Impact of heat stress on electrocardiographic changes in New Zealand white rabbits

A study was conducted on twelve clinically normal and healthy New Zealand White rabbits of both sexes, weighing between 2-3 kg, and aged between 1-3 years to observe the normal electrocardiogram and its changes during heat stress. The normal electrocardiogram and changes during heat stress were evaluated for the three bipolar standard limb leads (I, II and III) and three unipolar augmented limb leads (aVR, aVL and aVF). ECG recordings were made in sternal recumbency using a multi channel electrocardiograph. The normal heart rate with a mean of 204±7 beats/min was recorded. The mean amplitude observed was 0.05±0.002 mV for P wave; 0.19±0.008 mV for QRS; 0.14±0.007 mV for T wave. The mean duration observed was 0.03±0.002 sec for P wave; 0.06±0.002 sec for PR interval; 0.05±0.003 sec for QRS complex; 0.13±0.004 sec for QT interval; 0.07±0.002 sec for T wave. During heat stress tachycardia was observed with progressive rise in temperature along with ventricular fibrillation, ventricular extra-systole and atrial fibrillation. At 43°C, ventricular fibrillation was observed in Lead II, III, aVL and aVF. At 45°C, ventricular extra-systole was recorded in Lead III, aVL and aVF. At 45°C (30 mins more exposure), ventricular extra-systole in Lead I and ventricular fibrillation in Lead III was observed. At 47°C, ventricular fibrillation was seen in Lead II, III, aVR, aVL and aVF. At 47°C (30 mins more exposure), atrial fibrillation in all the leads were observed The mean cardiac axis recorded was 90°±0.065 without significant alterations throughout the study.