hrv mean values. mood and anxiety disorders low rmssd, thought to represent vagal tone, have been associated with major depression. lower sdnn and elevated lf/hf were found in those with bipolar disorder, and in particular those characterized as having greater illness severity due to greater number of episodes, illness duration and whether there had been psychosis. patients with ptsd also had lower hf, a measure of vagal tone. modifications by specific interventions interventions that augment hrv may be protective against cardiac mortality and sudden cardiac death. although the rationale for changing hrv is sound, it also contains the inherent danger of leading to the unwarranted assumption that modification of hrv translates directly into cardiac protection, which may not be the case. despite the growing consensus that increases in vagal activity can be beneficial, it is not as yet known how much vagal activity (or hrv as a marker) has to increase in order to provide adequate protection. β-adrenergic blockade the data on the effect of β-blockers on hrv in post-mi patients are surprisingly scant. despite the observation of statistically significant increases, the actual changes are very modest. in conscious post-mi dogs, β-blockers do not modify hrv. the unexpected observation that before mi, β-blockade increases hrv only in the animals destined to be at low risk for lethal arrhythmias after mi may suggest novel approaches to post-mi risk stratification. antiarrhythmic drugs data exist for several antiarrhythmic drugs. flecainide and propafenone but not amiodarone were reported to decrease time domain measures of hrv in patients with chronic ventricular arrhythmia. in another study, propafenone reduced hrv and decreased lf much more than hf. a larger study confirmed that flecainide, also encainide and moricizine, decreased hrv in post-mi patients but found no correlation between the change in hrv and mortality during follow-up. thus, some antiarrhythmic drugs associated with increased mortality can reduce hrv. however, it is not known whether these changes in hrv have any direct prognostic significance. scopolamine low-dose muscarinic receptor blockers, such as atropine and scopolamine, may produce a paradoxical increase in vagal effects on the heart, as suggested by a decrease in heart rate. in addition, scopolamine and low dose atropine can markedly increase hrv. however, though the heart rate slowing in proportional to the (low) dose of atropine, the increase in hrv varies widely across and within individuals. this suggests that even for vagal activity to the heart, hrv may be a limited marker. thrombolysis the effect of thrombolysis on hrv (assessed by pnn50) was reported in 95 patients with acute mi. hrv was higher 90 minutes after thrombolysis in the patients with patency of the infarct-related artery. however, this difference was no longer evident when the entire 24 hours were analyzed. exercise training exercise training may decrease cardiovascular mortality and sudden cardiac death. regular exercise training is also thought to modify cardiac autonomic control. individuals who exercise regularly have a 'training bradycardia' (i.e., low resting heart rate) and generally have higher hrv than sedentary individuals. biofeedback the technique called resonant breathing biofeedback teaches how to recognize and control involuntary heart rate variability. a randomized study by sutarto et al. assessed the effect of resonant breathing biofeedback among manufacturing operators; depression, anxiety and stress significantly decreased. a first overall meta-analysis by goessl vc et al. (24 studies, 484 individuals, 2017) indicates ''hrv biofeedback training is associated with a large reduction in self-reported stress and anxiety'', while mentioning that more well-controlled studies are needed. wind instruments one study that surveyed the physiological effects of playing native american flutes found a significant hrv increase when playing both low-pitched and high-pitched flutes. normal hrv values even though there are no widely accepted standard values for hrv that can be used for clinical purposes, multiple studies have measured and reported normal values for various populations: normal values of standard measures of hrv time domain analysis spectral analysis variable units normal values (mean±sd) variable units normal values (mean±sd) ibi ms 926±90 lf ms2 519±291 sdnn ms 50±16 hf ms2 657±777 rmssd ms 42±15 lf nu 52±10 hf nu 40±10 lf/hf ratio 2.8±2.6