Please explain what is factored into an equation to determine left ventricular stroke work index and what the equation itself is. Also, how is pulmonary capillary wedge pressure measured and please define it. Thank you.
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Dear Jon, thank you for your question. Left ventricular stroke work index
measures the output of the left ventricle in terms of power per square unit
of area of myocardium. First, the stroke volume has to be calculated which
is cardiac output / heart rate. The cardiac output is determined with a
right heart catheterization by either a thermodilution technique or by directly
measuring the oxygen saturation of venous and arterial blood and using a formula
called the Fick equation. Stroke work = (Mean left ventricular systolic
pressure - mean left ventricular diastolic pressure) X Stroke Volume X 0.0144
(correction factor). Stroke work describes the amount of power generated by
the left ventricle with each contraction. The pulmonary capillary wedge pressure
is a direct measurement of the intravascular blood volume. Just before ventricular
contraction, the mitral valve is open and the left atrial and left ventricular
end diastolic pressures are equilibrated. At this point in the cardiac cycle,
the intravascular blood volume determines the end diastolic pressure. This
value can be measured by directly inserting a catheter into the left ventricle
but this is usually only done during a left heart cardiac catheterization. A right heart
catheterization involves inserting a catheter into the venous or right side
of the heart and can be performed at the patient's bedside. The end diastolic
pressure can be measured indirectly with this technique by positioning the
catheter into the pulmonary artery to the point where the balloon on the tip of
the catheter completely occludes the lumen of the pulmonary artery. At this
point, there is a static pressure column between the left atrium and the balloon
tip since no blood flow occurs past the balloon tip. This is called the
pulmonary capillary wedge measurement because the left atrial pressure is reflected
back to the catheter through the pulmonary capillary bed. The "wedge" is increased
in conditions of congestive heart failure where the venous pressure in the lung
increases and causes "fluid" or excess blood to fill the lungs. There are
no reliable methods for determining the "wedge" non-invasively. I'm not sure
why you asked this question, but if you would like more information on the normal
values for these tests, how the values are interpreted, or how therapeutic
decisions are made with these values, please don't hesitate to write back. I
hope this information is useful.
Information provided in the heart forum is for general purposes only. Specific
diagnoses and therapies can only be provided by your physician.
