Question:  a patient presents with AF, eventually leading to intubation. is this a primary cardiac problem, or is the AF secondary to an underlying disease? The answer is hiding on this ventilator screen!

Answer:  The key finding here is the volumetric capnography data at the bottom right of the ventilator screen showing a VCO2 to be 435 ml/min (this measures the total amount of CO2 excreted by the patient each minute, taken as the time integral of the end tidal CO2 curve over time).  This VCO2 is substantially elevated, suggesting a state of systemic hypermetabolism.  In this case, the high metabolic rate and difficult-to-control AF were caused by thyroid storm.

In a patient with purely cardiogenic atrial fibrillation, a roughly normal VCO2 would be expected.  VCO2 rates vary based on body size and age.  Normal values fall more into the 150-300 ml/min range.

So the elevated VCO2 was a nice clue to suggest that the patient's AF was secondary to an underlying problem.  Of course, VCO2 elevation isn't specific for thyroid storm.  This could be caused by a variety of hypermetabolic states (e.g., acute sympathomimetic intoxication).

Volumetric capnography is a really neat tool that can also be used to evaluate for fluid responsiveness, or as a very early indicator of pending cardiac arrest (if the VCO2 starts steadily falling in a peri-arrest patient, this predicts imminent arrest).

My opinion is that all intubated patients should be continuously monitored with waveform capnography and volumetric capnography.  If the VCO2 changes (especially abruptly), that is something worth paying attention to.  An advantage of VCO2 compared to capnography alone is that VCO2 provides information about the total CO2 clearance which isn't affected by respiratory rate and tidal volume (making it a more stable metric in the face of ventilator changes).  For example, VCO2 would probably be the best early indicator of malignant hyperthermia (although this hasn't yet been studied).

To further underscore that last point, let's take another look at the ventilator screen.  The end tidal CO2 here is 34 mm, which is unremarkable.  So simply based on the etCO2 you might not realize that the metabolic rate is high.  However, the minute ventilation is really huge (17 liters/minute, normal maybe being ~8 liters/minute), so the total CO2 clearance (VCO2) is quite elevated.

Of course, if you don't have volumetric capnography at your shop, you could probably figure out that it's super abnormal to require a huge minute ventilation of 17 liters/minute to achieve an end tidal CO2 of 34 mm (e.g., a minute ventilation of 17 liters/minute applied to a patient with a normal metabolic rate would push the end tidal CO2 down a lot lower, maybe into the low 20's).  So this mismatch of high minute ventilation and normal etCO2 alone could suggest an elevated VCO2, but in a less precise fashion.

Enough on that for now, we will be exploring volumetric capnography in more detail in the IBCC in a little while.

cheers, josh