The echocardiographic assessment of the CO can be obtained by measuring cross-sectional area of the left outflow tract (LVOTd) at the level of aortic annulus and by measuring the velocity time integral (VTI) at the level of aortic valve by pulsed wave Doppler, respectively. The CO is calculated by using the following equation:

Echo CO = Stroke volume (SV) × HR, where SV= [(3.1416) × (LVOTd/2) 2 ] × VTI.

• LVOTd is measured from a parasternal long-axis view. The distance from the inner edge to inner edge of the LVOT was measured in a line parallel to the aortic annulus from the base of the right aortic valve coronary cusp to the base of the non-coronary cusp.
• LVOT VTI was measured by obtaining an apical 5-chamber view and then placing a pulsed-wave Doppler cursor in the LVOT below the aortic valve annulus (average of 3 beats).

The cardiac index (CI) can be derived by dividing the CO by BSA (body surface area, m2). The normal values are 150–400 ml/kg/day in infants and children.

This method is valid for CO of RV using RVLOTd and RVOT VTI.