Critical medicine topic-CMT 008 

COMBINED AORTIC REGURGITATION (AR) AND AORTIC STENOSIS (AS):

CALCULATION OF AORTIC VALVE AREA

 

Clinical Problem

In patients with combined Aortic Regurgitation (AR) and Aortic Stenosis (AS), accurate calculation of the aortic valve area (AVA) is challenging because part of the blood ejected by the left ventricle regurgitates back into the ventricle. Therefore, the total left ventricular output is greater than the forward systemic cardiac output.

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Why Forward Cardiac Output is Inaccurate

The Fick method and dye dilution method measure only the effective forward cardiac output delivered to the systemic circulation.

These methods do not include the regurgitant volume that repeatedly crosses the diseased aortic valve.

As a result:

·        Forward cardiac output is less than the total LV output.

·        Using this lower value in the Gorlin formula falsely reduces the calculated valve area.

·        The patient may appear to have more severe aortic stenosis than is actually present.

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Methods of Cardiac Output Measurement

1. Fick Method

·        Measures oxygen consumption.

·        Uses arterial and mixed venous oxygen content.

·        Calculates forward cardiac output only.

·        Underestimates total LV output in AR.

2. Dye Dilution Method

·        Measures dilution of injected indicator.

·        Also measures effective forward cardiac output only.

·        Does not include regurgitant flow.

3. Angiographic Left Ventricular Cardiac Output (Preferred)

This method measures the total volume ejected by the left ventricle, including both:

·        Forward systemic flow

·        Regurgitant flow back into the LV

Formula:

LV Cardiac Output = LV Stroke Volume × Heart Rate

This value should be used in the Gorlin formula for accurate calculation of aortic valve area.

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Gorlin Formula

The Gorlin formula calculates the anatomical aortic valve area using:

·        Cardiac output

·        Systolic ejection period

·        Mean transvalvular pressure gradient

For patients with combined AR and AS:

·        Use total LV cardiac output, not forward cardiac output.

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Why Angiographic LV Output is Preferred

Because it measures:

·        Total blood ejected from the left ventricle

·        Forward flow to the body

·        Regurgitant flow returning through the incompetent aortic valve

Thus, it provides the correct flow across the stenotic valve.

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Clinical Importance

Using only forward cardiac output:

·        Underestimates valve area.

·        Overestimates severity of aortic stenosis.

·        May lead to inappropriate clinical decisions regarding valve replacement.

Using angiographic LV cardiac output provides a more accurate assessment of stenosis severity.

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Key Examination Points

·        Combined AR + AS requires measurement of total LV output.

·        Fick method measures only forward cardiac output.

·        Dye dilution method also measures only forward output.

·        Regurgitant volume is excluded by these methods.

·        Angiographic LV cardiac output includes both forward and regurgitant flow.

·        LV Cardiac Output = Stroke Volume × Heart Rate.

·        The Gorlin formula should use total LV output for accurate AVA calculation.

·        Failure to do so results in an artificially small calculated valve area and overestimation of AS severity.

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Summary

In patients with combined aortic stenosis and aortic regurgitation, the forward cardiac output measured by the Fick or dye dilution methods is inadequate for calculating the aortic valve area because these techniques exclude regurgitant flow. Accurate assessment requires the total left ventricular cardiac output, best obtained by angiographic measurement of LV stroke volume multiplied by heart rate. Using this value in the Gorlin formula avoids underestimation of the valve area and provides a true assessment of the severity of aortic stenosis.

 

 

DR.C.GANESAN M.D.,

PROFESSOR OF MEDICINE