Plethysmography

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 are based on a fundamental principle of hemodynamics: blood filling of a tissue at any given time represents the difference between the amount of blood flowing in through arterial vessels and the amount of blood draining through the venous bed. Because arterial blood inflow is pulsatile and uneven, the volume of the studied body part also experiences rhythmic fluctuations, which are recorded by the device.

Time characteristics and delay effect

An important feature of hemodynamics is that changes in peripheral blood filling do not occur simultaneously with heart contractions, but with a certain delay. This phenomenon is called delay time.

Let us consider a classic example of calculating the delay 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 is, on average, equal to 1 meter.
• Based on the formula ($t = S / v$), the delay time of peripheral blood filling relative to cardiac activity is about 2 seconds.

Plethysmogram analysis

The graphical representation of volume fluctuations is called a plethysmogram. In its shape, 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 distinguished:

1. Anacrote — the ascending part of the curve (rise period), reflecting a rapid increase in tissue volume due to arterial inflow.
2. Catacrote — the descending part of the curve (fall period), showing a decrease in blood filling.
3. Incisura — a characteristic notch or indentation on the descending part of the graph.
4. Dicrotic wave — a secondary, less pronounced wave of volume increase immediately following the incisura.

Differences from sphygmography and clinical significance

Although plethysmogram and sphygmogram graphs visually consist of the same elements, there is a fundamental physiological difference between them.

If both parameters are recorded simultaneously, one can notice that analogous segments of the curves (e.g., anacrotes) occur at different times. The reason for this lies in hemodynamic mechanisms:

• Sphygmography records the propagation velocity of the pulse wave (mechanical oscillation of the vascular wall), which is very fast.
• Plethysmography records the actual influx of blood mass, meaning it depends on the linear velocity of the blood flow itself, which is significantly slower.

In clinical practice, this distinction makes plethysmography an indispensable tool. The method allows physicians to reliably assess the volumetric blood flow rate in a specific area under study, diagnosing peripheral circulation disorders.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Dependence of blood filling on the phases of the cardiac cycle (systole and diastole)
• Physics of pulse wave propagation compared to linear blood flow
• Hemodynamic causes of incisura and dicrotic wave formation
• Methods for calculating linear and volumetric blood flow velocity

Other topics in "Vascular Bed Hemodynamics"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.