P38 Differences in Form Factor Calculated from Oscillometric or Waveform Mean Arterial Pressure

Abstract

Background

Oscillometric mean arterial pressure (MAP) agrees closely with invasive MAP, [1] but most devices do not report MAP and it is usually estimated by a form factor (FF). However, blood pressure (BP) measurement errors will affect FF, its correlations with exposures, and introduce errors into MAP estimated from the BP waveform.

Methods

Brachial BP was measured using a Pulsecor device in 1,112 participants in the Southall and Brent Revisited study (68.8 ± 6.1 y; 78.2% male; 47.4% White-European; 38.3% South-Asian; 14.3% African-Caribbean). Form factors (FFosc and FFwave) were calculated as (MAP-diastolic BP)/(systolic BP-diastolic BP) by oscillometry (MAPosc) or from the BP waveform (MAPwave).

Results

FFosc and FFwave differed (0.28 (SD = 0.02) vs. 0.36 (SD = 0.04); p <0.001) and were negligibly correlated (r = 0.07). Neither FF_osc nor FF_wave were associated with ethnicity, prevalent cardiovascular disease or current smoking status, and neither showed significant correlations with age, total-or HDL-cholesterol, or physical activity. Both FF_osc and FF_wave were lower in men (difference (Δ) = −0.005(95% CI = −0.007, −0.002) vs −0.015(95% CI = −0.020, 0.009) respectively) and were negatively correlated with height (r = −0.14 both), but only FF_wave correlated with body mass index (r = 0.02 vs r = 0.10) and heart rate (r = −0.06 vs r = 0.20). ΔMAP_osc-MAP_wave correlated with age (r = 0.10), height (r = 0.15) and heart rate (0.17) and was greater in women (0.9(95% CI = 0.5, 1.3) mmHg).

Conclusions

FF_wave agrees poorly with FF_osc probably due to measurement errors. This creates spurious associations between exposures and FF and causes systematic errors in estimated MAP_wave. These errors have the potential to confound associations in epidemiological studies.