Correlation among cardiac autonomic function tests in normal subjects

Cardiac autonomic function tests such as the Orthostatic Challenge Test (OCT) and Valsalva Manoeuvre (VM) test are popular clinical neurophysiological investigations used to diagnose autonomic disorders. Analysis of heart rate responses in OCT (3015 ratio) and VM (Valsalva ratio) evaluates the parasympathetic responses of the cardiac autonomic nervous system.The indices of short-term Heart Variability Analysis (HRV) that reflects the cardiac parasympathetic activity are pNN50, RMSSD and HF power. The current study investigated whether there was any correlation between (a) OCT 3015 ratio and Valsalva ratio (b) OCT 3015 ratio and pNN50, RMSSD and HF power (c) Valsalva ratio and pNN50, RMSSD and HF power. Towards this, the data acquired from 22 healthy male volunteers were analysed Five min ECG and respiration were recorded for short-term HRV analysis. Following this, the OCT was performed where the subjects were instructed to stand abruptly from the supine posture. For VM the subjects were instructed to blow through a mouthpiece attached to a mercury manometer to sustain a pressure of 40mm of Hg for 10 seconds. The correlation of short-term HRV parameters and OCT 3015 ratio and Valsalva ratio was done using Spearmans correlation test. The results of our study showed that there was a weak positive correlation between the OCT and VM test results (r0.431. p0.045). The Valsalva ratio showed a weak but highly significant correlation with the HF power (r0.559, p 0.007) while the OCT results showed no correlation with any of the HRV indices considered.Thus, in conclusion, though all the tests studied were evaluating the cardiac parasympathetic activity, strikingly, the OCT results showed no correlation with any of the short- term HRV indices while the VM test results showed highly significant correlation with HF power. Thus, even though VM is more difficult to perform, the Valsalva ratio appears to be better correlated with short-term HRV index of HF power.