The effect of respiratory activity on the QRS complex in the ECG in clinically healthy horses and horses with chronic lung diseases

Interrelations between respiration and changes in amplitudes of the QRS-complex of the ECG of horses were investigated. The value of the R-vector and both its components Rx and Ry decrease during inspiration and increase in the expiration phase. It means, that the R-waves in leads from cranial to caudal (y, II, aVF) and from right to left (I) become smaller with inspiration and larger with expiration. The difference of the amplitudes may be important, especially in horses with respiratory problems, so that it has to be noted when evaluating electrocardiograms. For that purpose it is not necessary to draw respiratory curves. It is proposed to evaluate the heart-cycle with the largest R-wave (expiration) only when there are differences in the R-amplitudes. Pulmonary air content is discussed as a possible cause.