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The Case for Capnography
Monitoring end tidal CO2 provides early warnings of airway obstructions.
Larry Snyder
Publish Date: November 17, 2007   |  Tags:   Anesthesia

I consider capnography to be the ultimate safety net for anesthesia providers during MAC or IV sedation cases, but not everyone agrees. According to an April 2006 Outpatient Surgery Magazine reader survey, only 40 percent of respondents used capnography during these cases. The American Society of Anesthesiologists doesn't mandate the use of capnography for procedures not involving general anesthesia. Here's why the monitoring should be an integral part of patient care during all conscious sedation cases.

Tidal waves
Capnography measures the CO2 present in the gas exhaled by the patient, in millimeters of mercury. As the airway becomes obstructed, your anesthesia provider would see a drop in indicated end tidal CO2 (ETCO2). The longer this is allowed to happen, the more acidotic the patient becomes. After three to five minutes, this condition can become life-threatening. The goal of ETCO2 monitoring is to recognize a drop in the ETCO2 indication well in advance of a low oxygen saturation (SaO2) that indicates hypoxia. These parameters are intimately related.

The presence of CO2 is not an absolute value but rather a relative measurement marking changes in the respiratory patterns of individual patients. A safe rate of CO2 expulsion for one patient may indicate imminent danger for another. The anesthesia provider must therefore establish a baseline measurement by observing the capnograph's movements during the patient's first few monitored breaths.

A typical capnograph resembles a square wave on an oscilloscope, with the upslope representing the initial expiratory phase before the constant top of the square leads into a rapid downslope. Airway obstructions or restrictions appear as variations to the square wave. If, during monitoring, the wave disappears completely, your anesthesia provider will surely see a drop in oxygen saturation within 45 to 60 seconds. The assessment of wave patterns provides an accurate indication of the patient's respiratory condition and is the primary reason for analyzing the waveform rather than capnography's absolute numbers.

When sedation kicks in, the patient's breathing rate and end tidal volume drop, two physiological reactions that cause the soft tissue in the back of the throat to relax and collapse on the airway. The airway can quickly become occluded with the pressure of the throat's muscles.

Capnography serves as an early indicator to this potentially dangerous change in a patient's respiratory rate and loss of responsiveness. The anesthesia provider must quickly assess the patient's condition when the capnograph's waveform begins to differ from the patient's baseline cycle measured at the beginning of the case. Measuring the level of oxygen in the blood is an alternate means of monitoring a patient's cardiovascular function, with normal levels falling between 96 and 100 percent. But CO2 disappears well in advance of the SaO2 dropping. Keep in mind that the ETCO2 number is not accurate with a split cannula having O2 running on the other side, but does provide at least an indication of the basic "air goes in and out" theory.

Hypoxic patients experience increased coronary stress with the passing of each second. Those precious seconds of cardiac compromise demand that the anesthesia provider immediately recognize changes in the patient's condition, changes that are quickly indicated by the capnograph.

Make it required
Capnography monitors run between $700 and $2,000, according to a manufacturer's rep. Throw in the approximate $10 spent on the nasal cannula needed for each case and cost could become a factor in the decision to forego ETCO2 monitoring.

That's a mistake, in my opinion. End tidal CO2 monitoring is a valuable tool during and immediately after surgery. Propofol, for example, has no reversal agents and a narrow margin between conscious sedation and the patient needing airway adjuncts or even intubation. The need for an early warning system is imperative when administering the drug, especially when it is administered by RNs. Respiratory problems are also a very real risk in the PACU, especially with the recent increase in the number of obese patients presenting for surgery with obstructive sleep apnea. The PACU staff should observe ETCO2 closely. As is the case in the OR, a complete understanding of the capnograph's cycles will optimize the timing of necessary interventions in order to prevent airway complications.