Use of pulsed-wave Doppler in which of the following views allows for calculation of cardiac output?
A. Transesophageal mid-esophageal 4-chamberCorrect Answer: B
Cardiac ultrasound provides an opportunity to estimate cardiac output in a noninvasive manner. This can be a useful tool in the assessment of undifferentiated shock and correlates very well with thermodilution cardiac output measurements by PA catheter. The technique involves obtaining the velocity time integral (VTI) at the LVOT. When pulsed-wave Doppler is applied to a specific area of blood flow, a tracing is produced that has velocity of blood flow on the y-axis and time on the x-axis, as shown in the image below. This is the blood flow velocity versus time at that specific location, in this case the LVOT. When the integral (area under the curve) of that tracing is obtained for a single systolic period, the result is a specific distance. This is the distance that a disc that has the cross sectional area of the LVOT travels in systole. Thus, we end up with a 2D area of the LVOT and a height, allowing us to calculate the LV SV. The LVOT area is calculated by measuring the diameter in the parasternal long axis (transthoracic echocardiogram) or mid esophageal long axis (TEE). Assuming there is no aortic regurgitation, the SV calculated here multiplied by the heart rate provides us the cardiac output.
LVOT Area = (LVOT diameter/2) 2 × 3.14SV = LVOT VTI × LVOT AreaCO = HR × SV
To accurately measure the LVOT VTI, we must align the probe with the direction of blood flow. In typical hearts, this is best done in the apical 5- chamber view (transthoracic echocardiogram) and the deep transgastric 5- chamber view (TEE), making B the correct answer here. In the other answer choices, the Doppler beam will not align with the direction of blood flow and therefore the VTI will be underestimated.
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