Class
Visible structures
1
Full visibility of tonsils, uvula, faucial pillars, and soft palate
2
Visibility of hard and soft palate, upper portion of tonsils, and uvula
3
Soft and hard palate and base of the uvula
3
Only hard palate visible
Mallampati scores 3 and 4 are associated with difficult airways; however, the scoring is dependent upon the investigator.
Due to the poor correlation of individual factors in predicting a difficult intubation, the Wilson risk score was developed which combines various scores and indicators (Table 16.2) [13].
Table 16.2
Wilson score
Points | 0 | 1 | 2 |
Weight [kg] | <90 | 90–110 | >110 |
Range of motion in head and neck | >90° | 90° | <90° |
Maximum mouth opening [cm] or maximum jaw protrusion | >5 or lower jaw subluxation of upper jaw | <5 or lower jaw equal to over jaw | <5 or receding mandible |
Protruding maxillary anterior teeth | Normal | Moderate | Severe |
Receding chin | Normal | Moderate | Severe |
If the total of points is ≥2, a difficult intubation should be expected. The Wilson score has higher sensitivity than the Mallampati score but often leads to an overestimation of difficult intubation. The same goes for a combination of both scores. Although a reliable prediction of intubation difficulties is not possible, regular use of the scores generates important team awareness of the problem of unexpected difficult airways. The use of algorithms, such as the ASA Difficult Airway Algorithm [2], is immensely helpful if an unexpected intubation or ventilation problem occurs (see Sect. 1.1.6 and Fig. 1.1).
>> On her way to the hospital, Ms. Hall took the 7.5 mg midazolam PO she had been prescribed because of severe anxiety. The monitor in the preoperative area revealed a heart rate of 92 beats/min and a blood pressure of 164/96 mmHg.
“Are you still nervous?” asked Dr. Sven. Ms. Hall nodded and said, “Well, the pill worked well at first, but now I have the feeling that the effect is gone.” Anesthesia tech Carol had already placed an IV, and Dr. Sven called his attending Dr. Eldridge to tell him he was heading back to the OR.
Dr. Sven was preoxygenating Ms. Hall as Dr. Eldridge walked into the OR. “Please give 300 μg fentanyl, and in 2 min, 200 mg propofol,” he said to Dr. Sven. Mask ventilation was less than desirable, even with the help of an oropharyngeal airway. Dr. Eldridge asked anesthesia tech Carol to give 50 mg propofol. Afterward Dr. Sven could ventilate the patient much better. Finally, Ms. Hall received 8 mg vecuronium. Dr. Sven was a little nervous, as he was handed the laryngoscope, but the intubation was unremarkable. “Cormack II,” he announced loudly, and attending anesthesiologist Dr. Eldridge nodded he had understood. After auscultation to check the position of the endotracheal tube, anesthesia tech Carol secured the tube, and Dr. Sven placed a nasogastric tube. Dr. Eldridge commented, “I’ll be moving on,” and he was gone.
For maintenance of anesthesia, Dr. Sven chose sevoflurane 1 MAC in 50 % oxygen. He set the ventilator to volume–controlled ventilation with a tidal volume of about 600 ml, a frequency of 12/min, and a PEEP of 7 cmH 2 O and an I:E ratio of 1:1.5.
After the surgical prep, Dr. Buster placed the first trocar into Ms. Hall’s abdomen. “Turn on the gas” he said to the circulating nurse. “Thomas, can you place the patient in reverse Trendelenburg?” Dr. Sven did as requested. “So far, so good! Now I just have to fill out the anesthesia record,” thought Dr. Sven as he began to chart on the computer screen.
16.1.2 Which Gas Is Usually Used for a Pneumoperitoneum?
In laparoscopic surgery, carbon dioxide (CO2) is standard. CO2 has the benefit of not supporting combustion, as opposed to oxygen or nitrous oxide. In addition, CO2 is highly soluble in blood, as opposed to other gases like nitrous oxide or helium. This offers protection against gas embolism. The peritoneum absorbs CO2, which can then be eliminated by the lungs.
16.1.3 Explain the Effects of a Pneumoperitoneum!
With insufflation of gas into the abdominal cavity, the intra-abdominal pressure increases, and in addition some gas is absorbed. The effects of increased intra-abdominal pressure must be differentiated from the effects of CO2.
16.1.3.1 Effects of Pressure
Pulmonary Effects
Increased intra-abdominal pressure leads to cranial displacement of the diaphragm, which compresses the lungs and reduces lung volume. The result appears similar to restrictive lung disease with a reduction in compliance. In volume-controlled ventilation, peak pressures increase, as well as the mean airway and plateau pressure. Compression of the lungs increases the risk of atelectasis formation and ventilation–perfusion mismatch with the resulting hypoxia.
