Physiological Basis of the Method
The volume of any organ (such as a finger or an entire limb) is not a static value. It constantly changes depending on the phase of the cardiac cycle — systole and diastole.
These fluctuations rely on a fundamental hemodynamic principle: tissue blood volume at any given moment is the difference between the amount of blood entering via arterial vessels and the amount leaving through the venous bed. Because arterial blood inflow is pulsatile and uneven, the volume of the examined body part also undergoes rhythmic oscillations, which are recorded by the device.
Temporal Characteristics and the Lag Effect
An important hemodynamic feature is that peripheral blood volume changes do not occur simultaneously with cardiac contraction, but with a specific delay. This phenomenon is called lag time.
Consider the classic example of lag calculation for hand vessels:
- The average linear blood flow velocity in major vessels is about 0.5 m/s.
- The distance blood must travel from the heart to the fingers averages 1 meter.
- Based on the formula ($t = S / v$), the lag time of peripheral blood filling relative to cardiac activity is about 2 seconds.
Plethysmogram Analysis
The graphic representation of volume oscillations is called a plethysmogram. Morphologically, the resulting curve bears a striking resemblance to aortic pressure changes and a typical pulse wave (sphygmogram).
When interpreting a plethysmogram, four main elements are identified:
- Anacrot — the ascending limb of the curve (rise period), reflecting a rapid increase in tissue volume due to arterial inflow.
- Catacrot — the descending limb of the curve (descent period), showing a decrease in blood volume.
- Incisura — a characteristic notch or indentation on the descending part of the graph.
- Dicrotic wave — a secondary, less pronounced wave of volume increase immediately following the incisura.
Differences from Sphygmography and Clinical Significance
Although plethysmogram and sphygmogram tracings visually consist of the same elements, a fundamental physiological difference exists between them.
If both parameters are recorded simultaneously, one can observe that analogous curve segments (e.g., anacrots) occur at different times. This is due to underlying hemodynamic mechanisms:
- Sphygmography records the propagation velocity of the pulse wave (mechanical oscillation of the vessel wall), which is very fast.
- Plethysmography records the actual delivery of blood mass, thus depending on the linear flow velocity of the blood itself, which is significantly slower.
In clinical practice, this distinction makes plethysmography an indispensable tool. The method allows clinicians to reliably assess the volumetric blood flow rate in a specific area of interest, aiding in the diagnosis of peripheral vascular disorders.