Electrocardiographic changes associated with haematocrit variations

The electrical resistivity of intracardiac blood is less than the resistivity of the surrounding tissues. This affects the transmission of cardiac forces to the body surface: the radial forces are enhanced, whereas the transmission of tangential forces is diminished (the Brody effect). Blood resistivity is directly related to haematocrit, hence, haematocrit changes are expected to affect the transmission of cardiac forces, resulting in changes in QRS complex voltage. To assess this hypothesis, a 12-lead electrocardiogram was recorded in 40 patients affected by thalassaemia before and after a transfusion of concentrated red cells. The voltage of each QRS component was carefully measured in every lead, and the sum of all R wave amplitudes (sigma R) was calculated. The post-transfusional electrocardiogram reflected a significant decrease in the R wave amplitude in every lead. sigma R also decreased, whereas S wave amplitude in lead V6 increased. A negative correlation between the ratio of haematocrit pre/post transfusion and that of the corresponding sigma R values was also observed (r = -0.434; P less than 0.01). An increase in haematocrit is therefore associated with a decrease in R wave amplitude. These findings explain why several patients with high haematocrit manifest relatively low voltage QRS complexes.