High variability in the reproducibility of key hemodynamic responses to head-up tilt.

Abstract

Increased blood pressure upon standing is considered a cardiovascular risk factor. We investigated the reproducibility of changes in aortic blood pressure, heart rate, stroke volume, cardiac output, and systemic vascular resistance during three passive head-up tilts (HUT) in 223 participants without cardiovascular medications (mean age 46 years, BMI 28 kg/m2, 54% male). Median time gap between the first and the second HUT was 9 weeks and the second and the third HUT 4 weeks. We utilized whole-body impedance cardiography and radial artery tonometry as methods. The participants were divided into quartiles of the changes in each hemodynamic variable during the first HUT, and the reproducibility of these changes was tested during successive HUTs. During the first HUT, significant differences were present in all between-quartile comparisons (n=6) of all variables. The differences persisted as follows: reduction of stroke volume in six out of six (6/6) between-quartile comparisons (p<0.001), decrease in cardiac output (p<0.001) and increase in heart rate in 5/6 comparisons (p<0.001), change in systemic vascular resistance in 3/6 comparisons (p<0.001), change in aortic systolic blood pressure in 1/6 comparisons (p=0.043), and change in aortic diastolic blood pressure in none (p=0.266). To conclude, the reproducibility of upright posture-induced changes is high for stroke volume, cardiac output, and heart rate, moderate for systemic vascular resistance, and modest for aortic blood pressure. While an increase in blood pressure during upright posture may be a cardiovascular risk factor, this effect may be attributed to other underlying hemodynamic variables that exhibit more reproducible posture-related changes.