Cardiovascular Effects
Venous return to the heart is dependent upon intra-abdominal pressure. As gas flows into the abdominal cavity, venous return increases due to compression and emptying of abdominal veins. As intra-abdominal pressure increases above 15 mmHg, the reduction in venous return increases, and cardiac output decreases. Systemic vascular resistance also increases with increasing intra-abdominal pressure. In the pulmonary circulation, compression of the lungs leads to an increase in vascular resistance.
Intra-abdominal pressures of <15 mmHg are well tolerated by healthy patients and patients with minimal comorbidities. ASA III or IV patients, however, can be adversely affected by the pneumoperitoneum. For these patients, the smallest amount of abdominal distention possible should be applied, or a gas-free or open technique should be used if hemodynamic or pulmonary complications arise.
Effects on Other Organs
The intra-abdominal and retroperitoneal blood flow decreases as intra-abdominal pressure increases. The hemodynamic effect is especially pronounced when intra-abdominal pressures exceed >15 mmHg.
16.1.3.2 Effects of the Hypercapnia
The absorption of CO2, without an adjustment in ventilation, leads to hypercapnia and respiratory acidosis. The amount of CO2 absorbed by the system also depends upon other things in addition to intra-abdominal pressure. The largest amounts of CO2 resorption are during extraperitoneal endoscopic operations, such as endoscopic extraperitoneal radical prostatectomy (see Case 31). Although hypercapnia directly causes systemic vasodilation, there is an indirect sympathetic stimulating effect, which increases heart rate, cardiac output, and arterial blood pressure. Additionally, the danger of cardiac arrhythmia increases. In contrast to CO2’s vasodilation effects in the systemic circulation, hypercapnia leads to vasoconstriction in the pulmonary circulation.
16.1.4 What Effect Does Patient Positioning Have on Pulmonary Function?
After induction of anesthesia, respiratory compliance is reduced and resistance is increased. The functional residual capacity decreases due to the diaphragm being displaced cranially and a decrease in the thorax diameter.
16.1.4.1 Supine Positioning
Due to the cranial dislocation of abdominal organs, lung volume is reduced. The decrease in functional residual capacity leads to a decrease in compliance, increase in respiratory resistance, and a possible impairment of gas exchange due to ventilation/perfusion mismatch. Gas exchange is especially impaired when the functional residual capacity volume decreases below the alveolar closing capacity.
16.1.4.2 Trendelenburg Position
The pulmonary physiological changes in the supine position are exacerbated by the Trendelenburg position. A further decrease in the functional residual capacity leads to increased atelectasis formation.
16.1.4.3 Reverse Trendelenburg Position
Compared to supine or Trendelenburg positioning, the reverse Trendelenburg position increases lung volume and compliance, thereby reducing respiratory resistance.
>> Dr. Sven had just begun charting on the computer when the ventilator sounded a high pressure alarm. The preset pressure maximum of 30 cmH 2 O had been exceeded. “Oh,” he thought to himself, “I’d better adjust the ventilation.” He decreased the tidal volume from 600 to 500 ml and increased the ventilation frequency from 12 to 15/min.
16.1.5 Would You Have Adjusted the Ventilator Differently?
In Sect. 16.1.3, the effects of capnoperitoneum and the resulting CO2 resorption was described. In order to avoid the adverse effects of CO2 resorption and maintain normocapnia, the minute volume must be increased by about 20–30 % for the duration of the pneumoperitoneum. Choosing volume-controlled ventilation makes it easier to achieve this goal. The decision to reduce the tidal volume was appropriate, in order to avoid potentially dangerous airway pressures. However, the increase of the frequency of respiration was not sufficient and increased the minute ventilation only from 7.2 to 7.5 l.
>> Even after readjusting the ventilator settings, the high pressure alarm sounded again. The ventilator was still delivering only about 400 ml as tidal volume. Dr. Sven raised his eyebrows and thought, “Then I will adjust the level of the high pressure alarm.” He thought about it for a second then increased it to 35 cmH 2 O. However, even under this new setting, the alarm sounded again. “Is there a problem?” asked Dr. Buster from the other side of the green drapes.
Dr. Sven responded after a small pause “I can’t get a big enough tidal volume into the patient. How high have you got the abdominal pressure?” Dr. Buster glanced at the pressure display of the insufflator and answered, “13 mmHg – as always. Maybe the problem is the patient’s build. The visibility isn’t the greatest in here either.”
16.1.6 Is the Surgeon Correct?
Obesity influences the mechanics of respiration as well as gas exchange, mimicking restrictive lung disease. The cranial positioning of the diaphragm plays an important role, resulting in:
Reduction in lung volume
Reduction in compliance
Increase in airway resistance
The reduction in compliance as well as anatomical changes, such as limited movement of the diaphragm, lead to increased work of breathing in spontaneously breathing patients. The reduction of compliance in volume-controlled ventilation leads to increased airway pressures. Visibility conditions during a capnoperitoneum with unchanged intra-abdominal pressures can be affected by obesity. The surgeon Dr. Buster could be correct.
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