- 1.
Cardiac Output and Shunt Determination (see Chapter 3 ).
- A.
Fick’s Principle.
“The total uptake or release of a substance by an organ is the product of the blood flow to the organ and the arteriovenous concentration difference of that substance.”
- B.
Fick’s Method for Determining Cardiac Output.
The Fick method is based on Fick’s principle. In the absence of a shunt, pulmonary blood flow equals systemic blood flow. The arteriovenous oxygen difference is equal to arterial saturation minus mixed venous saturation. Oxygen content is determined as the product of blood oxygen saturation, the hemoglobin concentration (in grams per deciliter), and the amount of oxygen carried per gram of hemoglobin (Hgb) (1.36 mL oxygen per g of Hgb). This value is multiplied by 10 to correct the units. Thus Fick’s formula for determining cardiac output is as follows:
Cardiac output = Oxygen consumption ( Arterial saturation – mixed venus saturation ) × ( Hgb ) × 13.6
Note: Oxygen consumption is often assumed (125 mL/min/m 2 for average individuals and 110 mL/min/m 2 for elderly patients).
- C.
Shunt Calculation.
- A.
Left-to-right shunt size is described as the ratio of pulmonary to systemic flow, or Qp/Qs. The absolute flow for each circuit can be calculated using the Fick method, or the relationship can be simplified as follows:
In the presence of an atrial septal defect, the empiric “Flamm formula” is used to define the mixed venous saturation as follows:
where SVC is the superior vena cava and IVC is the inferior vena cava.
A Qp/Qs ratio of 1.3 to 1 is the minimally detectable shunt.
A Qp/Qs ratio of 1.0 to 1.5 indicates a small left-to-right shunt.
A Qp/Qs greater than 2.0 indicates a large left-to-right shunt.
A Qp/Qs less than 1.0 indicates a net right-to-left shunt.
- 2.
Calculation of Valve Orifice Area (see Chapter 5 , Chapter 6 ).
- A.
The Gorlin Formula for Mitral Valve Area.
Mitral valve area = Cardiac output ( Diastolic filling period ) ( heart rate ) ( 37.9 ) ( Pressure gradient )
The mitral valve area is in centimeters squared, the cardiac output is in liters per minute, the heart rate is in beats per minute, and the pressure gradient is the mean gradient across the mitral valve in millimeters of mercury.
- B.
The Gorlin Formula for Aortic Valve Area.
where SEP is the systolic ejection period.
The aortic valve area is in centimeters squared, the cardiac output is in liters per minute, the heart rate is in beats per minute, and the pressure gradient is the mean gradient across the aortic valve in millimeters of mercury.
- C.
The Hakki Formula for Aortic Valve Area.
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- A.
