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First let's talk about EtCO2 and how Cardiac Output (CO) is influenced by ETCO2. This is confusing to some, and to further confuse this issue, a large importer and medical exam table manufacturer recently ran a "talking dog ad" making some very weak and confusing claims stating that their small hand held ETCO2 monitor can display Cardiac Output and Depth of Anesthesia. Wouldn't life be nice if things were that simple. Unfortunately "a little knowledge is a dangerous thing". ETCO2 at best would be a indirect indicator of Cardiac Output". Wouldn't it be nice if you were able to determine a patients Cardiac Output by looking at a CO2 value on a hand held CO2 monitor?
Over time people like Adolf Eugen Fick in 1870 have been trying to determine an easy method of getting to this value. If there was an easy and reliable way to get this information it would be a major medical breakthrough. Although there is a correlation to ETCO2, as Fick pointed out in 1870, and ETCO2 is valuable in resuscitation efforts in getting feedback information in cardio pulmonary resuscitation, a CO2 monitor cannot possibly give you a Cardiac Output Index Value. This is not to suggest that ETCO2 is not tremendously valuable during anesthesia but it is very important that a diagnostic value such as Cardiac Output is not to be confused with getting valuable subjective trending information and quantitative CO2 values to know what the patient expired CO2 value is. In this respect it is imperative that the most amount of CO2 waveform information should be displayed on the screen for an anesthesia case.
Cardiac Output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. An average resting cardiac output would be 5.6 L/min for a HUMAN male and 4.9 L/min for a female. Already we see a problem. What is the Cardiac Output for a Dog, a Cat, a Horse? There are too many variables.
For many years I sold thermal dilution cardiac output monitors and have seen this technology used in ICU's on very high acuity patients, unfortunately, always with one foot in the grave. I have also watched excitedly, the advances of several non invasive technologies which have all disappointed the medical community. There is no doubt that knowing the CO of a critical patient is tremendously valuable but only a bed manufacture can conclude that a Cardiac Output value can be arrived at from a battery operated hand held CO2 monitor. In fact, although I have BS in Biology and over twenty five years experience with Cardiac Output monitoring, I would quickly refer you to a veterinary anesthesiologist for any serious discussion on this very exciting but very complicated topic.
However, let’s talk about my favorite topic, the "real value" of ETCO2 without making ridiculous, ignorant, insulting claims of Depth of Anesthesia and Cardiac Output. For several years I worked for a very well known company that developed, and manufactured only ETCO2. My job was to find applications for measuring ETCO2 outside of the large human hospitals. This was twenty years ago, and the biggest problem was the weight, size and price of an ETCO2 monitor. Yet, the most significant clinical area for ETCO2 was EMS. I was able to convince Boston Fire to put several of these big, heavy units in their EMS trucks.
I point this out because some of the most significant strides made in EMS are directly a result of being able to monitor ETCO2. Interestingly, these same benefits apply to anesthesia. One of the most significant causes of death in EMS has always been airway related. Poor initial intubations, extubation on the ride to the hospital etc. When I arrived on the scene with my expensive CO2 monitor, Boston Fire was already using a calorimetric device that was litmus paper in a plastic casing, to test ph or in this case the presence of CO2. Once they saw the color change, they threw the device on the floor of the truck and would quickly start resuscitation efforts, with a peace of mind knowing they were in the airway.
Then comes my turn, with this big heavy CO2 monitor. They positioned it at the head of the patient and placed the sensor on the ET tube after intubation... they saw a CO2 WAVEFORM AND a CO2 NUMBER (Oh Yes! Greater than canned beer!) and then put the BVM on the sensor. Now we had a CO2 sensor in line while they resuscitate this guy. It was a beautiful thing.... while they were doing CPR they could monitor how effective the efforts were etc. As time went on they started realizing the tremendous importance of seeing a waveform. They were able to know everything from that number and waveform. Than the medic realized that when they were told to hyperventilate a head injury they were able to do it without under or over ventilating the patient. My monitor was a hit.
So without making ridiculous unrelated claims by some non medical person, the reality is that just knowing the expired CO2 of a patient has tremendous airway and ventilator value, and additionally knowing the inspired CO2 lets you know if the patient is rebreathing CO2.
My suggestion would be to read about how to interpret the capnographic waveform. There are several good articles but if you would like I can send you a very well written book on ETCO2.
In summation: An ETCO2 monitor for anesthesia should not be a small portable unit unless you are doing anesthesia in the wild or marine biology. The important thing is that you have a large screen with logically organized waveform and numbers clearly presented.
Thank you, I hope this information is helpful and above all I hope it improves a patient outcome. Please check back for more discussions on the world of monitoring.
Jim Arnold
Director of Marketing and Sales
System VET
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