[Respiratory function diagnosis during sleep: possibilities and limitations]

Diagnosis of the respiratory function during sleep is based on a continuous recording of all necessary signals. These are basically respiratory flow, respiratory effort and blood gases. In addition, sleep stages, body position and cardiovascular function have to be monitored to distinguish the different forms of sleep related breathing disorders as there are obstructive, mixed and central apnea and hypoventilation. The main evaluation is a simple counting of events and the calculation of an index related to total sleep time. These indices are of limited value because all information about the duration of the events is lost and about the severity which depends on the actual blood gas values. A computer assisted evaluation of respiration can overcome these limitations and allows to evaluate sleep stage dependencies in addition. Based on the duration of respiratory events only it was possible to distinguish patient groups which also provided different blood gas values at daytime. Determination of the critical closing pressure of the upper airways during sleep can help to quantify effects of body weight loss and other therapeutic interventions beside ventilation therapy in patients with sleep related breathing disorders. Quantitative characterisation of respiration during sleep is limited by the fact that most methods available induce changes in sleep or respiration itself. A study on healthy subjects proved an increased number of arousals during cardiorespiratory polysomnography. Therefore the added value obtained by quantitative methodology (pneumotachograph and esophageal pressure) must be weighted against non-invasive technology (inductive plethysmography). Non-invasive methods can provide comprehensive information about the respiratory function, especially if extensive computer-assisted evaluation is used. These evaluations still need further improvements. Nevertheless they can help to clarify the different types of respiratory regulation present in NREM-, REM-sleep and awake state.