Respiratory sinus arrhythmia magnitude quantification as a potential marker of stress and pain in cows and sheep

Respiratory sinus arrhythmia (RSA) provides a quantitative evaluation of 'vagal tone' and may be used for pain and stress assessment in livestock. The aim was to quantify the magnitude of RSA in cows and sheep. In 7 cows and 4 sheep standing at rest we measured the 3-lead electrocardiogram (ECG) together with the pneumogram, to identify inspiration and expiration. For each breath, RSA was the difference in instantaneous heart rate (HR) between the inspiratory peak and the expiratory trough, in percent of mean HR. The resting breathing rates (28 ± 2 and 32 ± 5 breaths/min in cows and sheep, respectively) were about twice those expected for similar size non-ruminants, in conformity with previous reports. Both species had long-period (>15 s) HR fluctuations. The average values of RSA, 1.4 ± 0.2% in cows and 7.8 ± 3.1 in sheep, were lower than those previously computed by an identical approach in humans (12%), dogs (40%) and horses (9%). In conclusion, by breath-by-breath analysis of instantaneous HR we measured RSA in both cows and sheep. Results from the present study represent a preliminary step in assessing whether or not RSA could be used as a biomarker for stress or pain in ruminants.