- Cardiac Output/Stroke Volume
- Qp/Qs (Shunt)
- Aortic Valve Area (Stenosis)
- Mitral Regurgitation (EROA/PISA)
Details:
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.
Recall that:
Cardiac output is = Heart Rate x Stroke Volume
and
Stroke Volume = LVOT area x LVOT VTI
= Pi(LVOT diameter/2)^2 x LVOT VTI.
Example:
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
Pitfalls:
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.
Example:
You are asked to calculated the shunt fraction Qp/Qs for a secundum ASD.
For this, you need to obtain the following measurements:
LVOT diameter (parasternal long axis) | LVOT velocity (Apical 5 or 3 chamber, PW) |
RVOT diameter (PSAX) | RVOT VTI (PSAX, PW) |
We use the fact that
Flow (Q) = area of the tube in cm^2 x VTI stroke distance in cm
In this case the "tubes" are the LVOT and RVOT. We assume they are circles with the same diameter throughout systole. Since the area of a circle is 2(Pi)radius^2 we can calculate:
Qp = 2(Pi)(RVOT diameter/2)^2 * RVOT VTI
Qs = 2(Pi)(LVOT diameter/2)^2 * LVOT VTI:
In the example shown above we have:Qp = 2Pi(2.41/2)^2 X 23
Qs= 2Pi(1.9/2)^2 x 22
= 1.7
Note that for the ratio Qp/Qs the 2(Pi) terms cancel out.
Important Points
1) This ratio is highly sensitive to errors in measurement of the LVOT and RVOT diameter, since both of these values are halved and then squared.
2) Measuring the RVOT diameter can be challenging without adequate echo windows. The Diameter should be measured at the RVOT, proximal to the pulmonic valve, which is where the VTI measurement takes place.
3) This formula only works well in cases where there is pure left to right or shunting. It does not work with bidirectional shunting.
Description and Example:
To calculate Aortic Valve Area by the continuity equation you need the following:
1) LVOT diameter from the PLAX view (in most people this is near 2 cm). This is assumed to be constant throughout systole.
2) LVOT velocity and/or VTI from the 5 chamber or apical long axis view.
3) The velocity of VTI at the aortic valve from the 5 chamber or apical long axis view.
LVOT diameter from PLAX view | LVOT VTI from Apical Long axis or 5 chamber view |
Aortic VTI from Apical long axis or 5 chamber view | |
By the continuity Equation: Area1xVTI1 = Area2x VTI2LVOT Area x LVOT VTI = AV Area x AV VTI 3.14(2.0cm/2)2 x 19cm = AV Area x 82.1cmAV Area = 0.72cm^2, which is consistent with severe aortic stenosis. |
Example Calculation:
Here is a sample calculation from a real example:
Important Points:
1. It is important to shift the color baseline to get a lower aliasing velocity in order to get an adequate hemispherical flow convergance profile. An aliasing velocity around 40cm/s is recommended.
2. Whether you are doing TEE or TTE a simple rule of thumb is to shift the baseline in the direction of the MR jet (this means changing the lower velocity in TTE apical views and the upper aliasing velocity in the mid esophageal TEE views.)
3. This method is only reliable for quantifying a single centrallly directed jet. In patients with multiple MR jets, or those which are eccentric the PISA method is not reliable.
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