Never been terribly impressed by transcutaneous CO2 monitors.

The lag lag time can be significant and the accuracy is often questionable especially in patients with poor perfusion and/or those on vasoconstrictors.

I think that PICU was always the best place for them but obviously as an adult intensivist I stay well clear!

I'm a little confused by the comments about "burn sites" because modern transcutaneous oxygen and carbon dioxide monitors don't burn the skin. When I was an anesthesia resident at UCLA, I had the opportunity to try the earliest version of a transcutaneous oxygen monitor, and with these primitive machines there was some concern about leaving the sensor in the same location for prolonged periods of time due to the possibility of causing tissue damage. They warmed the skin to "arterialize" capillary flow, but the heating element was not regulated by a thermostat. It was impossible to calibrate these original monitors. They provided a numerical "readout" for oxygen concentration that rose with increased tissue oxygenation, but they could not actually measure the partial pressure. There has been considerable improvment in the technology during the 50 years since I was a resident. I have purchased one of the new machines. They now measure transcutaneous CO2 as well as oxygen, and can be calibrated to measure the partial pressures of both. However, they remain far from ideal. They are very "tricky" to use. Their main advantage is that they are non-invasive, and can provide continuous measurement of the partial pressures of oxygen and carbon dioxide. The sensors are not likely to cause tissue damage, because they are now governed by a thermostat that maintains the skin temperature around 45 degrees celsius, so patient injury is no longer a concern.

I am now working with the Plexus software company to enable automatic incorporation of TcO2 and TcCO2 data into my anesthetic records, and capture readings every five minutes.

Carbon dioxide chemistry and pathophysiology is very tricky stuff. I once had an engineer who helped design modern capnographs tell me that it was necessary to install a fan in the measurement chamber of the machines to constant stir the gas mixture, because carbon dioxide has a vexing tendency to "settle" and thereby frustrate accurate measurement. Transcutaneous CO2 measurement is similarly vexed by variables. The sensor is mounted in a small plastic cup that is pasted to the skin. About five drops of saline solution must be installed in the cup before the sensor is attached. The skin must be carefully "prepped" to prevent the water from leaking out of the plastic cup "chamber" where the measurement takes place. The temperature must rise to 45 degrees before the machine begins to operate. Too much or too little water in the chamber will disrupt measurement. The sensor must be calibated each time it is applied, and if you are using it in an ICU setting it's probably a good idea to re-calibrate the sensor every hour or so. I use the machine only during short dental surgeries, and must re-calibrate between each case. It's essential to have a clear understanding of the role of carbon dioxide in the pathophysiology of oxygen transport and delivery, and CO2 data alone is meaningless in the absence of O2 data.

Exactly what are you trying to accomplish in the ICU setting? Are you performing some sort of study, or are you using the machine to manage critically ill patients?

Inagree with evan - same issue is with continous lactate measurement- as it can depend upon the local blood flow.

Also co-2 and bicarb are bystanders - i really wish we can have a good continous ph monitoring devices.

But they also have limitations.

Thanks @Ian Ewart for your enlightening answer!

While there are several pitfalls to avoid with these devices, I do hope the newer versions will make them more valuable for our critically ill children!

Thanks @Lewis S Coleman for your enlightening and elaborative comments! I really enjoyed the update!

In our PICU settings, they were very helpful for ventilated children with type 2 respiratory failure, especially when they connected to the HFOV, where continuous end tidal CO2 capnography is not possible. HFOV also tends to have more mucus plugging in these small Airways, especially that we try avoiding open suction of the ventilator circuit to maintain the lung recruitment. We have several scenarios where things seemed "stable" just to be surprised with a scheduled blood gases showing significant respiratory acidosis.

Thanks @s. Sharma for your enlightening answer.

Though many medical equipment got their intrensic limitations, yet we can utilize them as an additional tool to improve our patients outcomes.

