WHY DO BRACHIAL SYSTOLIC BLOOD PRESSURE AND PULSE PRESSURE REMAIN RELATIVELY CONSTANT BETWEEN SECOND AND FIFTH DECADE OF LIFE, YET RISE STEEPLY FROM INFANCY TO AGE 18 AND OVER AGE 50?
M. O'Rourke*, A. Avolio2, A. Qasem2.
Medical Professorial Unit, St Vincent's Hospital1 and Graduate School of Biomedical Engineer, University of New South Wales2, Sydney, New South Wales.
Major epidemiological studies of arterial pressure have shown a relative plateau of systolic pressure (SP) and pulse pressure (PP) between age 18 and 50 years. In adults a progressive increase in pressure occurs after 50 years, whilst in children a steep increase in pressure is seen from infancy to adolescence. The cause and implications of this phenomenon are unexplained. We sought an explanation by modelling wave transmission in the trunk and upper limb. Brachial pulse wave velocity (PWV) remains relatively constant throughout adult life so that the pressure transfer function between ascending aorta and brachial artery also remains relatively constant. In contrast, aortic PWV increases by 40% between age 18 and 50.
Methods: Upper limb pressure waves for the second (D2) and fifth decade (D5) were obtained from ensemble averaged signals from 100 subjects and used with the brachial artery transfer function to estimate ascending aortic pressure waves. Pressure values were calibrated from data of large epidemiological studies. Aortic impedance and indices of wave reflection were determined, assuming a constant heart rate (68 b/min) and stroke volume for D2 and D5.
Results: Brachial pressure of 125/75 mmHg (D2) and 132/86 (D5) corresponded to an aortic pressure of 108/78 (D2) and 124/85 (D5) respectively. That is, between ages 17 and 50 brachial SP increased by 5.6% while aortic SP increased by 14.9%. Separation of pressure into forward and backward waves showed that at age 50 the intensity of wave reflection is 13.8% higher than at age 18. The energy at low frequencies (where there is minimal pulse amplification, but most of the energy [>90%]) is greater at age 50 (14% and 9% increase respectively for the first two harmonics of heart frequency). This is consistent with the known increased stiffness of the aortic trunk with age.
Conclusion: The relative plateau in SP and PP between ages 18 and 50 years is due to progressive decrease in amplification of the pressure wave between aorta and brachial artery, even as amplitude of the aortic pressure wave progressively increases. A steep rise of brachial systolic pressure from infancy to age 18 is partially due to the same phenomenon, with increasing body length causing increase in brachial pressure wave amplification.
