Mathematical modelling of post-exercise heart rate recovery in ponies

Using a recently developed nonlinear algorithm which identifies reference points on left ventricular pressure waveforms and other waveforms, we analyzed heart rate recovery in ponies after strenuous treadmill exercise. Each pony performed exercise tests with and without premedication with atropine methyl nitrate. Resting and exercise heart rates were 54 +/- 3 and 209 +/- 6 beats/min, respectively, without premedication and 85 +/- 8 and 212 +/- 5 beats/min, respectively, following atropine premedication. Post-exercise heart rate recovery without premedication was best described by a biexponential curve with a rapid initial period (duration = 55 +/- 6 sec, Tl/e = 15 +/- 2 sec) and a slower secondary period (duration = 267 +/- 5 sec, Tl/e = 455 +/- 78 sec). After atropine premedication, heart rate recovery followed a monoexponential curve (Tl/e = 174 +/- 46 sec) in 4 ponies, but remained biexponential (Part 1 duration = 74 sec, Tl/e = 20 sec; Part 2 duration = 243 sec, Tl/e = 342 sec) in the other ponies. The calculated time required for heart rate to return to resting levels after atropine premedication was 11.3 +/- 2.2 min, compared to 26.3 +/- 3.9 min without premedication. However, without premedication, only 6.6 +/- 1.6 min were required to reach the post-atropine resting heart rate, indicating that the time difference was due to the higher resting heart rate after atropine premedication. Thus, heart rate recovery in the non-premedicated pony follows a biexponential curve similar to that observed in other species.(ABSTRACT TRUNCATED AT 250 WORDS)