The 4-second exercise test (T4s) is pharmacologically validated to assess cardiac vagal tone, and consists in pedaling, as fast as possible, a cycle-ergometer unloaded, from the 4th to the 8th second of a 12-second maximum inspiratory apnea. An dimensionless cardiac vagal index (CVI) is calculated from the ratio of the duration of the cardiac cycles (RR intervals at the electrocardiogram) from immediately before the exercise and the shorter of the exercise. Our objective was to determine T4s intra and interdays reliability, and the actual need for two trials, as described on the original protocol. In study 1, the interday reliability of the results was assessed prospectively from 15 asymptomatic subjects (28 ± 6 years) submitted to T4s for five consecutive days, being two trials carried out at each day. To determine CVI intraday reliability, in one of the five days, randomly selected, nine T4s consecutive trials were made. In study 2, the CVI intraday reliability was calculated from 1699 subjects (47 ± 17 years) in two trials. CVI presented high intraday and interday reliability (r i = 0.92; 95%CI = 0.84 to 0.97 and r i = 0.77; 95%CI = 0.49 to 0.92, respectively) for study 1 and for study 2 (r i = 0.89; 95%CI = 0.88 to 0.90). In spite of high reliability, there were some minor differences between the means (mean ± SEM = 1.32 ± 0.01 vs 1.37 ± 0.01; p < 0.001), and in only 15% of the cases this difference was higher than 0.20, thus not representing major clinical meaning. It was also observed that in 65% of the cases, the second trial was considered the best and, with only one trial, clinical misinterpretation could occur in 27% of the data. In summary, this study evidenced high CVI reliability assessed by T4s, and confirmed the need for two consecutive trials, as prescribed in its protocol.
4-second exercise test; Physical exercise; Heart rate; Cardiac vagal tone; Reliability