Effects of the equine metabolic syndrome (EMS) on the cardiac autonomic function and on the arterial blood pressure in the horse

The human metabolic syndrome is accompanied by obesity, insulin dysregulation, arterial hypertension and cardiac dysfunction leading to an increased risk for heart diseases in the affected individuals. Insulin dysregulation can lead to an increased tone of the sympathetic nervous system. With the subsequent increased vascular tone, the perfusion in peripheral tissue might be reduced. Furthermore, increased sympathetic tone reduced the heart rate variability (HRV) and a reduction in HRV has been shown in patients suffering from metabolic syndrome even before an insulin resistance becomes clinical apparent. The equine metabolic syndrome (EMS) has several similitudes with the human metabolic syndrome. For instance, increased blood pressure has been reported in ponies with endocrine induced laminitis and this finding led to the hypothesis that endocrine induced laminitis was related to altered vascular tone in the dermal laminae within the hoof wall. Another study found an increased heart rate (HR) in ponies with EMS in comparison to a control population and a significant correlation between the left ventricular wall thickness and parameters of HRV as well as between left ventricular wall thickness and arterial blood pressure. However, no changes in the HRV could be shown in this population of ponies with EMS. Nevertheless, the aim of the presented study was to investigate further whether EMS had an effect on arterial blood pressure or the autonomic cardiac function in horses and whether the severity of general or regional adiposity was related to changes in parameters of HRV or arterial blood pressure. Therefore, the severity of general adiposity, regional adiposity and insulin resistance were assessed in horse with EMS and pooled in an EMS score. Further, heart rate variability was analyzed on recorded electrocardiograms to assess indirectly the autonomic function. Time domain parameters (mean HR; mean RR intervals; standard deviation of normal to normal RR intervals, SDNN; square root of mean squared differences between successive RR intervals, RMSSD) as well as frequency domain parameters (low frequency band, LF; high frequency band, HF; LF/HF ratio) were analyzed. Blood pressure was evaluated non-invasively at the coccygeal artery by oscillometry. 32 horses with EMS could be recruited for the study. The comparison between groups based on t tests or one-way ANOVAs showed that the HR (p = 0.048) and pulse frequency (p = 0.007) were increased in horses with more severe EMS score. Similarly, horses with severe insulin resistance (p = 0.024) as well as obese ones (p = 0.047) had increased HR. No difference could be documented in the analyzed parameters of HRV. Nevertheless, in our population older horses showed a shift of autonomic nervous tone towards sympathetic predominance (LF/HF, LF, HF each with p = 0.014). In conclusion, we could show that the presence of severe obesity, the degree of insulin resistance and the EMS score had an influence on the HR. Horses with more severe degree of EMS had higher HR in our population. However, it was not possible to show a decrease in the parameters of HRV due to altered autonomous tone or an increased arterial blood pressure in the examined horses with EMS.