From Echocardiography in ICU
Calculation of left ventricle stroke volume
In the absence of significant valvular regurgitation, the left ventricle stroke volume can be calculated by measuring the left ventricle outflow tract (LVOT) area and the amount of blood going through this area:
Stroke volume = LVOT area x Quantity of blood across LVOT
+++ For serial measurements of stroke volume, always keep the same value of LVOT area +++
- Non-alignment of Doppler beam: when there is an angle between the Doppler beam and the LVOT direction, the VTI will be underestimated.
- Approximation in LVOT diameter measurement. It is important to have a good image with the true long axis of the LVOT (make sure that the walls of the LVOT and of the aortic root are parallel) and to use the zoom, to minimize the errors in the measurement. The LVOT diameter is normally related to body surface area and should be around 20mm for an average size person.
- Irregular heart rhythm or atrial fibrillation. It is important to average several measurements for each patient, and at least 10 average measurements in patients with irregular heart rhythm.
Calculation of cardiac output
From the calculation of the left ventricle stroke volume, cardiac output and cardiac index can easily be calculated:
Systolic index (ml/m2)= Stroke volume / body surface area
Cardiac output (l/min) = Stroke volume x Heart Rate
Cardiac index (l/min/m2) = Cardiac output / body surface area
JF Lewis, LC Kuo, JG Nelson, MC Limacher and MA Quinones. Pulsed Doppler echocardiographic determination of stroke volume and cardiac output: clinical validation of two new methods using the apical window. Circulation 1984;70;425-431.
Zoghbi WA, Quinones MA. Determination of cardiac output by Doppler echocardiography: a critical appraisal. Herz 1986;11:258-68