Towards using photo-plethysmogram amplitude to measure blood pressure during sleep.

There is considerable interest in non-intrusive and reliable continuous ambulatory blood pressure measurement systems. Pulse amplitude is the peak to trough amplitude of the photo-plethysmogram signal. We compared pulse amplitude with a currently popular parameter, the pulse arrival time (PAT), for estimating continuous systolic blood pressure (SBP). Overnight sleep data from 18 young, healthy subjects (14 M 4 F, age 24+/-5 years, BMI 23.8+/-4.0 kg/m2) was analyzed. We found that pulse amplitude was more effective than PAT for estimating SBP during sleep. Mean coherence between pulse amplitude and SBP was significantly stronger than that for PAT [p<0.001, 95% CI: 0.21-0.25 (finger), 0.11-0.14 (wrist)]. Correlation between pulse amplitude and SBP was significantly stronger than that for PAT [p<0.001, 95% CI: 0.46-0.53 (finger), 0.13-0.20 (wrist)]. SBP estimation errors were significantly lower using pulse amplitude [p<0.001, 95% CI: -1.55 to -1.29 mmHg (finger), -0.53 to -0.36 mmHg (wrist)]. We also found that while pulse amplitude was closely related to SBP, the relationship weakened during and around REM sleep (ANOVA of REM, transitional Wake-REM and transitional REM-Sleep versus other sleep states: F=24.7, p<0.001). These results suggest that pulse amplitude is potentially a more suitable measure than pulse arrival time for estimating continuous blood pressure.