Relationships of left side systolic time intervals to beat-by-beat heart rate and blood pressure variables in some cardiac arrhythmias of the horse.

This study examines the relationships between left side systolic time intervals and heart rate and blood pressure variables in horses with certain cardiac arrhythmias. It concludes that heart rate and left ventricular ejection time may be directly and inversely related, respectively, and that these relationships may be affected by conditions such as atrial fibrillation.

Methodology and Data Collection

• The researchers studied systolic time intervals (STIs), isovolumic contraction time (ICT) and left ventricular ejection time (LVET) in seven horses with supraventricular arrhythmia, a condition that causes an irregular heartbeat.
• The STIs were measured over multiple beats using two catheter-mounted transducers to record pressure contours from the left ventricle (LV) and the aorta (Ao) simultaneously.

Findings and Results

• The data analysis showed that the ICT was significantly and positively associated with beat-to-beat heart rate, while the LVET was significantly and inversely related to heart rate in five of the horses.
• In two of the horses studied, both suffering from atrial fibrillation, LVET decreased more rapidly at heart rates exceeding 70 to 80 beats per minute. This suggests that ventricular filling might be compromised at this heart rate range in horses with this type of arrhythmia.
• Different regression analyses were performed with ICT, LVET, the ratio of LVET to ICT, and mechanical systole (the sum of ICT and LVET) against various pressure parameters and heart rate.
• The researchers identified some relationships between these parameters and heart rate, including a direct relationship between ICT and aortic end-diastolic pressure, and an inverse relationship between it and left ventricular end-diastolic pressure and LV dP/dtmax.
• Likewise, LVET was found to be directly related to aortic systolic pulse pressure and inversely related to aortic end-diastolic pressure. However, this was the case only in two out of seven horses in relation to heart rate.

Conclusion

• The significant relationships identified in this research, including those between ICT and heart rate, between LVET and heart rate, and among various pressure parameters, provide insights into the potential origins and effects of cardiac arrhythmias in horses.