Occasional reviews Pulse transit time: an appraisal of potential clinical applications
چکیده
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 diYculty 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 diYcult 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 eVort. The measurement of swings in pleural pressure by oesophageal manometry is the current gold standard technique for detecting changes in inspiratory eVort. However, the placement of an oesophageal catheter is often uncomfortable and unacceptable, it may modify the upper airway dynamics, and some believe that it contributes to the sleep disturbance during the sleep study, though this is disputed. 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 diYcult 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. It is likely that “micro-arousals” which fall short of the standard EEG criteria for arousal may contribute significantly to the patient’s hypersomnolence. It is well recognised that not all obstructive respiratory events end with a recognisable arousal. Indeed, in normal individuals “non-visible” sleep fragmentation caused by repetitive auditory stimulation has been shown to cause significant daytime sleepiness in normal subjects. It has also been shown that some stimuli, which are not suYcient to produce cortical arousals detectable on the EEG, may nevertheless result in an acute rise in blood pressure. The clinical significance of these “subcortical” autonomic arousals is not clear and is the focus of much research activity. If both “micro-arousals” and “subcortical” arousals turn out to be important in terms of excessive daytime sleepiness and cardiovascular risk, then methods which detect the associated sympathetic and cardiovascular activation may be more relevant than cortical arousals detected with EEG. It is clear that new techniques for detecting signs of arousals and changes in respiratory eVort are needed. Ideally they should be less invasive, more practical, and cheaper than (but as reliable as) current tests used for investigating patients with suspected sleep disordered breathing. Pulse transit time (PTT) is a physiological measure which shows promise in these respects, not only in the sleep laboratory but also in domiciliary sleep studies. In this review we will briefly describe how it is measured and where its possible clinical applications might be. Also discussed are the limitations of PTT and areas requiring validation and further development.