is confirmed by blood flow, place the ultrasound transducer on the field to free up the nondominant hand.42,43 The number of attempts with additional arterial punctures increases the size of the developing hematoma and the real risk for vessel wall damage, thrombosis, and even loss of arterial flow through the vessel. Despite the added trauma, there is no reported increase in complications when both walls, rather than one, are punctured in a single cannulation attempt.44–46
Arterial Puncture and Cannulation
Historical Perspective
Indications and Contraindications
Arterial versus Venous Analysis
Equipment: Arterial Puncture
Continuous Monitoring via Arterial Catheter
Preparation for Arterial Cannulation
Techniques
Ultrasound
Percutaneous Technique for Arterial Cannulation
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