Of the three types of sleep apnea, OSA is the most common, though anesthesiologists will treat a patient with mixed sleep apnea as they would a patient with OSA. Very few patients have pure central sleep apnea and, in general, patients with central sleep apnea are not of special concern to the anesthesiologist, except under three special circumstances. Those circumstances are first, central sleep apnea with snoring, in which case the patients should be treated as patients with OSA; second, central sleep apnea due to heart failure in which case the precautions for these patients will relate to their underlying heart disease. The third circumstance is central sleep apnea with hypoventilation syndrome; these patients may require unanticipated assisted ventilation during surgery and also post-op.
An anatomic and physiologic familiarity with OSA is important to understand why anesthesia can be problematic in these patients. The syndrome of OSA is characterized by repetitive episodes of upper airway obstruction during sleep which may be accompanied by sleep disruption, hypoxemia, and arterial oxygen desaturation. Obstruction or anatomic narrowing can occur at one or more points in the upper airway and may be due to a variety of factors including abnormal neuromuscular tone, redundant soft tissue or an increase in upper airway adipose tissue. Classical OSA patients tend to be obese. However, non-obese patients can have OSA from tonsillar hypertrophy or craniofacial abnormalities. Overtime, recurrent or prolonged arterial oxygen desaturation leads to secondary cardiac and lung abnormalities including systemic and pulmonary hypertension, cardiac rhythm disturbances and, in extreme cases, right ventricular failure which is known as corpulmonale. Therefore, the first step in successfully anesthetizing an OSA patient is to conduct a thorough preoperative assessment.
The preoperative assessment includes a thorough history and physical examination. As the vast majority of sleep apnea patients are undiagnosed, it is not sufficient to simply ask if the patient has sleep apnea or disturbance. The typical patient with sleep apnea is male, overweight, and over the age of 40, but sleep apnea does occur in both sexes, in thin individuals, and in all age categories.1 Children, particularly those with tonsillar hypertrophy, can also be at risk. Key questions to ask a patient are:
Once the presence of sleep apnea is suspected, the anesthesiologist should ascertain whether the patient has had a previous sleep study and, if so, review the results. If no sleep study has ever been conducted - or if one has been conducted before significant weight gain or another current potentially associated factor - a sleep study may be warranted. The severity of OSA may be learned by questioning the patient regarding the degree of nighttime sleep disruption and daytime sleepiness, but often patients are not aware of their sleepiness or the extent of their nighttime disruption.
Of particular importance is any previous history of anesthesia or surgery. Records should be reviewed for information pertaining to the anesthetic technique employed or any adverse intraoperative or postoperative events. A history of obesity and cardiac or pulmonary disease should be noted.
The physical examination provides an important complement to the historical review. Obesity, particularly upper body obesity which places the patient at risk for OSA, should be noted. The presence of a large neck circumference, even in the non-obese, increases the risk for sleep apnea and should be noted. A formal assessment for the potential difficulty of endotracheal intubation is essential and is usually accomplished utilizing the Mallampati classification of difficult airways.2 Attention is paid to the length and range of motion of the neck, the size of the tongue and teeth, and the presence of any skeletal deformity. Evaluation of the ability to see into the hypopharynx yields a numerical rating. Examination of the heart and lungs focuses on the presence of physical findings suggestive of systemic or pulmonary hypertension, heart failure or impaired oxygenation. If additional studies are required to clarify the findings on physical examination, specialized tests such as an echocardiogram or pulmonary function studies can be ordered. A general rule is that patients with OSA should not undergo elective procedures until after a thorough preoperative assessment along the lines described here.
Many patients with OSA are morbidly obese (e.g.: more than two times their ideal body weight). This places them at increased risk for, among other things, aspiration of acidic gastric fluid at the time of induction of anesthesia. It is for this reason that many of these patients receive medications to suppress gastric acid production, to neutralize the acid, or to stimulate emptying of the stomach. Other potential preoperative challenges with patients who are obese can include obtaining adequate reliable intravenous access.
It is customary for anesthesiologists to prescribe sedative medications preoperatively. However, this practice may be problematic for patients with OSA, as they are often sensitive to sedative medications, especially if the OSA is untreated. Even minimal sedation can cause airway obstruction and ventilatory arrest. Therefore, many anesthesiologists do not give preoperative sedatives to patients with OSA.
The most serious perioperative misadventure is the loss of airway control after induction of general anesthesia. Because of reduced oxygen reserve due to obesity-related decreases in lung volume, morbidly obese patients cannot tolerate a lack of ventilation for appreciable periods before hypoxemia results. Tracheostomy (usually emergency cricotyrotomy) can be performed in critical emergency situations to secure the airway. However, the overall results are frequently suboptimal when this procedure is performed in urgent circumstances. It is precisely for this reason that many anesthesiologists prefer to intubate these patients awake, using a fiberoptic laryngoscope. Under certain extreme circumstances, it may be prudent to have an experienced surgeon available in the operating room at the time of induction of general anesthesia in case tracheostomy becomes necessary. An alternative to general anesthesia, particularly for extremity surgery, is regional anesthesia (spinal, epidural, intravenous regional, or peripheral nerve block). Regardless of the primary anesthetic technique chosen, airway maintenance, especially with sedation, remains a fundamental concern to the anesthesiologist.
The period of awakening from anesthesia can be problematic for patients with OSA. In patients who have just undergone surgery for the treatment of their OSA, the airway can be narrowed from swelling and inflammation. Also, the lingering sedative and ventilatory depressant effects of the anesthetic can pose difficulty. Perioperative vigilance should continue into the postoperative period. Many patients require postoperative intubation and mechanical ventilation until fully awake. A CPAP (Continuous Positive Airway Pressure) machine can be employed in some patients postoperatively to support breathing. For certain patients, it may be prudent to admit them to an intermediate care or intensive care area postoperatively to facilitate close monitoring and airway support measures. Narcotic analgesics can precipitate or potentiate apnea that may result in a ventilatory arrest. If narcotics are deemed necessary in the postoperative period, appropriate monitoring of oxygenation, ventilation, and cardiac rhythm should be provided.
Obstructive sleep apnea patients undergo surgery for a variety of reasons. Airway maintenance issues and frequently associated cardiopulmonary abnormalities place OSA patients at risk for perioperative complications. Safe anesthetic care can be provided by thorough preoperative assessment, a thoughtful and well-executed anesthetic plan, and vigilance which extends well into the postoperative period.