The autonomic nervous system (ANS) undergoes important adaptations to aerobic physical training and alterations in its function are related to the genesis and prognosis of various diseases. The heart rate fluctuation in response to respiration is a natural phenomenon in which is based the respiratory sinus arrhythmia test (RSA), utilized to evaluate the function of the parasympathetic branch of the ANS in regulating the cardiovascular system. The RSA is calculated by the coefficient between the longer R-R interval from the electrocardiogram (ECG) during the expiration (E) and the shorter R-R interval during inspiration. So far, there is no standard way to select the R-R intervals when the ECG is registered during two consecutive respiratory cycles. The purpose of this study was to compare the methods used to select these intervals, determining the most effective way to quantify the RSA. Fifty-five young volunteers performed two consecutive RSA tests, each one composed by two consecutive respiratory cycles. The two largest E/I indexes calculated by either one of these two ways were then compared: 1) both E and I derived from the same respiratory cycle (intra-cycle); 2) E and I from any one of the two respiratory cycles (cycle-independent). The cycle-independent E/I indexes (1st test = 1.49 ± 0.03; 2nd test = 1.44 ± 0.03) were higher than the intra-cycle E/I indexes [1st test = 1.44 ± 0.03; 2nd test = 1.41 ± 0.03 (P < 0,001)]. The cycle-independent E/I indexes from the 1st test were higher than those from the 2nd test (P = 0.04). The use of the E and I intervals chosen independently of the respiratory cycle where they occurred allowed for the observation of the maximum variation of the parasympathetic activity, denoting the maximal magnitude of the RSA. In addition, the results suggest a reflex accommodation during the 2nd test, making it unnecessary to perform a 2nd test when the 1st one is performed without any technical problem.
Autonomic nervous system; Respiratory sinus arrhythmia; Autonomic cardiovascular tests
