I like to read the ASCA blog every morning. When topics come up that have to do with anesthesia, I will frequently respond. As an anesthesiologist with over 30 years of experience doing everything from open heart and liver transplant, to pain management, OB and fast-paced ambulatory surgery, I’ve pretty much done it all.
One topic that comes up every once and a while on the ASCA blog is capnography, i.e., the measurement of end-tidal carbon dioxide (CO2). There seems to be a lot of misunderstanding about capnography, and a frequent theme seems to be: “Why do we need capnography, we already measure oxygen saturation?”.
The reason is simple but requires a brief refresher on pulmonary physiology. Oxygenation is the process by which oxygen from the lungs is transferred into the bloodstream in the alveoli. Ventilation, in contrast, is the exchange of air from the atmosphere for carbon dioxide in the lungs. A patient can stop breathing (ventilating) for a long time before oxygen saturation will fall. That is because the body has stores of oxygen in the lungs (the functional residual capacity or FRC for you physiology mavens) that it can tap into to maintain appropriate oxygen levels in arterial blood before oxygen levels fall. However, once the FRC is depleted, the fall in oxygenation will be rapid and precipitous. The smaller the FRC, the less period of time between cessation of breathing and fall in oxygenation. Patients with small FRCs generally are those with large abdomens that push up the diaphragm squashing the lower parts of the lungs. Patients who are obese, pregnant or have ascites are 3 groups with low FRCs.
So what’s the bottom line? It’s simple: capnography is a very early indicator of a problem in respiratory function, oximetry is a very late indicator of inadequate respiratory function. Put another way, by the time the O2 sat falls, you’re already in trouble.
Here’s an everyday example from my work as an anesthesiologist. For the last 5 years, I occasionally work in an GI suite at a community hospital in Massachusetts that does more bariatric surgery than any other facility in New England. You guessed it: Lots of morbidly obese patients having endoscopies. After a little propofol, the patients frequently obstruct and go apneic (stop breathing, i.e. ventilating). However, the O2 sat (for a brief period) will still be 100%. All that is usually required is a jaw lift to relieve the obstruction, restore ventilation, and maintain adequate oxygenation. In extreme cases, an oral airway needs to be inserted, but that is rare. In a dimly lit GI room with an obese patient, it’s not always easy to tell when the patient is apneic.
So what is a capnograph and how much does it cost? The capnograph, an additional plug-in module to our Philips monitor solves that problem. There are also free-standing units available if your monitors don’t have the capability to measure end-tidal CO2. The nasal cannula tubing we get has an extra line in it to sample expiratory CO2 while simultaneously delivering oxygen. Those cannula cost a few pennies more than standard nasal cannula oxygen and are only used for MAC and conscious sedation cases.
In summary, with capnography, you can catch respiratory problems BEFORE they happen. Isn’t that worth a few pennies per patient?