Plethysmography (from Greek Plethysmos – filling, enhancement, graphō – write, depict) is the technique allowing recording of the heart and cardiovascular system parameters on the basis of the blood supply rate to finger capillaries using Infra-red transmitter and light receptor. The technique is called photoplethysmography and based on registration of the ECG interval sequence between systoles, measurement and comparison of their duration followed by statistical analysis of the data for diagnosis and identification of major indicators of the heart and vascular system condition.
Clinical importance of photoplethysmography
In diagnostics the photoplethysmography technique is used for objective assessment of the state and pathologies of the regional blood flow, artery and vein tonus, differential diagnostics of the vascular organic and functional pathologies, as well as assessment of the efficiency of medical treatment prescribed for rehabilitation of vascular functions.
Contrary to the beat-to-beat rate-metering technique (cardiointervalgraphy), photoplethysmography presents data on the basis of several parameters not just on one parameter (heart-beat rate); most important out of these parameters are the tonus of peripheral vessels, heart-beat rate and stroke volume (systolic).
For diagnostic purposes the photoplethysmography technique is most valuable as it is most informative. Distal finger and toe phalanges demonstrate highest intensity of arterial and venous circulation. One square centimeter of the skin on the end of the wrist phalanx has as many as 500 arteriovenous anastomoses. Transcapillary flow rate in distal phalanxes is analogous to that in the brain tissue. Besides, distal phalanxes contain comparatively inconsiderable amount of muscular tissue able to actively absorb infrared radiation.
A signal received from an on-finger clip on the hand is processed by the device, and the data are recorded in the computer for subsequent analysis. The curve displayed on the screen represents the dynamics of the blood flow rate in the finger capillaries. The capillary blood flow rate dynamics depends on many factors including pulse pressure, vascular elasticity and hand surface temperature. This approach allows collection of data on the state of a cardiac vascular system and in combination with today’s mathematical signal processing techniques provides a complete set of variance pulsimetry parameters.