Physicians must carefully note each epoch in all-night polysomnograms to understand how position affects the patient’s condition and to determine proper treatment
When discussing sleep, there are four positions that are commonly referred to as supine (on the back), lateral (left or right side), and prone (on the stomach). Some sleep patients find it impossible to sleep and breathe except in an almost upright or vertical position, which is normally the position of wakefulness. If sleep positions are compared to radii in a circle, then there are an infinite number of intermediate points or angles.
John B. West1 was among the first to scientifically diagram the effects of position on ventilation, blood flow, and gas exchange. While he studied these effects, in both the normal and diseased lung, he paid scant attention to what happens to these factors during sleep. While Wests dictums prevail during sleep and the awake state, there are added effects of positioning on ventilation during the sleep state that can greatly compromise the ability of the cardiorespiratory system to adequately oxygenate the blood. This makes the time when patients are asleep the riskiest time of the day because these effects can precipitate hypertensive crisis, congestive heart failure, and potentially fatal arrhythmias ending in cardiac arrest and death. If a person with a history of coronary disease were out shoveling snow or engaging in some other strenuous exercise, a heart attack might be an understandable consequence. If that same person were asleep at home in a nice warm bed and the same thing happened, it becomes questionable. A significant percentage of all heart attacks occurs precisely under these circumstances between 4 am and 7 am. And round-the-clock Holter monitoring of heart rhythms indicates a preponderance of cardiac rhythm abnormalities during this period as well.2 This is no coincidence.
The answer to this question is an evolutionary anomaly involving human anatomy. The structures in the posterior portion of the oropharynxthe portal through which air passesserve people well when they are rigid and awake, but block the ingress and egress of air while flaccid and asleep. This anatomy blocks airflow even more efficiently when lying in the supine position. When one is positioned laterally or semirecumbent (in a slightly upright position), the obstructing effect on airflow may be lessened but not always eliminated.
Other anatomical factors enhance the obstructing property of a low-hanging soft palate and uvulaan enlarged uvula and oversized tonsils and adenoids (if present). First invented by an anesthesiologist to predict the difficulty anticipated in performing endotracheal intubation, a simple procedure called a Mallampatti score can help clinicians screen for this problem. A quick peek with a flashlight at the posterior oropharynx (see Figure 1, page 18) can rate the patency of the airway even if the person is awake. If one has a particularly heavy neck, the weight of this added tissue also impinges on the patency of the airway as does the weight of the abdominal cavity in a morbidly obese individual.
FIGURE 1. Upper airway showing the different segments of the pharynx and indicating the variety of upper airway abnormalities reported to cause obstructive sleep apnea.
Thus, position, gravity, and anatomy join together to conspire against us when we are asleep. Position interacts with gravity, which is Wests underlying thesis. Add to this the loss of muscle tone and rigidity that occurs during sleep, particularly rapid eye movement (REM) sleep, and there may be a recipe for disaster. It is a foregone conclusion that in sleep tests, the greatest number of obstructive events and oxygen desaturations (if they occur at all) happen while in the supine position during REM sleep.3 In addition, during an obstructive event, with the airway blocked or partially blocked, the individual is straining to breathe, creating an increased negative pressure in the thoracic cavity. This favors the return of more blood to the right side of the heart, raises blood pressure, and overloads the system. This abnormally increased volume of blood fluid coursing through the system finds its way to the kidneys, batters the fine network of vessels there, and increases blood pressure and urinary output. It is not unusual for sufferers of obstructive sleep apnea (OSA) to wake several times during the night to urinate.
Position is of paramount importance during an all night polysomnogram to diagnose OSA and to titrate continuous positive airway pressure (CPAP) treatment. Although some patients refuse to sleep in any position other than elevated or laterally (some even insist on sleeping in chairs), sleep apnea may or may not be seen in such positions, whereas it does occur if the patient is made to lie supine. Its severity may also go from mild to severe if the subject switches position from lateral and/or upright to supine. It is helpful for sleep laboratories to have electric hospital-type beds so that patients can be put to sleep sitting upright and then lowered by the technologist after sleep onset to help make the diagnosis. There are manufacturers that distribute queen-sized electric beds that are reinforced to accommodate morbidly obese patients weighing up to 1,000 pounds. In our facility this special bed plays a critical role in obtaining accurate diagnostic data. In addition, CPAP may be undertitrated unless the technologist can evaluate pressure levels in different positions, especially while supine in REM stage. The selection of a bed, therefore, can be as important as the polysomnographic system in collecting data during the night. The availability of equipment to obtain positioning is an important reason for the sleep study and CPAP titration to be performed in a properly equipped facility as opposed to the home. In unattended home studies, there is little or no control over a patients position. The consequences of this can be grave as the degree or even existence of OSA can be underestimated or missed entirely. A particular CPAP pressure may be adequate to arrest all events in the lateral or upright position but fall far below what may be required when patients roll onto their backs. Evaluating this difficulty in the sleep laboratory eliminates the problem, although CPAP machines capable of delivering pressure limits that vary in response to respiratory events may eventually become useful in such cases.
Position is an integral component of all-night polysomnograms and must be carefully noted, either automatically or manually, on each epoch collected so that scoring personnel and interpreting physicians understand exactly how it has affected the test subjects condition. It is not only important to document the effect of position to meet diagnostic standards (Medicare and insurance company requirements) but, more important, to adequately treat the patient. And if a patient with OSA fails to tolerate CPAP and forgoes a surgical option, then positional therapy remains the final, although not necessarily the best, weapon in the current arsenal against OSA.
Thomas Kilkenny, DO, FAASM, medical director, and Steve Grenard, RRT, clinical coordinator, are both at the Sleep Apnea Center, Staten Island University Hospital, Staten Island, NY.
1. West JB. Ventilation/Blood Flow and Gas Exchange. Oxford, England: Blackwell Inc; 1965.
2. Guilleminault C, Connolly S, Winkle R. Cyclical variation in the heart rate in sleep apnea syndrome: mechanisms and usefulness of 24-hour electrocardiography as a screening technique. Lancet. 1984:126-131.
3. Cartwright RD. Effect of sleep position on sleep apnea severity. Sleep. 1985;7:110-114.