Introduction To characterize changes in maternal cardiovascular variables throughout the reproductive cycle in healthy bitches and determine whether magnitude of pregnancy-induced changes correlates to litter size.

Animals Eleven client-owned breeding bitches.

Materials and Methods Bitches were enrolled prospectively and followed longitudinally throughout a single reproductive cycle. Physical examination, echocardiography, blood pressure, and plasma volume estimation were performed during proestrus, diestrus (early and late pregnancy), and anestrus. Fetal echocardiography was performed during late pregnancy. Data were compared across visits using a linear mixed-effects model, and correlation between variables was assessed.

Results Compared to proestrus, no significant changes were observed at any phase of the cycle in heart rate, blood pressure, echocardiographic measurements of left ventricular size or function, or echocardiographic calculations of stroke volume or cardiac output. Estimated plasma volume increased by 29.6% in early pregnancy (p < 0.001) and 70.7% in late pregnancy (p < 0.001). Fetal echocardiography was feasible in a subset of fetuses for each bitch. There was a significant correlation between estimated total fetal cardiac output and late pregnancy increase in maternal cardiac output (p = 0.0025). The incidence of physiologic heart murmurs ranged from 5/11 (45%) bitches in proestrus to 2/11 (18%) in late pregnancy, attributed to variations in aortic outflow velocity.

Conclusions Hemodynamic alterations in pregnant bitches do not result in consistently detectable echocardiographic changes, suggesting that cardiac screening could be diagnostic at any time during a reproductive cycle. Physiologic heart murmurs were common in this study population and not obviously associated with reproductive cycle.