Polysomnography is the study of several physiological variables during sleep, including the recording of sleep itself. The precise items recorded will vary but usually include electroen-cephalography, electro-oculography, electromyography, respiratory pattern, snoring, oxygen saturation, transcutaneous carbon dioxide tension, electrocardiography, and body posture (49-51). This allows the recognition of events that occur during sleep and offers the advantage over other techniques that sleep can be identified with certainty.
In patients with marked respiratory muscle weakness, detection of respiratory movement during obstructive events from external sensors may be very difficult or impossible. Indeed, true obstructive events may be scored even by skilled observers as "central." There are three ways around this problem. First, esophageal pressure may be recorded during polysomnogra-phy in all such patients (52, 53). Second, flattening of the flow-time curve during these events in sleeping subjects may be helpful (54). Third, some centers have adopted the approach that patients with multiple "central" apneas should be treated with continuous positive airway pressure to determine whether these events respond to continuous positive airway pressure.
Computerized systems are generally used, which allows the recording of the all-night data on to optical or compact disk formats. Detection of fluctuation in nasal pressure during inspiration and expiration reflects changes in inspiratory and expiratory airflow and therefore is a promising method for the detection of hypopneas (50, 55). Studies have shown that nasal pressure is more sensitive than thermal sensors for detecting hypo-pneas and that the square root of the pressure signal improves the estimate of airflow (56, 57).
Polysomnography allows identification of sleep. It can detect coincidence of different events, such as respiratory change and sleep disturbance. It is possible to identify the extent of decline in arterial oxygen saturation (SaO2) during different sleep stages. The sequelae of hypoventilation due to respiratory muscle insufficiency during sleep can be identified.
Polysomnography is expensive relative to limited sleep study equipment, both in terms of capital and staff cost. Expert technical staff are needed to perform and analyze the studies.
Polysomnography is a valuable method of identifying sleep-related hypoventilation in patients with respiratory muscle problems and whether they have related sleep apnea-hypo-pnea syndrome as well. It is useful in some patients to monitor progress on treatment, such as noninvasive positive pressure ventilation.
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