A Comparison of the Indirect Estimate of Mean Arterial Pressure Calculated by the Conventional Equation and Calculated to Compensate for a Change in Heart Rate

The standard equation used to calculate mean arterial pressure (MAP) assumes that diastole persists for 2/3 and systole for 1/3 of each cardiac cycle. This ratio is altered when heart rate increases, and therefore we investigated the efficacy of predicting MAP during exercise using non-invasive indirect methods. Eight subjects exercised on a cycle ergometer for 3 minute intervals to elicit heart rates between 100 - 110, 120 - 130, 140 - 150, 160 - 170, and 180 - 190 beats/min. In the last minute of each 3 min interval an ECG recording was taken and systolic (SP) and diastolic (DP) blood pressure was measured by manual auscultation. MAP was calculated for each heart rate interval by: MAP = DP + 1/3(SP - DP) (method A), and MAP =DP + F_S(SP - DP) (method B), where Fs is the fraction of the cardiac cycle comprising systole, measured from the ECG. F_S increased from 0.35 ± 0.049 at rest to 0.47 ± 0.039 at a heart rate of 180 - 190 beats/min. MAP measured by method B was consistently greater than MAP calculated by method A at all heart rates greater than resting heart rate (p < 0.01). The error incurred when using the standard MAP equation (method A) to derive MAP during exercise (measured as the percentage difference between method A and B) increased linearly with heart rate (r = 0.98). The standard MAP equation should not be applied during exercise, as it does not account for the change in the systolic : diastolic period ratio as heart rate increases.

Key words:

Blood pressure - heart rate - arterial systolic period - arterial diastolic period

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