Abstract: 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)
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