Influence of different respiratory maneuvers on exercise-induced cardiac vagal inhibition

Physical exercise inhibits cardiac vagal activity. To study the relationship between heart rate (HR) and respiratory pattern, we applied the 4-s exercise test (4sET) and measured cardiac vagal index (CVI) in 30 healthy subjects who served as their own controls, using the standard plus three additional variations, essentially respiratory, of the original protocol: (a) a maximum inspiratory apnea of 16 s, of which 8 s were in the pre-exercise phase (4sET_insp); (b) free respiratory pattern (4sET_unc); and (c) maximum expiratory apnea of 12 s (4sET_exp). The respective results were expressed by the following CVIs: CVI_insp, CVI_unc and CVI_exp. CVI was determined in a continuous digital ECG recording through a specific ratio of two RR interval durations. The results [(mean ± SEM)] for the four different maneuvers were as follows: CVI (1.56 ± 0.05), CVI_insp (1.55 ± 0.05), CVI_unc (1.63 ± 0.05) and CVI_exp (1.37 ± 0.02). ANOVA-Bonferroni significant differences were only found between CVI_exp and CVI_insp (P = 0.009), CVI_unc (P < 0.001) and CVI (P = 0.003). Dividing our sample in terciles according to CVI values, those with lower CVI, showed an attenuation of biphasic HR response after a 15 s maximum inspiratory apnea. We conclude that cardiac vagal reflex seems to be influencing the biphasic HR response modulation after a 12 s inspiratory apnea as described in the original protocol of 4sET, and this appears to be the option that best discriminates the cardiac vagal reflex, with less variability in the maneuvers when subjects are divided in terciles.