If I were to take away all of your fancy anesthesia monitoring equipment save ONE monitoring parameter, which would you choose to keep? Between heart rate/ECG, ETC02, Sp02, blood pressure (non-invasive but you can choose Doppler unit or oscillometric), and temperature which would you choose? Thankfully many of us have the luxury of using all of these parameters plus our own eyeballs and fingers to monitor our anesthetized patients but if I were down to just one, I do not want to monitor anesthesia without capnometry to measure end tidal C02.
End tidal C02 is the measurement of carbon dioxide in each exhaled breath. Before getting into everything we can gain by monitoring this value, let’s think about why it is important to monitor. Carbon dioxide is the gas that drives respiration. We (and our patients) inhale because the respiratory center in our brain detects higher than normal levels of carbon dioxide in the blood. We inhale oxygen, and then exhale that carbon dioxide every minute of every day of our lives. If carbon dioxide levels get too high, our respiratory rate will increase so that we are exhaling more C02. If levels get too low, our respiratory rate decreases so that we hang on to more C02. In our normal patients without lung disease or metabolic disease, this process is sufficient to keep their C02 levels perfectly normal. When we anesthetize that patient, the drugs we use can decrease the ventilatory drive in the brain and relax the intercostal muscles which can cause changes in ETC02. Changes that, because they are anesthetized, the patient cannot correct on their own.
Do we really need to monitor ETC02? We can see our patient breathing, we have the Sp02 giving us good numbers, why bother? First of all, our trusty pulse oximeter is only giving us part of the picture. With some fancy new models, we can get some impressive perfusion information from our pulse oximeter, but it is still only giving us oxygenation status of our patient. It is measuring the percentage of hemoglobin that is saturated with oxygen. This number tells us that the patient is receiving enough oxygen. When that patient is anesthetized and breathing 100% oxygen, a low patient Sp02 may be masked by the increase in inhaled oxygen. And the Sp02 monitor has its limitations – ambient light, probe placement, movement, and decreased peripheral perfusion can all alter the reading. By monitoring ETC02 we are able to determine our patient’s ventilation status. This is the physical movement of air in and out of the lungs and upper respiratory system. By using both ETC02 and Sp02 we are getting a more complete picture of our patient under anesthesia.
Depending on who you read and where you work, the normal range for ETC02 may differ slightly, but I prefer to use 35mmHg-45mmHg as my ideal range for an anesthetized patient. Not only are we monitoring ventilation and respiratory drive with that normal range, we are also protecting the patient from acid/base changes. Elevated levels of carbon dioxide can lead to acidosis which can bring additional problems to our anesthetized patient. By monitoring, we can intervene to keep the ETC02 within that normal range.
An elevated ETC02 (>45mmHg), or hypercapnia, signifies that the patient is hypoventilating. Common causes for this include: too deep a plane of anesthesia, an airway obstruction, pneumothorax, body position of the patient, and disease process (remember that obesity is a disease, especially when we place those patients in dorsal recumbency). To correct hypercapnia, increase the patient’s respiratory rate until the ETC02 reaches a normal level, and adjust anesthesia as needed. Troubleshooting the patient may be necessary if a pneumothorax is present or the patient is not responding as anticipated.
A decreased ETC02 (<35mmHG) or hypocapnia, signifies that the patient is hyperventilating. Common causes for this include: too light a plane of anesthesia, pain resulting in tachypnea, panting, pronounced hypothermia, decreased cardiac output, or excessive dead space in the anesthetic circuit. To correct hypocapnia, pain management or deeper anesthesia may be required to allow a lower respiratory rate, as well as monitoring other vital signs (such as temperature). Further troubleshooting may be necessary if the patient is not responding as anticipated.
In addition to exhaled carbon dioxide, a capnometer will also display the inhaled C02 with each breath. This number is ideally zero, but it is acceptable for a patient to be rebreathing a small amount of C02, so a value of 1mmHg or 2mmHg is tolerable. Higher inhaled C02 numbers can indicate exhaustion of C02 granules or a malfunction with the anesthetic machine or circuit. In very small or debilitated patients, increased inhaled C02 numbers may signify a need for mechanical ventilation or switching to a non-rebreathing circuit.
Many capnometers will also display each breath as a waveform, called a capnograph. Interpreting capnography is outside the scope of this article, but can give valuable information about breathing patterns, the presence of an airway obstruction, an airway leak, and breathing over a ventilator.
As mentioned, an end tidal C02 monitor reads the amount of C02 exhaled with each breath. Whether a mainstream or side stream machine, you are looking at the result of not only ventilation, but also blood flow, cellular metabolism, and alveolar ventilation. In order for C02 to make it out of the lungs and into your capnometer, your patient must be perfusing cells and transporting C02 back to the lungs to be exhaled. ETC02 is reliant on ventilation and perfusion. It is also an instantaneous result, giving you up to the minute results of what is happening with your patient. We have discussed the respiratory monitoring, but ETC02 numbers are also a clue as to the patient’s perfusion and circulation. Decreased cardiac output can lead to decreased ETC02. The patient continues to ventilate, exhaling C02, and if perfusion decreases there is less C02 being brought back to the lungs to be exhaled. A rapid drop in ETC02 is cause for alarm, as this can signify impending arrest.
Watching ETC02 in relation to other vital signs under anesthesia will help you as the anesthetist gain a better overall understanding of your patient. For instance, you gather the following vitals on a 5 year old MN Doberman who is undergoing an elective procedure. He has been under anesthesia for 30 minutes when you record the following:
- Heart Rate – 52bpm (ECG normal)
- Respiratory Rate – 10bpm (on an anesthesia ventilator)
- Mucous Membranes – Pink, CRT 1-2 seconds
- SP02 – 99%
- Temperature – 97.6°F
- Blood Pressure – 112/78 (MAP 84)
- ETC02 – 33mmHg
This is a young, healthy dog, and looking at his vitals you might be concerned about his bradycardia, but his blood pressure looks good, his gums are pink, he is doing fine, right? His ETC02 of 33mmHg is pretty close to normal - does his bradycardia really need to be addressed? Remember that ETC02 is also a measurement of metabolism and perfusion. The dog is hypothermic but not severe, the respiratory rate is not increased, but this patient may be hypoperfused due to his bradycardia. After treatment for his bradycardia with glycopyrrolate, this same patient then had the following vital signs:
- Heart Rate – 104bpm
- Respiratory Rate – 10bpm
- Mucous Membranes – Pink, CRT 1-2 seconds
- Sp02 – 99%
- Temperature – 97.6°F
- Blood Pressure – 119/81 (MAP 89)
- ETC02 – 41mmHg
By increasing the heart rate we were able to see a slight increase in blood pressure, but the ETC02 came up to normal. As we improved perfusion, we improved ETC02.
As mentioned previously, watch ETC02 for sudden changes, especially dropping. A level that is normal and suddenly decreases can signal impending arrest. As the animal stops perfusing, they exhale their C02 and blood flow is too poor to bring any new C02 to the lungs to be exhaled. That patient is in danger and needs help immediately. Using that same logic, monitoring ETC02 on a patient undergoing CPR can let you know when that patient has a return of spontaneous circulation. ETC02 readings in a patient that has arrested will be low, into the low teens or maybe even single digits, but as that animal begins perfusing their cells again, the number will begin to slowly rise.
Even if you are not convinced enough to say that ETC02 is your one and only monitoring parameter if you are forced to pick only one, hopefully you are convinced that monitoring ventilation and perfusion is a good idea, and worth the investment in an end tidal C02 monitor for your multi-parameter anesthesia monitor.