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Question 7#

The thermodilution technique for determining cardiac output:

A. Calculates Q_T with the Fick equation
B. Underestimates cardiac output at low values
C. Should be performed with a cold indicator liquid to increase the signal-to-noise ratio
D. Is influenced by respiratory cycle due to changes in blood temperature and Q_T

Correct Answer is D

Comment:

The relationship used by the thermodilution technique for calculating QT is called the Stewart-Hamilton equation:

Q_T = [V X (T_B - T₁) X K₁ X K₂] /fT_B(t)dt 

where V is the volume of the indicator injected, T_B is the temperature of blood, T₁ is the temperature of the indicator, K, is a constant that is the function of the specific heats of blood and the indicator, K₂ is an empirically derived constant, and fT_B(t)dt is the area under the time-temperature curve. Determination of cardiac output by the thermodilution method is generally quite accurate, although it tends to systematically overestimate Q_T at low values. Changes in blood temperature and Q_T during the respiratory cycle can influence the measurement. Therefore, results generally should be recorded as the mean of two or three determinations obtained at random points in the respiratory cycle. Using cold injectate widens the difference between T_B and T₁ and thereby increases signal-to-noise ratio. Nevertheless, most authorities recommend using room temperature injectate (normal saline or 5% dextrose in water) to minimize errors resulting from warming of the fluid as it is transferred from its reservoir to a syringe for injection.