Photoplethysmography

Abstract

The photoplethysmographic (PPG)waveform, also known as the pulse oximeter waveform, is one of the most commonly displayed clinical waveforms. First described in the 1930s, the technology behind the waveform is simple. Thewaveform, as displayed on themodern pulse oximeter, is an amplified and highly filtered measurement of light absorption by the local tissue over time. It is optimized by medical device manufacturers to accentuate its pulsatile components. Physiologically, it is the result of a complex, and not well understood, interaction between the cardiovascular, respiratory, and autonomic systems. Allmodernpulse oximeters extract and display the heart rate and oxygen saturation derived from the PPG measurements at multiple wavelengths. "As is," the PPG is an excellent monitor for cardiac arrhythmia, particularly when used in conjunction with the electrocardiogram( ECG).With slight modifications in the display of the PPG (either toa stripchart recorderor sloweddownonthemonitor screen), the PPG can be used to measure the ventilator-induced modulationswhich have been associated with hypovolemia. Research efforts are underway to analyze the PPGusing improved digital signal processing methods todevelopnewphysiologicparameters. It ishoped that when these newphysiologic parameters are combinedwith amore modern understanding of cardiovascular physiology (functional hemodynamics) the potential utility of the PPG will be expanded. The clinical researcher's objective is the use of the PPG to guide early goal-directed therapeutic interventions (fluid, vasopressors, and inotropes), in effect to extract from the simple PPG the information and therapeutic guidance thatwaspreviously only obtainable froman arterial pressure line and the pulmonary artery catheter.