Abstract P349: Establishing Correction Factors for Oscar and SpaceLabs 90207 Oscillometric Ambulatory Blood Pressure Monitors

Abstract

Purpose and Background: To obtain a more accurate picture of diurnal variations in blood pressure (BP), we establish correction factors based on differences between mercury (Hg) column & 24-hr ambulatory blood pressure (ABPM) measurements in the lab. In normotensives, hypertensives & alcoholics, an auscultatory ABPM underestimated diastolic BP (DBP) & misclassified DBPs in > 75% of cases. Our goal was to explore oscillometric ABPMs accuracy & reliability. Methods: In the lab, simultaneous opposite arm BPs by the Oscar 2 and SpaceLabs 90207 ABPMs & simultaneous same arm BPs by observers (O1, O2) using a ThinkLabs digital electronic stethoscope with ambient noise reduction, were obtained in 5 subjects seated with arms at the phlebostatic axis. Postural tests included simultaneous, same arm Hg BPs by two observers & 45 sequential recordings by each ABPMs (SBP range 92-183; DBP range 30-124 mm Hg). Results: Baseline SBPs for O1, O2, Oscar & SpaceLabs were 112.8 ± 22.3, 114.0 ± 23.4 (O1 - O2 Δ = - 1.2), 123.7 ± 23.4 & 119.0 ± 19.4 (Oscar - SpaceLabs Δ 4.7) mm Hg, respectively. The Oscar overestimated O1O2 SBPs by 10.3 mm Hg (P < 0.05). DBPs for O1, O2, Oscar & SpaceLabs were 61.7 ± 22.2, 62.4 ± 22.4 (O1 - O2 Δ = -0.7), 66.5 ± 16.6 & 67.4 ± 15.7 (Oscar - SpaceLabs Δ -0.9) mm Hg, respectively. From postural tests, average SBP/DBP correction factors were -4.5/-3.2 mm Hg for the Oscar and -1.3/-2.4 mm Hg for the SpaceLabs, but with extreme variation from -25 to +16 mm Hg based on supine, seated, standing and normotensive vs. hypertensive conditions. The Oscar significantly (P <0.01) overestimated SpaceLabs 24-hr SBP (154 ± 11 vs 146 ± 10, Δ = 8 mm Hg) in a hypertensive, but overestimations were less pronounced for 24-hr DBP (101 ± 9 vs 98 ± 8, Δ 3 mm Hg). In a normotensive, SBP & DBP corrections for 24-hr ABPMs were less clinically significant, except during sleep. Conclusions: Our results though limited, indicate that recordings obtained may depend entirely upon the ABPM selected and the BP range and postures assumed by the subject over 24 hr, demanding further investigation and assessment of postural BPs. Unless correction factors are developed on an individual basis and subjects happen to match the population from which oscillometric nomograms were developed, the accuracy of 24-hr ABPMs should be questioned.