Rapid Declines in Systolic Blood Pressure Are Associated With an Increase in Pulse Transit Time

Background : Substantial investigation has been made into the correlation between Pulse Transit Time (PTT) and Blood Pressure (BP), as a possible route to achieve continuous non-invasive measurement of BP (cNIBP). We investigated whether PTT-trends could model BP-trends during episodes of rapid declines in Systolic Blood Pressure (SBP). Methods: From the freely available Medical Information Mart for Intensive Care (MIMIC-III) waveform database, we identified subjects who experienced a reduction in SBP from ≥ 120mmHg to ≤ 90 mmHg during a period of ≤ 15 minutes, for whom complete peak detection was possible. SBP was extracted from the Arterial Blood Pressure (ABP) waveform, and PTT was calculated from the R-peak of the ECG to the peak of the ABP waveform. Both SBP and PTT were processed using a moving average filter, yielding the variables SBP AV and PTT-RA AV . A moving average of continuous heart rate (HR AV ) was also analysed as a negative control to assess the effect of averaging. The intra-individual association between variables was assessed per subject using linear regression. Results: 511 patients were included for the main analysis. Median correlation coefficients (r) obtained from linear regression versus SBP AV were as follows: PTT-RA AV -0.93 (IQR -0.98 to -0.76), HR AV 0.46 (IQR -0.16 to 0.83). Regression slopes for the relationship between SBP AV and PTT-RA AV displayed a median of -2.46 mmHg/ms (IQR -3.47 mmHg/ms to -1.61 mmHg/ms). In supplementary analysis, results did not differ substantially when widening inclusion criteria, but the results were not always consistent within subjects across episodes of hypotension. Conclusions: In a large cohort of critically ill patients experiencing episodes of rapid declines in systolic blood pressure, there was a moderate-strong intra-individual correlation between averaged systolic blood pressure and averaged pulse transit time as measured from ECG R-peak to the peak of the arterial blood pressure waveform. Our findings encourage further investigation into using the pulse transit time for non-invasive real-time detection of hypotension.