Effect of body incline on cardiac performance

Maximal cardiac performance is improved in man during upright compared to supine exercise. Whether cardiac performance in quadrupeds is dependent upon body position is unknown. Therefore, we undertook the present investigation to determine if peak cardiac output (Qpeak) would be influenced by body inclination in the Thoroughbred horse. To test the hypothesis, four Thoroughbred horses performed an incremental exercise protocol (speed increased by 1 m/s/min to fatigue) on both a level (L) and inclined (I: 6 degrees) treadmill. Specifically, we hypothesised that Qpeak would be increased on the incline, as this represents a progression towards upright exercise. Cardiac output was determined using the Fick relationship from continuous measurements of pulmonary VO2 and paired arterial (carotid artery or transverse facial) and mixed venous (pulmonary artery) samples. Qpeak was significantly increased on the incline (L: 279 +/- 20; I: 336 +/- 17 l/min; P<0.05), while CaO2 was not significantly different (L: 25.5 +/- 1.1; I: 25.4 +/- 1.9 ml/100 ml), and therefore, whole body O2 delivery (QO2) was significantly increased (L: 70.7 +/- 4.9; I: 84.4 +/- 3.1 l/min; P<0.05). In conclusion, within the scope of this investigation, these data suggest that cardiac performance, as judged by increased Qpeak and QO2, is enhanced in the inclined body position. Furthermore, these findings provide preliminary information that level and incline treadmill exercise tests may yield significantly different results in the Thoroughbred horse and consequently this factor should be considered when interpreting exercise testing and performance data.