Automated blood pressure measurement: Reasons for measurement variability uncovered

The most common automated blood pressure devices determine systolic, diastolic and mean arterial blood pressure (SBP, DBP and MAP) by analysing the oscillometric pulse waveform. The aim of this study was to assess the variability of the amplitude of oscillometric pulse waveform characteristics at SBP, DBP and MAP. Sixty oscillometric waveforms from twenty subjects were analysed. Manual SBP and DBP were obtained with a manual sphygmomanometer, from which manual MAP was calculated from the empirical equation. Automated MAP was estimated from the maximum oscillometric pulse. The oscillometric pulse amplitudes, normalised to the maximum pulse, at above SBP, SBP, MAP, DBP and below DBP were then determined. The cuff pressures at half the maximum oscillometric pulse amplitude were also measured. There were significant differences in normalised oscillometric pulse amplitude between SBP and DBP (mean ± SD: 0.45 ± 0.10 vs 0.80 ± 0.12, P < 0.001), and between manual and automated MAP (0.89 ± 0.09 vs 1.00 ± 0, P < 0.001). Manual SBP (118 ± 11 mmHg) and DBP (76 ± 9 mmHg) were significantly different from the cuff pressures at half the maximum oscillometric pulse amplitude (117 ± 12 mmHg and 66 ± 10 mmHg), with the paired differences of 1 ± 5 mmHg (P < 0.05) and 10 ± 7 mmHg (P < 0.001) respectively. Significant differences between manual and automated MAP were also observed, with the paired difference of 3 ± 6 mmHg (P < 0.001). In conclusion, there are large variations in the pulse characteristics at SBP, DBP and MAP. This complicates a reliable automatic estimation of these values.