Differential effects of parasympathetic blockade and parasympathetic withdrawal on heart rate variability

Parasympathetic Withdrawal and Blockade Effects on HRV, Introduction: Low h eart rate variability (HRV) has been shown to have important prognostic sig nificance in multiple settings. although this is believed to reflect reduce d parasympathetic tone, the physiology of reduced parasympathetic tone has not been elucidated. Methods and Results: To evaluate whether parasympathetic withdrawal and par tial parasympathetic blockade result in similar changes in HRV, 27 normal v olunteers underwent complete beta-adrenergic blockade and then were given ( 1) graded doses of nitroprusside to achieve baroreflex-mediated parasympath etic withdrawal and (2) low-dose atropine (0.01 mg/kg) to achieve partial p arasympathetic blockade. Five-minute ECG recordings were obtained for HRV a nalysis, In 19 subjects, paired 5-minute recordings from each condition wer e available with mean RR intervals that differed by < 50 msec (low-dose atr opine: 869 +/- 96 msec and nitroprusside 875 +/- 99 msec), The root mean sq uare of the successive RR interval differences was lower following low-dose atropine than following parasympathetic withdrawal with nitroprusside (16 +/- 11 msec vs 22 +/- 15 msec; P < 0.02), During parasympathetic withdrawal , the low-frequency (LF) power was 0.917 +/- 0.602 bpm(2) and the high-freq uency (HF) power was 0.501 +/- 0.521 bpm(2). During partial parasympathetic blockade, the LP and HF powers were significantly lower (0.443 +/- 0.474 b pm(2), P < 0.005; and 0.198 +/- 0.207 bpm(2), P < 0.02). Conclusion: These data confirm that HRV reflects the character of parasympa thetic modulation of the heart rate rather than parasympathetic tone per se . Furthermore, this study identifies two distinct physiologic explanations for the finding of low HRV, namely, diminished vagal discharge and resistan ce of cardiac muscarinic receptors to vagal discharge. Further delineation of the relationships between parasympathetic tone and HRV will allow for be tter understanding of the pathophysiologic derangements associated with low HRV.