Relationship of QRS amplitude to left ventricular dimensions after acute blood volume reduction in normal subjects

The relationship between the acute reduction in left ventricular dimensions induced by intravenous frusemide and scalar electrocardiographic QRS amplitude was studied in 14 normal subjects. The major electrocardiographic change was a significant increase in QRS voltages inversely proportional to the changes in left ventricular dimensions. This effect was opposite to the change predicted by the Brody effect. Surface electrocardiographic voltages are greatly influenced by myocardial cell orientation and the effective dipoles of the wave of activation are parallel to the long axes of cardiac fibres. As in the normal ventricle the arrangement of fibres is mainly circumferential, the body surface potentials are strongly affected by longitudinal wave front voltages. We suggest that during acute reduction of blood volume the inverse relationship of left ventricular volume and QRS amplitude can be explained by a reduced short-circuiting effect of blood mass on a longitudinally directed wave of activation.