ESTIMATION OF MEAN BLOOD PRESSURE BY THE CONVENTIONAL FORMULA, COMPARED WITH THAT OBTAINED WITH THE CALCULATION TAKING ACCOUNT THE DURATION OF EJECTION TIME AND CARDIAC CYCLE

Objective: The mean arterial pressure (MAP) is generally calculated with the values of systolic blood pressure (SBP) and diastolic blood pressure (DBP), according to the conventional formula MAP=SBP-DBP x 0.33 +DBP. However, some studies show the inaccuracy of this calculation when compared with direct or invasive measurements of blood pressure. Our study aims to analyze the extent of the differences between the MAP obtained conventionally and that resulting from considering the real length of the systolic and diastolic periods (weighted MAP). Design and method: During the echocardiogram recording of 110 subjects of both sexes, in sinus rhythm and without valve lesions, SBP and DBP were measured with oscillometric equipment. Cardiac cycle length (Cycle), heart rate (HR), and aortic ejection period duration (Ao-e-Systole) were measured on aortic Doppler. MAP was calculated with the formula: MAP = SBP-DBP x (Ao-e-Systole /Cycle) + DBP. This latter value was compared with the MAP obtained conventionally. Results: The value of the Ao-e-Systole/Cycle relationship was higher as HR was higher (r=0.70, p<0.001). The MAP discrepancy between the two calculation methods was significantly correlated with HR (r=0.71, p<0.001). Conventional MAP shows that at lower HRs, its values are higher than those resulting from weighted MAP, but at high HRs, the traditional formula shows lower values than those obtained with the formula proposed in our study. At intermediate HRs, these discrepancies are minimal. Conclusions: The conventional formula for calculating MAP overestimates its true value at lower HRs, and underestimates it at higher HRs. These discrepancies are negligible at intermediate and usual HRs. The heart rate value should be considered when estimating MAP in subjects with tachycardia or bradycardia, particularly in patients in which organ perfusion needs to be known with precision.