A plethysmograph is an instrument for measuring changes in volume within an organ or whole body (usually resulting from fluctuations in the amount of blood or air it contains). A photoplethysmograph (PPG) is a plethysmograph that uses optical techniques. A pulse oximeter is a PPG and can mesure the change in the volume of arterial blood with each pulse beat. This change in blood volume can be detected in peripheral parts of the body such as the fingertip or ear lobe using a technique called photoplethysmography. The pulse oximeter that detects the signal is called a plethysmograph (or 'Pleth' for short).

Because of the nature of pulse oximeter, measuring the pressure from a pleth waveform has not been possible. However, the information has shown to be useful in diagosis and in gauging the severity of illness in newborns. Many doctors use the pulse oximeter's pleth waveform as an early indication for cyclic changes in physiology. If the variability increases, it indicates a change in the intrathoracic/blood volume relationship.

The blood flow is normally shown as a wareform using a bar or graph. It can provide useful information regarding the heart condition.

Pleth is useful to detect irregular heart rhythm. The primary function of the heart is to supply blood and nutrients to the body. The regular beating, or contraction, of the heart moves the blood throughout the body. By watching the pleth waveform, the heart beating pattern is clearly displayed. When a heart patient is having discomfort, it is important to observe the pleth waveform and relate the information to his doctor. Since the discomfort is generally transient, a doctor may not be able to see the waveform during an office visit.

Only more advanced pulse oximeter would provide the waveform. Read this article "Photoplethysmograph" for more information how the waveform is related to breathing, hypovolemia, and other circulatory conditions (premature ventricular contraction or PVC and venous pulsations). The following diagram illustrates the relationship between PPG and PVC.

Besides the handheld pulse oximeter MD300I2, finger pulse oximeters MD300C3, MD300C310 and MD300C21 all have this feature.