When a patient is connected to a hospital monitor, the screen displays various lines and numbers. One continuous, rhythmic line is labeled “Pleth,” short for plethysmograph. This waveform provides a visual, beat-by-beat representation of the patient’s pulse and how blood is flowing through the extremities. It offers immediate, non-invasive insight into the patient’s circulatory status and peripheral circulation.
Defining the Pleth Waveform
The term plethysmography refers to measuring changes in volume within an organ or the body, usually resulting from fluctuations in blood volume. In the hospital setting, the monitor displays a photoplethysmograph (PPG), which uses optical techniques to track these changes in a peripheral area like a fingertip or earlobe. The resulting wavy line, or pleth waveform, directly represents the volume of arterial blood flowing into the tissue with each heartbeat. This continuous tracing indicates the quality of blood flow in the body’s smaller vessels. The line rises and falls rhythmically, with each peak corresponding to the heart’s contraction and the subsequent rush of blood.
How the Monitor Creates the Line
The pleth waveform is generated using the technology within a pulse oximeter, a device traditionally used to measure oxygen saturation. The sensor works by emitting two different wavelengths of light, typically red and infrared, through the patient’s tissue, while a photodetector measures the light that passes through. The amount of light absorbed changes with each heartbeat because arterial blood is pulsatile, meaning its volume changes cyclically. When the heart contracts, a surge of blood flows into the tissue, momentarily increasing the volume and absorbing more light. The pulse oximeter separates this rhythmic change from the constant background absorption, translating it into the visible, oscillating pleth waveform.
What the Pleth Wave Reveals
The shape and consistency of the pleth waveform are used by clinicians to interpret the patient’s physiological state. A strong, well-defined waveform with clear, sharp peaks suggests a robust pulse and adequate blood perfusion at the sensor site. The regular interval between the peaks also confirms a consistent heart rhythm.
Conversely, an erratic, flattened, or small waveform can signal several issues. A small amplitude may indicate poor perfusion, perhaps due to low blood pressure or the body constricting blood vessels. The waveform’s presence confirms that the oxygen saturation reading displayed by the pulse oximeter is reliable. If the pleth line is distorted or absent, the corresponding numerical oxygen reading is likely inaccurate, prompting investigation into the poor signal quality or circulation.