Measuring Cardiac Output by Swan-Ganz

[asaha]

Hi all,

I'm a medical student reading about Swan-Ganz catheters and the use of thermodilution technique to assess cardiac output. I know the catheter is fed into the right side of the heart, cold fluid is released from a proximal port, and the change in temperature is measured by a thermistor located distally on the catheter. The change in blood temperature over time is used as an indicator of blood flow: when flow is low, the temperature will be decreased for a longer period of time since the cold fluid isn't being cleared fast enough. The flow rate is equivalent to the cardiac output.

I can see how this technique is useful for measuring right heart cardiac output. However, there is an underlying assumption that right heart cardiac output is equivalent to left heart cardiac output. But this is often not the case as there are many causes of left ventricular dysfunction which result in decreased left heart cardiac output -- all of which are more likely in any patient who needs a Swan-Ganz in the first place!

Granted, a dysfunctioning left heart will ultimately result in a dysfunctioning right heart, in which case the two cardiac outputs would be equal. But it seems that acutely, such as with an MI, Swan-Ganz thermodilution is not a good indicator of left-heart cardiac output.

Does this make sense?

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[Myostatin]

You're right that thermodillution assumes Rt sided CO = Left sided CO.

Places where this breaks down:
1) sig left to right shunt (asd,vsd)
2) severe MR
3) severe TR (your measured rt sided output will be sig greater than the actual left sided co)

Short of these however, in steady state RT sided CO must equal left sided CO. Otherwise you would have massive pooling in the rt or left venous systems. Even in decompensated CHF, assuming the pt is not decompensating over the course of a few minutes, the assumption is largely true - if there were a sig mismatch in rt and left output, say 10%, with a cardiac output of 3-6 L/min you're talking about pooling .3-.6 L / min of blood in the pulmonary venous or systemic venous systems - this can't go on for more than a few minutes given normal circulating blood volume.

Its a good exercise to actually observe the practical aspects of thermodillution - overall its very innacurate given the degree of operator dependence - measurements depend on how fast/consistently saline is injected into the proximal port, and the standard deviation in measurement is very large.

CO calculation by the fick principle is in general much more accurate, and also avoids the innacuracies posed by thermodiluation in the conditions I mentioned above.

 

[asaha]

Thanks.