Pulse transit time: validation of blood pressure measurement under positive airway pressure ventilation

The objective of the present study was to validate noninvasive, continuous blood pressure (BP) measurement using pulse transit time (PTT) to represent absolute values and detect BP changes under continuous positive airway pressure (CPAP) treatment. We applied CPAP to 78 patients of a cardiological sleep lab using 0 (baseline), 4, 8, and 12 cmH(2)O for 10 min at every level and measured BP simultaneous via PTT-based method and standard oscillometric method (OM). Quality of signal perception was acceptable to convert PTT into BP values in 64 patients (82%). When comparing both methods, we found a strong linear correlation of systolic and diastolic BP (baseline, r = 0.94 for systolic BP; r = 0.95 for diastolic BP, p < 0.001) while no significant differences between absolute values obtained with OM and PTT measurement. Mean bias at baseline was 4.1 +/- 3.2 mmHg for systolic BP and 2.3 +/- 2.2 mmHg for diastolic BP. With increasing CPAP levels, PTT and OM measurements differed continuously up to a systolic difference of 6.6 +/- 4.9 mmHg and a diastolic difference of 4.4 +/- 3.5 mmHg. There was no definite trend of PTT method to either over- or underestimate BP. We found that PTT- and OM-based BP results are closely correlated while applying CPAP over a period of 40 min. With higher CPAP level, bias between both methods increased.