Continuous Non-invasive Blood Pressure Measurement Using The Pulse Wave Transit Time

Arterial blood pressure is a fundamental physiological parameter for the assessment of cardiovascular haemodynamic function in basic medical research as well as in clinical practice. A new continuous non-invasive blood pressure measurement method was described, which monitored the BP by measuring pulse wave transit time(PWTT).The physiological and physical principle for BP monitoring applying PWTT was presented and a formula BP = a + b×PWTT describing the relation between the BP and the PWTT was deduced. Two methods— hydrostatics method and Korotkoff's sound delay time method were proposed to determine the characteristic parameters a and b of BP equation.It has been validated that the relation between artery blood pressure and PWTT can be described using a linear equation, which was determined by parameters a and b . But the parameters among different individuals were not uniform. The parameters must be obtained before we monitor the BP with the equation. Two BP values and the corresponding PWTT is necessary to determine the patameters.In hydrostatics method , one leg was elevated and the blood pressure in the artery of the elevated leg altered.The PWTT in the artery elevated changed correspondingly.The ΔBP which is the change of BP,can be calculated by the height of the elevated leg.The ΔPWTT which was the change of PWTT,was the difference of the two PWTT values at two posture.Accordingto ΔBP and ΔPWTT ,the coefficient of leg b_leg can be obtained. There were three methods to deduce the coefficient b corresponding to the wholePWTT:(1) according to the ratio of the length of the artery in the trunk and leg.(2) according to the PWTT in the artery in the trunk and the leg. (3)according to the PWTT and the modulus in the trunk artery and in theleg.After obtained parameter b, combined with the blood pressure at basicposture,parameter a can also be obtained.Using the blood pressure equation,we can continuously estimate the blood pressure by PWTT in every cardiaccycle.The relation between Korotkoff's Sound Delay time (T_K) and cuff pressure (P ) was firstly proposed. T_K became smaller with the cuff pressuredecreased.A function T_K = f(P) was used to describe the law. In fact, thechange of the cuff pressure reflected the change of artery transmural pressure.If the cuff pressure keeps constant, the change of transmural pressurereflects the change of artery blood pressure. dT_K / dP is the change of T_K for 1mmHg transmural pressure change.Holding the cuff pressure at a relevant constant level, the alter of T_K represents the alter of the blood pressre.The beat-to-beat change of blood pressure ΔEBP can be calculated by dT_K/ dP .Accumulating the ΔEBP can get the blood pressure EBP. Withthe simultaneous PWTT, the parameter b can be get. There were two methods to calculate b :(1) Calculate the regress coefficient of EBP and PWTT.(2) Calculate the ratio of the peak-valley of EBP and PWTT. Moreover,when cuff pressure keeps constant, great blood pressurefluctuation induced by the strong respiration improves the accuracy.