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Ficks principle in cardiac output

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Fick principle cardiac output equation mixed venous oxygen official reference

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Fick principle for measuring cardiac output

Principle: At steady state, the body's oxygen consumption equals blood flow multiplied by the arterial to mixed-venous oxygen content difference.
[ \dot V O_2 = CO \times (C_aO_2-C_vO_2) ]
Therefore:
[ \boxed{CO=\frac{\dot V O_2}{C_aO_2-C_vO_2}} ]

Terms

  • CO: cardiac output
  • (\dot V O_2): whole-body oxygen consumption, in mL O(_2)/min
  • (C_aO_2): arterial oxygen content
  • (C_vO_2): mixed venous oxygen content, sampled from the pulmonary artery
When oxygen contents are expressed in mL O(_2)/dL blood:
[ \boxed{CO,(L/min)=\frac{\dot V O_2}{C_aO_2-C_vO_2}\times \frac{1}{10}} ]
Equivalently:
[ \boxed{CO,(L/min)=\frac{\dot V O_2}{(C_aO_2-C_vO_2)\times10}} ]
The factor 10 converts dL to L.

Example

If:
  • (\dot V O_2 = 250) mL/min
  • (C_aO_2 = 20) mL/dL
  • (C_vO_2 = 15) mL/dL
[ CO=\frac{250}{20-15}\times\frac{1}{10} =\frac{250}{5}\times\frac{1}{10} =5,L/min ]

Clinical measurement

  1. Measure oxygen uptake from expired air using a metabolic cart.
  2. Obtain an arterial blood sample for (C_aO_2).
  3. Obtain mixed venous blood from a pulmonary artery catheter for (C_vO_2).
  4. Apply the formula.
Direct Fick uses measured (\dot V O_2) and is the reference method. Indirect Fick estimates (\dot V O_2) from nomograms, which can cause clinically important error, especially when physiology differs from normal. Fuster and Hurst's describes direct Fick as the gold-standard method. Fick calculations assume steady-state oxygen consumption and no important intracardiac shunt.
Source: Costanzo Physiology, 7th ed., pp. 159-160; Fuster and Hurst's The Heart, 15th ed., p. 1748.
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