Background
Plethysmography is the study of changes in volume of an organ from the fluctuations in blood or air contained within it. Digital plethysmography uses air-filled cuffs connected to pressure transducers and photoplethysmographs (PPGs) to specifically evaluate changes in the blood volume of a finger or fingers. The digital plethysmograph is a secondary output of the pulse oximeter, and it provides a noninvasive, continuous, and real-time measurement of arterial pressure by transmitting an infrared light through the digits. The technique is easy to implement and does not require patient cooperation, which makes it a valuable diagnostic tool for detecting peripheral vascular diseases like Raynaud’s phenomenon and scleroderma.1,2
Historical Overview
Plethysmography has been used since the late 1800s to measure the pharmacological effects of drugs, primarily vasodilating effects of organic nitrates. The PPG waveform was first examined in the 1930s, but it was not until the 1980s that this waveform gained widespread use in clinical research and practice. It was in the 1980s that digital PPG was also first developed, at which time it was presented as a noninvasive technique for continuously measuring hemodynamic parameters in research and clinical settings. Today, digital PPG may be used alongside other established measurements of hemodynamic parameters.1,3
Description
The plethysmogram waveform represents pulsatile peripheral blood flow, which reflects both peripheral and central hemodynamics. Digital plethysmography uses infrared light transmitted through a finger to continuously measure arterial pressure. The commonly used method involves integration of the infrared light source into a pneumatic digital cuff serving as a transmitter-receiver unit, which is placed around the pulp of the tested digit. The instantaneous digital blood volume is reflected in the light intensity, and the light signal increases with cuff pressure elevation, and reduces when the intravascular pressure is exceeded based on changes in digital artery blood content and lumen dimension. Maximal amplitude occurs when the arterial wall is completely relaxed and the transmural pressure ceases.1,2
For finger plethysmography performed with upper-arm occlusion, the recommended occlusion time is 5 minutes to reach maximum response. Similar to Doppler readings, the tracing of a normal artery should produce a triphasic wave pattern, whereas stenotic and occluded vessels will produce classic, pathognomonic waveform patterns. Pre- and post-operative plethysmographs may also be obtained to evaluate effectiveness of interventions, such as digital periarterial sympathectomy for scleroderma or severe Raynaud’s disease.1,2
