LACK OF LOCOMOTOR-CARDIAC COUPLING IN TROTTING DOGS

Coupling of locomotor and cardiac cycles has been suggested to facilit ate effective arterial delivery and venous return during vigorous exer cise. In an attempt to document locomotor-cardiac coupling, we ran fiv e dogs on a motorized treadmill while monitoring heart activity with s urface electrocardiogram electrodes and locomotor events with high-spe ed video and an accelerometer mounted on the dog's back. Analysis of t he cardiac and locomotor frequencies revealed that heart rate was usua lly slightly greater than stride frequency. Hence the timing of the ca rdiac cycles varied with respect to the phase of the locomotor cycles, and therefore consistent coupling of the locomotor and cardiac cycles was not observed in any of the dogs. However, the period of the cardi ac cycle sometimes varied in a rhythmic way that caused brief periods of transient coupling of the locomotor and cardiac cycles in three of the five dogs. These brief periods of coupling (5-20 heartbeats) occur red at approximately the same phase relationship in each of the three dogs. We hypothesize that the variation in cardiac period and the resu lting transient coupling are a function of locomotor and ventilatory i nfluences on venous return and/or ventricular ejection. Because venous return and ejection fraction are likely to vary in an unpredictable m anner when animals run in a complex environment, we suggest that refle x control of heart rate will be important during locomotion and strict integer coupling of the locomotor and cardiac cycles is unlikely to e volve.