Height Correction for Carotid Blood Pressure Measurement During Positional Changes Is Important to Account for Gravitational Effects when Assessing Arterial Stiffness

Abstract

IntroductionArterial stiffness is one of the earliest detectable manifestations of adverse functional changes within the arterial wall. Carotid arterial stiffness and distensibility are usually expressed by pressure‐strain elastic modulus (Ep), beta‐stiffness index (β), and arterial compliance (AC). However, when measuring these parameters in a seated or semi‐recumbent position, it may be important to adjust measurement of the carotid blood pressure to account for the gravitational influence of the fluid column by correcting for the height difference between the carotid artery and the brachial artery,PurposeTo investigate the effect of change in orthostatic position on carotid blood pressure measurement, a common measurements of arterial health used in research and clinical practice.MethodsHealthy young adults’ (n=20, 25 yrs) were randomly positioned at three different torso angles (θ) on an adjustable table: 1) 0°, 2) 45°, and 3) 72°. Heart rate (HR), blood pressure (BP), β, Ep, AC and pulse wave velocity (PWV) were measured at all three positions after a 5 minute rest period. Vertical height (Dv) from heart to the carotid measurement site (calculated by sin(θ)*distance) was then used to correct carotid BP (HC) for gravity (Dv*9.806/133).ResultsUncorrected β decreased but HC β increased linearly with an increase in θ. HC carotid blood pressures and diameters were decreased at all three angles (p<0.05), while PWV increased at all three angles (p<0.05), see table. Mean arterial pressure (MAP) and diastolic blood pressure (DBP) were higher at 45 and 72 degrees compared to 0 degrees (p<0.05), but were not different from each other. No significant changes were noted in systolic blood pressure (SBP), HR, Ep, or AC with a change in angle.ConclusionThis data demonstrates the importance of correcting for the role that gravity plays on the carotid blood pressure in a seated/semi‐recumbent position. Not accounting for a change in pressure caused by gravitational influence on the fluid column results in an incorrect decrease in measured arterial stiffness with positional change. Correcting the carotid blood pressure dramatically changes the outcome of β, which appears to be increasing arterial stiffness in the carotid artery that is also reflected in central arterial stiffness (PWV). Further investigation is required to fully elucidate the effects of positional change on the validity of measures of arterial stiffness. Variables 0 Degrees 45 Degrees 72 Degrees Heart Rate (bpm) 58 ± 2 60 ± 2 60 ± 2 SBP (mmHg) 122 ± 2 125 ± 3 124 ± 2 DBP (mmHg) 75 ± 2 * 78 ± 2 77 ± 1 MAP (mmHg) 93 ± 2 * 96 ± 2 96 ± 1 cSBP (mmHg) 120 ± 3 125 ± 3 124 ± 3 cDBP (mmHg) 75 ± 2 * 78 ± 2 78 ± 1 Beta‐Stiffness Index 4.3 ± 0.3 4.0 ± 0.3 3.9 ± 0.2 Ep 54 ± 4 53 ± 4 50 ± 4 AC 1.19 ± 0.07 1.11 ± 0.07 1.10 ± 0.06 HC Beta‐Stiffness Index 4.3 ± 0.3 4.6 ± 0.3 4.7 ± 0.3 HC cSBP (mmHg) ** 120 ± 3 113 ± 3 107 ± 3 HC cDBP (mmHg) ** 75 ± 2 66 ± 2 61 ± 1 PWV (m/s) ** 5.0 ± 0.1 6.3 ± 0.2 6.6 ± 0.2 Max Diameter (mm) ** 6.67 ± 0.17 6.37 ± 0.14 6.20 ± 0.14 Minimum Diameter (mm) ** 6.00 ± 0.16 5.68 ± 0.14 5.52 ± 0.14 All data are mean ± SEM, 0 degrees different from 45 and 72 degrees, p<0.05, All three angles are different from each other, p<0.05.