Nonlinear relationship between systolic blood pressure and pulse transit time in anesthetized dogs

Pulse wave transit time (PTT), as measured between the ECG R-wave and the arterial pulse wave at a distal site, has been previously reported to linearly correlate with blood pressure (BP). This study reinvestigated the relationship where various doses of epinephrine were administered to three anesthetized dogs to invoke a wide range of hypotension. The invasive systolic blood pressure in the left ventricle, the noninvasive photoplethysmogram pulse wave and the ECG of lead II were recorded synchronously. The results indicated that the relationship between the systolic BP and the PTT was nonlinear and logarithmic: P=b₁-b₂ lnT, where b₁ and b₂ are coefficients. The estimated BP using the nonlinear relationship was highly correlated with the measured BP; the correlation coefficients were often greater than 0.96 in the 36 trials. The root mean square of the estimation error was less than 6.53 mmHg. However, the coefficients b₁ and b₂ varied between trials, doses, and dogs. An analysis of the error contribution revealed that the estimation error increased linearly with the coefficient errors, Δb₁ and Δb₂, and with the PTT error, ΔT. The definition of the pulse arrival time (PAT) also affected the performance of the estimation. The performance based on five PAT definitions was compared. This study shows that caution is necessary when using this relationship to monitor BP.