Behind the Numbers: Exploring the Impact of Age and Calibration Method on Diurnal Variations in 24-Hour Central Ambulatory Blood Pressure.

Objective: Diurnal fluctuations in brachial blood pressure (BP) are well-known. However, the extent to which these are reflected in central systolic BP (cSBP) estimations remains uncertain, particularly due to differences in brachial waveform calibration methods. Calibration using mean arterial and diastolic BP(MAP/DBP) has received growing interest; however, our understanding of the implications of such methods remains unclear. We further explored the impact of calibration method on diurnal variations in ambulatory cSBP and related waveform indices in a large population of healthy individuals. Design and method: Ambulatory BP monitoring was conducted in 1,045 adults (45% female; age range:18–88 years) using the Mobil-O-Graph device. Twenty-four-hour cSBP profiles were generated using two brachial waveform calibration methods: (1) MAP/DBP, and (2) SBP/DBP. Brachial waveform analysis was performed to characterise the form factor (FF) underlying MAP estimation. Results: Waveform calibration method exerted a marked impact on absolute values of both cSBP(Figure 1) and other waveform indices including non-augmented SBP (P1), across the adult age span. Moreover, significant day-night differences were apparent in heart rate; MAP; and cSBPSBP/DBP in both younger and older adults(P0.05). In contrast, there was a marked absence of nocturnal dipping of cSBPMAP/DBP in younger adults (Figure 1). Similarly, while P1SBP/DBP exhibited nocturnal dipping in all adults(P<0.001), this diurnal variation was absent in the younger males P1MAP/DBP(P=0.53). Waveform analysis determined that the FF used to calculate MAP was fixed, with no inter-individual variability. Conclusions: Calibration method significantly and puzzlingly influences the 24-hour profile of ambulatory cBP, particularly in young adults. A potential explanation is that MAP estimations based on a fixed FF may not account for age-related and diurnal variations in waveform morphology. As FF varies with pulse wave shape within and between individuals, by measurement site, age, and heart rate, reliance on a fixed FF may lead to systematic bias in MAP estimation and, in turn, MAP-derived cBP calibration. Our observations are unlikely to be explained by true physiology, i.e. exaggerated bSBP dipping, but rather may represent methodological artefact, warranting further investigation.