How It Works
What is measured?
PWTT is calculated for each beat from the ECG and peripheral pulse wave. The peripheral pulse wave is measured by an SpO2 probe on the finger or toe.
PEP and a-PWTT
PWTT includes PEP (Pre-Ejection Period) and a-PWTT (Pulse Wave Transit Time in the Artery).
a-PWTT is the time it takes the pulse wave to travel from the aorta to a peripheral artery. a-PWTT is directly related to blood pressure. Unfortunately, a-PWTT cannot be measured directly. We can only measure PWTT, which also includes PEP. PEP is the period just before the blood is pumped into the aorta. In general, PEP change over short periods of time is negligible in most cases so we can assume that PWTT corresponds to a-PWTT and therefore to blood pressure. However, vasoactive and other drugs can cause significant changes in PEP and affect the correlation between PWTT and blood pressure. Generally in most cases, we can say that PWTT corresponds to a-PWTT and blood pressure.
Relationship between blood pressure and pulse wave speed
When the heart pumps blood into the aorta, it also generates a pressure wave that travels along the arteries ahead of the pumped blood. This is the pulse wave.
The speed of the pulse wave depends on the tension of the arterial walls. When the blood pressure is high, the arterial walls are tense and hard and the pulse wave travels faster. When the blood pressure is low, the arterial walls have less tension and the pulse wave travels slower.
This can also be understood by the following example. When a ping-pong ball is thrown against a hard table, the rebound is strong and fast. If the ping-pong ball is thrown against a soft blanket, the blanket absorbs the force and the rebound is weak and slow.
How PWTT detects change in blood pressure
Although the actual blood pressure itself cannot be determined from the speed of the pulse wave, a change in blood pressure is indicated by change in the speed of the pulse wave. Therefore, PWTT is used to detect change in pressure.
Change in PWTT indicates potential change in blood pressure. PWTT for each beat is compared to the PWTT of the last NIBP measurement. When PWTT change exceeds a threshold, it triggers NIBP measurement to measure the actual blood pressure.
You can increase or decrease the PWTT threshold to respond to larger or smaller blood pressure changes and trigger less frequent or more frequent NIBP measurements.