Pulse transit time: an appraisal of potential clinical applications.

TLDR: Current techniques for investigating patients with suspected sleep disordered breathing are inadequate and the difficulty in identifying and quantifying the spectrum of respiratory events responsible for the sleep fragmentation and disabling excessive daytime sleepiness that is characteristic of obstructive sleep apnoea and its related conditions.

Abstract: Current techniques for investigating patients with suspected sleep disordered breathing are inadequate. Not only are many of the tests expensive, but they are also cumbersome and many centres require these investigations to be performed in the sleep laboratory. At the heart of the problem is the difficulty in identifying and quantifying the spectrum of respiratory events responsible for the sleep fragmentation and disabling excessive daytime sleepiness that is characteristic of obstructive sleep apnoea and its related conditions. Obstructive apnoeic episodes are not usually difficult to detect, even when only a basic measure of respiratory effort such as thoracic and abdominal movement is used. On the other hand, correctly identifying obstructive hypopnoeas and episodes of upper airway resistance needs a sensitive measure of airflow and inspiratory effort. The measurement of swings in pleural pressure by oesophageal manometry is the current gold standard technique for detecting changes in inspiratory effort. However, the placement of an oesophageal catheter is often uncomfortable and unacceptable, it may modify the upper airway dynamics,1 and some believe that it contributes to the sleep disturbance during the sleep study, though this is disputed.2 Furthermore, this technique is available in only a proportion of sleep laboratories and, if performed, adds significantly to the cost of the sleep study. These issues pose a clinical dilemma because it is important to detect and define hypopnoeas and upper airway resistance episodes as they are as relevant as apnoeas in producing sleep fragmentation. The assessment of sleep structure also creates problems. Not only is high quality electroencephalographic (EEG) monitoring difficult to achieve outside the laboratory, but the scoring is laborious in terms of technician time and is hence expensive. In addition, there remains uncertainty regarding the definition of an arousal.3 It is likely that “micro-arousals” which fall short of the …