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Background: Respiratory sinus arrhythmia (RSA) is a major component of heart rate variability (HRV) in the high frequency (HF) range (0.15 – 0.4Hz). It occurs due to the modulation of cardiac vagal outflow by respiration. The longer response time of sympathetic nervous system does not allow for a similar respiratory modulation. Instead, cardiac sympathetic outflow is modulated only in the low frequency (LF) range (0.04 – 0.14Hz), by arterial baroreflexes. Assessment of HRV during deep breathing at 0.1Hz frequency (HRVdb) in the supine resting state is a standard test to assess the cardiac parasympathetic function. At this frequency, there is a possibility of respiration modulating the sympathetic outflow. This study explores the same during standing, which is a sympathoexcited state. Materials and Methods: ECG and respiration were recorded continuously as ten healthy subjects breathed spontaneously and at 0.1Hz frequency in both supine and standing positions for 5 minutes each. The mean and standard deviation of the normal to normal RR intervals (meanRR, SDNN) and the average of the differences between the maximum and minimum RR intervals of 6 consecutive respiratory cycles during 0.1Hz breathing (HRVdb) were compared. Results: Upon standing from the supine position, HRV during spontaneous breathing decreased significantly (p<0.05) while there was no significant change in the HRVdb Conclusion: Although vagal activity is reduced in the standing position, HRVdb did not show a corresponding decrease. Hence, HRVdb is not a very reliable test of cardiovagal activity when there is sympathoexcitation. Key Words 0.1 Hz breathing, cardiovagal function, heart rate variability (HRV), HRVdb, respiratory sinus arrhythmia (RSA)

Heart rate variability during breathing at 0.1 Hz frequency in the standing position