AbstractThe failure of changes in autonomic drive to affect heart rate is a common finding in patients with the sick sinus syndrome (SSS). The normal respiratory sinus arrhythmia is vagally mediated and should therefore be attenuated in SSS. This hypothesis was tested in a series of 32 symptomatic patients with SSS, mean age 68 years, and an age‐matched control group of 75 normal subjects. The degree of sinus arrhythmia present in routine 12‐lead electrocardiograms was quantitated as (1) the coefficient of variation (CV) of the R‐R interval, and (2) the maximal R‐R interval difference (ΔR‐Rmax). Mean CV was 2.7% in SSS compared to 3.8% in the control group (p < 0.01). Mean ΔR‐Rmaxwas 72 ms in SSS and 140 ms in controls (p < 0.01). Sixteen of 32 patients (50%) compared to only 4/75 (5%) control subjects had ΔR‐Rmaxof less than 80 ms (p < 0.01). Thus, SSS is frequently associated with a significant attenuation of sinus arrhythmia and the simple measurement of ΔR‐Rmaxin standard ECG recordings provides useful diagnostic information.