Cardiovascular regulation: associations between exercise and head-up tilt

It was hypothesized that faster cardiorespiratory kinetics during exercise are associated with higher orthostatic tolerance. Cardiorespiratory kinetics of 14 healthy male subjects (30 ± 4 years, 179 ± 8 cm, 79 ± 8 kg) were tested on a cycle ergometer during exercise with changing work rates of 30 and 80 W. Pulmonary oxygen uptake (Formula: see text) was measured breath-by-breath and heart rate (HR), mean arterial blood pressure (MAP), and total peripheral resistance (TPR) were measured beat-to-beat. Muscular oxygen uptake (Formula: see text) was estimated from HR and Formula: see text. Kinetic parameters were determined by time-series analysis, using cross-correlation functions (CCF max (x)) between the parameter and the work rate. Cardiovascular regulations of MAP, HR, and TPR during orthostatic stress were measured beat-to-beat on a tilt seat. Changes between the minima and maxima during the 6° head-down tilt and the 90° head-up tilt positions were calculated for each parameter (Δ tilt-up ). Formula: see text correlated significantly with ΔTPR tilt-up (r = 0.790, p ≤ 0.001). CCF max (HR) was significantly correlated with ΔHR tilt-up (r = –0.705, p = 0.002) and the amplitude in HR from 30 to 80 W (r SP = –0.574, p = 0.016). The observed correlations between cardiorespiratory regulation in response to exercise and orthostatic stress during rest might allow for a more differential analysis of the underlying mechanisms of orthostatic intolerance in, for example, patient groups.