Background

Right ventricular mass indexed to body surface area (RVMI) decreases and left ventricular mass index (LVMI) increases rapidly and substantially during early infancy. The relationship between these sizeable mass transformations and simultaneous electrocardiographic changes have not been previously delineated.

Methods

Normal term infants (#45 initially enrolled) were prospectively evaluated at 2 days and at 2-week, 2-month, and 4-month clinic visits. Ventricular masses were estimated with 2D echocardiographic methods. QRS voltages were measured in leads V₁, V₆, I and aVF.

Results

Mean QRS axis shifted from 135 (95%CI 124, 146) to 65 degrees (95%CI 49, 81) and correlated with both RVMI decrease and LVMI increase (R = 0.46^⁎ vs. 0.25^†, respectively. *p < 0.01, ^†p < 0.05). As RVMI decreased from mean 28.1 (95%CI 27.1, 29.1) to 23.3 g/m2 (95%CI 21.4, 25.2) so did V₁R and V₆S voltages. RVMI changes correlated with V₁R, V₆S, and V₁R + V₆S voltages (R = 0.29*, 0.23† and 0.35*, respectively. *p < 0.01, †p < 0.05) but not with V₁R/S ratio. As LVMI increased from 44.6 (95%CI 42.9, 46.3) to 55.4 g/m2 (95%CI 52.3, 58.5) V₆R and V₆Q increased but V₁S voltage did not. LVMI changes correlated with V₆R, V₆R-S, and V₆(Q + R)-S voltages (R = 0.31*, 0.34*, and 0.38* respectively. *p < 0.01) but not with V₁S or V₆R/S (R = 0.01 and 0.18 respectively, p = NS).

Conclusions

During early infancy the RVMI decrease correlates best with the QRS axis shift and V₁R + V₆S voltage, and the LVMI increase correlates best with V₆R-S and V₆(Q + R)-S voltages.