Cardiac output can be calculated by echo across any structure where one can measure cross sectional area and some information about velocity of blood flow. This could be the aorta, the pulmonic artery, or across any of the valves.
The easiest and least variable place to measure cardiac output is at the left ventricular outflow tract (LVOT). The LVOT diameter changes very little through systole and diastole and is assumed to be constant and closely approximating a circle in shape, however this introduces some error as it is in fact elliptical in many patients.
The LVOT diameter is measured in the parasternal long axis view in systole. The LVOT velocity time intergral (VTI) provides information regarding blood velocity across the time period of systole and is in the units of cm. Typical values are close to 2 cm.
The LVOT pulsed wave Doppler in either the apical long axis or 5 chamber view and the VTI can be traced out on the ultrasound machine or using digital software offline.
For the cardiac output calculation therefore you need the following:
1) the LVOT VTI, 2) the LVOT diameter and 3) the heart rate.
Cardiac output is = Heart Rate x Stroke Volume
Stroke Volume = LVOT area x LVOT VTI
= Pi(LVOT diameter/2)^2 x LVOT VTI.
Heart Rate: 100beats/min
Hence LV stroke volume = 3.14 (2.0cm/2)^2 x 19cm = 60 cm^3 or 60 ml
Cardiac Output = HR x SV = 100 beats/min x 60 ml/beat = 6000 ml/min or 6.0L/min
1. The LVOT VTI may not be accurate when the rhythm is irregular as in atrial fibrillation. In this case one should average the VTIs of several beats.
2. The LVOT diameter if off by a little can introduce significant error in output values since the radius is squared. Regardless in the same patient using the same LVOT diameter will give useful information about changes in cardiac output.