Respiratory sinus arrhythmia in newborn and adult rats

Objectives: We compared respiratory sinus arrhythmia (RSA; the acceleration and deceleration of heart rate during inspiration and expiration, respectively) in spontaneously breathing newborn and adult rats. RSA has been recognized as a physiological phenomenon, which functions to match pulmonary blood flow and breath-by-breath ventilation. Hence, we hypothesized in this study that RSA in rats becomes more definite in the adults compared to the newborns.
Methods: Newborn rat (2 to 4-day-old) was placed fine ECG electrodes subcutaneously and adult rat (8-week-old) inserted tubes into a tail artery for heart rate (HR) recordings under isoflurane anesthesia. Rats under light restraint were then placed in chambers; a small double-parted for the newborns (50 mL) and one cylindrical for the adults (2500 mL) to monitor spontaneous breathing and heart beats in air without anesthesia. Recording was started once breathing rate (f_R) and HR stabilized and the animal’s body temperature had recovered to the level before anesthesia. Data were analyzed breath-by-breath and averaged for the 10 consecutive breaths selected toward the end of 5-min-long data collection. For each breath, RSA corresponded to the difference in HR between peak and trough (△HR, beats/min) in percent of mean HR (RSA = △HR/mean HR %).
Results: In spontaneously breathing rats, Mean RSA was 0.65 ± 0.43 SD in the newborns (n = 10) and 3.25 ± 1.02 SD in the adults (n = 3).
Conclusions: These results suggest that RSA in rat increases and more definite by growth in a manner consistent with its postnatal functional development. Since RSA may support organs to gain oxygen, smaller RSA values in newborns may reflect on their intolerance against cardiorespiratory disturbances.