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mortality [1–5] . However, these parameters of heart rate variability, heart rate recovery, and heart rate turbulence stand for different components of cardiac autonomic function. Heart rate variability represents the spontaneous activity of cardiac autonomic function. Heart rate turbulence is used for evaluation of the spontaneous response of baroreflex receptor activity against stress. Heart rate turbulence shows the recovery of parasympathetic tonus after maximal treadmill stress testing [2–5] . Piotrowicz et al. [5] reported that 8-week physical training in patients with heart failure changed the heart rate variability indices without significantly altering the indices of heart rate recovery or heart rate turbulence, indicating that various mechanisms affected the parameters of heart rate variability, heart rate recovery, and heart rate turbulence. I think the discussion of outcomes in the study by Yilmaz et al. [1] might have been more useful if the authors had considered the remarkable difference among heart rate variability, heart rate recovery, and heart rate turbulence measurements. Moreover, it would have been more helpful to perform the study using the combination of heart rate variability, heart rate recovery, and heart rate turbulence for a clearer determination of the mechanism of impairment of cardiac autonomic function in the mercury-exposed individuals. Dear Editor, I read with great interest the paper by Yilmaz et al. [1] entitled ‘Assessment of the Cardiac Autonomic Nervous System in Mercury-Exposed Individuals via Post-Exercise Heart Rate Recovery’. They aimed to investigate cardiac autonomic function in mercuryexposed individuals by exercise heart rate recovery. They demonstrated lower heart rate recovery indices in the mercury-exposed patients, implying that there was cardiac autonomic dysfunction in these patients. Cardiac autonomic dysfunction is an independent cardiovascular risk factor and can be evaluated by noninvasive methods such as heart rate variability, heart rate turbulence, or heart rate recovery [2–4] . One of the best noninvasive approaches to assess cardiac autonomic function is to analyze heart rate variability defined as the oscillation of the interval between successive R waves on the ECG between beats. Hence, heart rate variability provides numerical data about the cardiac autonomic function [3] . Heart rate recovery is defined as the difference between maximum heart rate and heart rate during the first minute of the recovery period after treadmill stress testing. A difference ≤ 12 beats is considered an abnormal response of the cardiac autonomic function [4] . Heart rate turbulence is the measurement of turbulence onset and turbulence slope according to an algorithm, and describes the return of the heart rate to equilibrium after a premature ventricular contraction [1] . Impairments on heart rate variability, heart rate turbulence, or heart rate recovery are associated with a greater risk of Published online: October 10, 2016

Heart Rate Variability, Heart Rate Recovery, and Heart Rate Turbulence Represent Different Components of the Cardiac Autonomic Nervous System