Effect of acute haemorrhage on QRS amplitude of the lead II canine electrocardiogram

Objective To examine the effect of acute haemorrhage on the QRS amplitude o f the canine lead II surface electrocardiograph (ECG). Design Ten adult racing Greyhounds were tranquilised, anaesthetised, positi oned in right lateral recumbency and connected to recording electrodes of a n ECG unit. Baseline six-lead ECG traces were recorded, and further traces were obtained after one unit (460 mL) of blood, and then a second unit, wer e collected from the femoral artery. Results There was a consistent and progressive reduction in amplitude of th e QRS complex in all leads during acute haemorrhage. QRS amplitude in lead II after removal of two units of blood averaged 74% of the baseline voltage . with individual values of 61 to 91% (P < 0.0001). There were even greater reductions in QRS amplitudes in lead aVL during haemorrhage. In three addi tional dogs, reductions in QRS voltages were shown to be accompanied by red uctions in end-diastolic left ventricular internal dimensions measured echo cardiographically. Furthermore, the effects of haemorrhage on the QRS ampli tude and echocardiographic measurements were reversed when circulating bloo d volume was restored by re-infusion of blood removed previously. Conclusion Acute haemorrhage corresponding to an approximately one-third re duction in blood volume caused a substantial reduction in QRS voltage of th e surface EGG. It is postulated that this resulted from diminished ventricu lar distension as a consequence of reduced venous return. a similar mechani sm may account for the small-amplitude ECG complexes associated with perica rdial effusion, severe dehydration and hypovolaemia.