An operative surgical site is intentionally elevated above the level of the heart to decrease bleeding in the operative field and to provide better surgical conditions in the sitting position. Although popular in the 1980s and early 1990s for posterior fossa neurosurgical procedures, the sitting position is now commonly used for surgical procedures on the shoulder. Advantages of the sitting position to the anesthesiologist include easier ventilation because of unimpeded diaphragmatic excursion, easier access to the endotracheal tube and airway, unimpeded access to the chest wall for resuscitative measures, and unobstructed view of the face for monitoring cranial nerve function. Disadvantages include the need for a coordinated effort from the operating team (nursing, surgical, and anesthesia) to establish this position safely, hypotension, venous air embolism, consequences of excessive neck flexion (kinking of endotracheal tube and swelling of face and tongue), nerve injuries, pneumocephalus, and blindness.
Pathophysiology of Sitting Position
Gravity and anesthetic agents have significant effects on the cardiovascular function in the sitting position. In a healthy adult patient, SV and CO are decreased by about 12% to 20%, and cerebral perfusion pressure (CPP) reduces by 15% without much change in the heart rate (HR).13
There is an overall increase in ventilation with increased vital capacity (VC) and FRC. However, with positive pressure ventilation and relative hypovolemia, there is reduced perfusion in the nondependent lung fields leading to an increase in physiologic dead space.
During neurosurgical procedures, upon opening of the arachnoid membrane there is loss of cerebrospinal fluid (CSF) allowing air to enter the intracranial CSF pathway leading to pneumocephalus and downward displacement of the brain. Although this gravitation of the brain may be tolerated in most patients, those with thin cerebral mantles may suffer from subdural hematoma.6
While positioning the patient, care should be exercised with the following basic principles: maintaining normal body alignment, protecting and padding all pressure points, avoiding placement of rigid oropharyngeal airways and excessive flexion of the neck, exercising care with extremities so their limits of passive range of motion are not exceeded, establishing final position slowly allowing time for hemodynamic compensation, and exercising extreme caution with a horseshoe frame if used for support of the head (Fig. 27-9).
Sitting position. [Redrawn from Martin JT. Positioning in Anesthesia and Surgery. Philadelphia, PA: WB Saunders; 1997. With permission.]
The frequent and most common complications are related to the hemodynamic and ventilatory effects as described in the previous section. Neurologic complications pertaining to neuropathies and blindness are detailed at the end of the chapter.
The incidence of venous air embolism (VAE) in posterior fossa surgery in the sitting position is reported to be 41% to 45% with routine monitoring.14 However, with the use of Doppler ultrasound, the reported incidence is as high as 42% to 85%.15 VAE is often clinically undetected and frequently not of serious concern in a healthy patient if the volume and rate of air entrainment are minimal. The amount of entrained air that is reported to be lethal in humans is approximately 300 mL.16 Children generally have greater clinically significant hemodynamic derangement from VAE than adults.
Significant morbidity and mortality from VAE is now less than 1%,17 predominantly as a result of better monitoring techniques, early detection, and prompt intervention. VAE has significant effects on the cardiopulmonary system, resulting in elevated pulmonary artery pressures, decreased CO, systemic hypotension, and increased dead space ventilation. These physiologic changes are due to the results of mechanical effects of obstructed pulmonary blood flow from the air pocket in cardiac chambers and chemical mediator release from the air–blood interface. The appearance of dysrhythmias can signal the presence of intracardiac air, and therefore a high index of suspicion is warranted.
Paradoxical air embolus occurs whenever there is a communication between the right and the left sides of the heart. Although left-sided pressures are generally higher than the right, right-sided pressures can exceed the left in pathologic conditions (pulmonary hypertension, pulmonic stenosis) and also in healthy subjects during certain phases of the cardiac cycle. Thus increased right-sided cardiac pressures could result in the appearance of the entrained air in the arterial circulation with its associated complications. Therefore, the sitting position is contraindicated in patients with documented intracardiac defects or arteriovenous malformations. Patent foramen ovale (PFO) is the most common congenital defect associated with a paradoxical air embolus.