Changes in the QRS segment during exercise: effects of acute β-blockade with propranolol

Summary. Changes in the QRS complex during exercise may provide information with respect to ischaemic heart disease. The intention with present investigation was to shed light on mechanisms behind QRS changes and to study the possibly confounding effects of β-blockade on such alterations with exercise. Placebo or propranolol respectively was infused in randomized and double-blinded order in seven young healthy men before a maximum exercise test. Advanced computerized vectorcardiography and impedance cardiography was recorded continuously together with blood pressures and blood samples.

The Y-lead magnitude increased significantly with propranolol infusion (P<0–05), but it tended to decrease in the Z-lead (P<0.07). While the serum potassium concentrations increased (P< 0.0005), the spatial QRS magnitude tended to decrease irrespective of treatment (P<0.07). These changes correlated with changes in QR-duration (adj r²>0.58).

With exercise, the mean spatial QRS magnitude decreased with similar amounts irrespective of treatment. However, propranolol made the magnitude decrease earlier (P<001). No effect of treatment was detected on the decrease in QRS-duration.

Immediately after exercise, the QRS complex continued to change as during exercise in the placebo investigations, but did not with propranolol (P<0.05). These different patterns were most obvious in the first half of the QRS complex in the Y-lead.

It is concluded that acute β-blockade modifies QRS alterations both during and after exercise in healthy subjects. This indicates that such drugs may have confounding effects in evaluations of the diagnostic value of QRS alterations.