PWTT

PWTT can detect sudden blood pressure change and trigger NIBP measurement to confirm it. Together with periodic NIBP measurement, this can provide more complete blood pressure monitoring. PWTT, pulse wave transit time, is a non-invasive parameter which can indicate sudden change in blood pressure. PWTT is measured by continuous monitoring of ECG and pulse wave. No additional sensors or modules are required. In most cases, change in PWTT correlates with change in blood pressure. When PWTT change exceeds a threshold, it triggers NIBP measurement to confirm blood pressure.

Principle of PWTT

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Blood pressure	PWTT
Increases	Shortens
Decreases	Lengthens

PWTT is the time from the R wave of the ECG to the appearance of the peripheral pulse wave. The peripheral pulse wave uses a photoplethysmographic signal obtained from a SpO₂ probe attached to a finger. Generally, the PWTT shortens as blood pressure increases and lengthens as blood pressure decreases.*

Why does PWTT reflect changes in blood pressure?

The speed of waves that propagate through a tube depends on the stiffness of the tube; the stiffer the tube, the faster the waves propagate, and the softer the tube, the slower the waves propagate (Figure 2).        
In the human body, the arterial wall becomes stiffer when blood pressure increases, which speeds up the propagation of the pulse wave and accelerates the time it takes for the pulse wave to reach the peripheral arteries.        
When blood pressure decreases, the arterial wall becomes softer, and the pulse wave travels slower to the peripheral arteries. Thus, changes in blood pressure result in changes in the time it takes for the pulse wave to travel through the blood vessels. Therefore, when a change in PWTT is observed, there is also a change in blood pressure.*

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References        
1) Sugo Y, Ogino K, Ochiai R, Iwao Y,. Evaluation of blood pressure monitoring using pulse wave transit time in the operating room (in Japanese). The Japanese journal of medical instrumentation. 2001; 71 (10),551-552        
2) Sugo Y, Ogino K, Mato T, Uehara J, Tsutsumi H. Blood pressure monitoring in the emergency room using pulse wave transit time. The Japanese journal of medical instrumentation. 2002; 72 (10), 551-552

Case Studies

These examples show some clinical applications of PWTT, as well as typical correlation of PWTT with blood pressure. The data is from actual cases. IBP, PWTT and periodic NIBP were actually measured but the PWTT triggered NIBP is a simulation based on the actual measured data.

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In this case, bleeding started shortly before a periodic NIBP measurement. The resulting BP drop was not detected by the periodic NIBP measurement, but PWTT immediately detected the BP change. Ephedrin was given to increase the BP.

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ICU

In this case, the BP of a conscious patient in the ICU was high and unstable. A sedative was administered and the BP dropped then became stable. 30 minute periodic NIBP measurement could not detect these BP changes but PWTT was able to detect it.

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Reference 
Sugo Y. From invasive blood pressure measurement to real time measurement ~PWTT (Pulse Wave Transit Time) triggered NIBP measurement~. The 3rd Workshop of the Japanese Society for Medical and Biological Engineering CE Safety Study Group. 2001.