CHAPTER 3: Echocardiographic Quantification

DOPPLER CALCULATIONS

• Stroke volume is calculated as the volume of a cylinder.

• The volume of a cylinder is calculated as the area of the base × height.

• Example: Left ventricular outflow stroke volume = LVOT area × LVOT VTI.

• LVOT VTI is the stroke distance (cylinder height).

• Stroke volume × heart rate = cardiac output.

• Stroke volume = end-diastolic volume × ejection fraction.

• Cardiac output = heart rate × stroke volume.

• Cardiac output = heart rate × end-diastolic volume × ejection fraction.

• Corollary:

  • - Cardiac output is affected by changes in heart rate, by end diastolic volume (preload), and by ejection fraction.

  • - Practical clinical value: 2D echo provides a look at the impact of preload in patients with Doppler evidence of constriction, restriction, etc., by showing right and left ventricular end-diastolic volumes.

Source

• Hurrell DG, Nishimura RA, Ilstrup DM, et al. Utility of preload alteration in assessment of left ventricular filling pressure by Doppler echocardiography: a simultaneous catheterization and Doppler echocardiographic study. J Am Coll Cardiol. 1997;30:459–467.

CARDIAC OUTPUT IN THE CATH LAB

Fick versus Thermodilution

• The Fick method uses the a-v O₂ difference. It works in atrial fibrillation because “steady state” is used. It is most accurate in low-output states where there is a wide a-v O₂ difference. Oxygen extraction is greater with decreased cardiac output. This results in a low pulmonary artery saturation. Pulmonary artery saturation <65% indicates decreased cardiac output.

• Thermodilution measures the “area under the curve” after a short set of cardiac cycles. It is less reliable in low-output states. The calculation is affected by significant tricuspid regurgitation, and possibly by the variable cardiac cycle length in atrial fibrillation.

Source

• Gonzalez J, Delafosse C, Fartoukh M, et al. Comparison of bedside measurement of cardiac output with the thermodilution method and the Fick method in mechanically ventilated patients. Crit Care. 2003;7:171–178.

DERIVATIONS FROM STROKE VOLUME CALCULATIONS

• Aortic stenosis area calculation by continuity:

  • - Because of flow continuity and conservation of mass: Stroke volume is the same below the stenotic aortic valve and the same at the stenosis.

  • - In other words, the stroke volume “cylinder” simply changes from “short and fat” to “long and thin.”

  • - The volume of both cylinders is identical.

• The two cylinders have a total of four components.

• If three components are known, it is possible to use a simple formula to solve for the ...