Variable day/night bias in 24-h non-invasive finger pressure against intrabrachial artery pressure is removed by waveform filtering and level correction

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Abstract
BACKGROUND Twenty-four-hour finger arterial pressure (FAP) recordings show a negative bias against intrabrachial artery pressure (BAP) and the bias is greater during the night, thereby overestimating the nocturnal blood pressure dip. We have available a methodology with which to reconstruct BAP from FAP by waveform filtering (transfer function) and generalized level (bias) correction that reduces the bias for short-term blood pressure records. OBJECTIVE To investigate if this methodology also decreases the extra bias during the night, thereby yielding a better estimate of the nocturnal dip. METHODS Twenty-four-hour FAP and BAP blood pressure recordings were simultaneously obtained in eight healthy normotensive volunteers and 14 patients with hypertension (ages 19-60 years), during standardized scheduled activities. The data were analysed off-line, applying the brachial reconstruction technique (reBAP) consisting of a waveform filter and level correction. Simultaneous beats yielded systolic, diastolic and mean pressures that were averaged per 30 min, per day, per night, per activity, over the 24-h period, and for volunteers and patients separately. RESULTS Over the full 24 h, FAP systolic, diastolic and mean values for the total group differed from BAP by +1 +/- 10, -8 +/- 7 and -10 +/- 8 mmHg (mean +/- SD), respectively. Similarly, reBAPs differed by +1 +/- 11, -2 +/- 7 and -2 +/- 7 mmHg. BAPs dipped by 20 +/- 8, 13 +/- 6 and 15 +/- 6 mmHg, respectively, during the night. These dips were overestimated by +8, +4 and +4 mmHg by FAP, but not by reBAP: -1, +1 and +1 mmHg. The volunteer and the patient groups showed slight differences in results, but these were not statistically significant. CONCLUSIONS The generalized reconstruction technique to obtain near-brachial pressure from non-invasive FAP almost completely removed bias over the full 24-h day-night period and improved tracking of diurnal changes for all three blood pressure values.