Assessment of the cardiac vagal reflex: validation of the 4-s exercise test during simulated wheelchair propulsion

PURPOSE: The rapid heart rate increase during the first seconds of exercise is primarily mediated by cardiac vagal withdrawal, which can be assessed by the 4-s exercise test (4sET), a pharmacologically validated and highly reliable procedure. However, the need for a cycle ergometer limits its applicability. Thus, we aimed to test the validity of a modified 4sET using wheelchair propulsion simulation (WPS) by comparing it with the traditional cycle ergometer protocol. METHODS: Sixty healthy adults (30 men, 30 women; 22 ± 2 years) performed, in randomized order, three repetitions each of the traditional (LEG 4sET) and modified (WPS) protocols. RR intervals were recorded via electrocardiography, and the cardiac vagal index (CVI) was calculated as the ratio of the last pre-exercise RR interval (RRB) to the shortest exercise RR interval (RRC). RESULTS: = 0.63; P < 0.001). After correction, CVI did not differ between protocols (1.48 ± 0.17 vs. 1.48 ± 0.20; P = 0.854). Furthermore, a high and significant intraclass correlation coefficient (ICC) was found for the CVI between the protocols (ICC = 0.87; [0.74-0.93]; P < 0.05), and Bland-Altman analysis showed negligible bias and acceptable limits of agreement for the corrected WPS CVI. CONCLUSION: The WPS shows strong agreement with the traditional cycle ergometer 4sET, supporting its validity as a simpler and more accessible method for assessing dynamic cardiac vagal control.