Respiratory monitoring using reflection mode photoplethysmography : clinical and physiological aspects

Abstract: Photoplethysmography (PPG) is a non-invasive optical technique for monitoring variations in blood volume and blood flow in skin and other tissues. Light from a light-emitting diode is absorbed, scattered and reflected from the skin, and detected by a photodetector as a plethysmogram. The plethysmogram contains variations of different frequencies. The most clinically utilised is the pulse synchronous variation in the PPG application pulse oximetry, but there is also a respiratory rate synchronous variation (PPGt).The aims of this work were to evaluate PPG as a clinical method for detection of breaths during anaesthesia, postoperative and intensive care, and to investigate possible circulatory factors in the physiological background of the respiratory synchronous part of the reflection mode PPG signal.It was concluded that respiratory variations in reflection mode PPG derived from the forearm could be detected with high sensitivity and specificity. PPG, was not significantly affected by awake or anaesthetised state, though indirect signs of reduced sympathetic tone in the anaesthetic state were present, or by spontaneous or positive pressure ventilation. There were no significant differences between men and women or between young and old subjects. Circulatory pressure variations in phase with respiration were present on both the arterial and venous side and they showed the same degree of variation, as did PPG when provoking respiration. Squared coherence, cross-correlation and visual techniques for evaluating time differences were equally good.Respiratory monitoring in the clinical setting is challenging and no golden standard exists. Methods based on airflow are mostly considered advantageous in detecting apnoea, but are less well tolerated by patients. The results indicate that PPG has a potential as a tool for monitoring respiratory rate. It is non-invasive, well tolerated and can be used for continuous monitoring.