PLETHYSMOGRAPH

Typical Number in Hospital: 5

Cost Bands: 2,3

References: 3,9

This measures changes in volume. It may be used to measure the changes in the volume of the whole body during respiration, changes in the chest volume due to blood flow or respiration, or changes in limb volumes due to the blood flow. The common methods of measurement include electrical impedance (see Impedance plethysmograph), optical transmission or reflection changes due to blood volume changes (see Photoplethysmograph), or strain gauges may be fitted around the segment being monitored (see Mercury plethysmograph). Short-term changes in blood volume occur because the venous flow is steady whereas the arterial flow is pulsatile. Therefore the volume of the segment will vary during the cardiac cycle. Venous occlusion plethysmography is used to estimate the perfusion of a body segment by applying pressure to the proximal part of the segment to prevent the return of venous blood and then noting the rate of change of volume of the segment when the pressure is released. Sometimes the limb or limb segment is included in a chamber (chamber plethysmograph) which seals around the segment. Volume changes are calculated from the air flow in and out of the chamber, or from pressure changes.

The whole body plethysmograph, using a body box, can be used to find the residual volume (RV) of the lungs. The body box consists of a sealed chamber like a telephone box. The patient breathes through a tube to the outside and changes in chest volume are detected by changes in air pressure in the box.

Impedance plethysmography uses low-frequency alternating current which may be applied through ECG-type electrodes. This technique is used to monitor breathing and in some cases to estimate cardiac output.

Photoplethysmography utilizes the transmission or reflection of light to demonstrate the changes in blood perfusion. Such devices might be used in the cardiology department, intensive care department or for diagnostic purposes related to vascular surgery.