Plethysmography is a non-invasive method to measure changes in the volume of an organ or body part. Medical professionals use it to monitor parameters such as oxygen saturation levels and heart rate. This diagnostic tool provides insights into a patient’s circulatory status.
How Plethysmography Works
Plethysmography operates on the principle of light absorption by blood. A device, often placed on a finger, toe, or earlobe, emits two types of light: red and infrared. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Conversely, deoxygenated hemoglobin absorbs more red light and less infrared light.
The amount of light transmitted through the tissue changes with each heartbeat as blood volume fluctuates. A photodetector measures the varying light absorption. These detected changes convert into an electrical signal, which generates a visual waveform. This waveform illustrates the pulsatile blood flow in the capillaries.
Characteristics of a Normal Plethysmographic Waveform
A healthy plethysmographic waveform displays a smooth, consistent, and pulsatile shape. It begins with a rapid upward deflection, the systolic upstroke, corresponding to blood ejection from the heart during systole. Following the peak, a gradual downslope features a distinct dip or indentation called the dicrotic notch. This notch signifies the closure of the aortic valve and momentary backward blood flow into the aorta.
The amplitude, or height, of the waveform reflects the relative change in blood volume with each cardiac cycle. A consistent amplitude indicates stable peripheral perfusion. The waveform then descends into a diastolic downslope, representing continued outflow of blood from the capillaries. A normal reading includes an oxygen saturation (SpO2) level between 95% and 100% and a regular heart rate, confirming adequate oxygen delivery and cardiac function.
Interpreting Abnormal Plethysmographic Readings
Deviations from the typical waveform signal various physiological issues. A diminished or flattened waveform suggests poor peripheral perfusion, caused by conditions such as low blood pressure (hypotension) or narrowing of blood vessels (vasoconstriction). This reduced amplitude indicates less blood volume reaching the measurement site.
A highly variable waveform, where amplitude changes significantly, could point to conditions such as hypovolemia (decreased blood volume) or cardiac arrhythmias (irregular heart rhythms). An absent waveform indicates a complete lack of pulsatile flow at the site, potentially due to severe circulatory compromise or a dislodged sensor. These waveforms indicate a need for further medical assessment to determine the underlying cause and appropriate intervention.
Factors Influencing Plethysmographic Accuracy
Several factors impact the accuracy of plethysmographic readings. Patient movement can introduce artifacts into the waveform, making interpretation difficult. Improper sensor placement, such as loose application or incorrect alignment, can hinder accurate light transmission and detection, leading to erroneous data.
Cold extremities can cause vasoconstriction, reducing blood flow to the measurement site and resulting in a diminished or unreadable waveform. External factors like nail polish or artificial nails can interfere with the light path, affecting accuracy. Strong ambient light, from surgical lamps or sunlight, can be detected by the sensor, corrupting the signal and producing inaccurate readings.