For example, even if the transcutaneous CO2 measurements were not corresponding to the blood gases, still if I find the trending up of transcutaneous measurement we can reasses the patient with another blood gas to make sure if it was a true rise and manage it earlier.

It sounds as if we have more in common that might be expected. I wasn't aware that HFOV was still being used in PICU settings. It was originally invented in the late 1980's using a conventional loudspeaker connected to AC electricity, whereupon it 1. demonstrated that "apneic oxygenation" effectively maintains tissue oxygenation and 2. led to the re-discovery of the therapeutic benefits of "permissive hypercarbia", that had been extensively researched by Yandell Henderson during the WWI era, but had been forgotten in association with the founding of professional anesthesiology. The history is fascinating, and you can find it in my published paper called "Four Forgotten Giants of Anesthesia History." There is a lot of useful medical science that has fallen into the "memory hole."

I am using the TcO2/TcCO2 monitor to test and document the benefits of my anesthetic technique, which combines elements of the old "mask technique" that prevailed during the "golden era of ether" with modern machines, medications, and monitors. It begins with mask induction with Nitrous oxide and oxygen, followed by "steal induction" with Sevoflurane, whereupon the N20 is shut off and the patient breathes high concentrations of oxygen to flush the nitrous oxide from the breathing circuit. Meanwhile, an IV is established and the patients are treated with 10 mcg/kg of fentanyl, which temporarily abolishes respiratory drive, and produces excellent intubating conditions without the need for paralysis or intravenous hypnosis. I perform elective nasal intubations for all our cases, which are mostly children for dental procedures, but we also have adult patients. There follows a period of "apneic oxygenation" where pulse oximetry readings remain close to 100% for 5-10 minutes, whereupon rising tissue levels of CO2 offset the opioid effects and spontaneous breathing resumes. During this interval I apply the TcO2/TcCO2 monitor, and it demonstrates rising levels of tissue oxygen and carbon dioxide. Blood pressure declines substantially during induction with this technique, but this is because the beneficial combination of fentanyl analgesia, inhalation anesthesia, and rising CO2 levels reduces sympathetic tone and microvascular flow resistance, which improves cardiac output, tissue perfusion, and tissue oxygenation. The transcutaneous oxygen levels rise progressively during the beginning of these cases, generally to between 200-300 torr (resting TcO2 in healthy children ranges between 70-80 torr). I've seen TcO2 readings as high as 600 torr. The TcCO2 rises until spontaneous breathing resumes, and generally approaches 100 torr. End-tidal CO2 simultaneously rises to around 100 torr, and declines as breathing resumes. Based on available studies in dogs, CO2 should not disrupt oxygen loading in the lung until it rises above 300 torr, and probably even then it should be safe in the presence of high concentrations of inspired oxygen. I am working with the Plexus software people to automatically incorporate the data in the electronic anesthesia record every five minutes along with other routine monitoring parameters. The technique produces excellent outcomes, with little PONV or pain, and safe exubation within ten minutes of the close of surgery. There have been no problems with respiratory depression due to the generous dose of fentanyl, because we never hyperventilate patients.

Are you working in a setting where you can perform clinical research? I believe that your critically ill pediatric patients would benefit greatly from a combination of synergistic treatments consisting of 1. elective intubation 2. HFOV 3. permissive hypercarbia (which opens the capillary gate, enhances cardiac output and the release of O2 from the hemoglobin molecule into tissues) 4. general anesthesia using 1/2 MAC inhalation agent to control the harmful sympathetic stimulation due to consciousness, fear, and anxiety 5. generous synthetic opioid treatment to control harmful nociception and reduce anesthetic requirements to 1/2 MAC and 6. intravenous magnesium sulphate to inhibit thrombin generation that causes systemic inflammation and excessive production of pulmonary exudates. I believe that this treatment strategy should produce prompt restoration of homeostasis and organ function in your critically ill patients. Let me know if you would like more information about the theoretical basis for these combined treatments. You can communicate with me directly at [email protected] or call me at 661-900-2390. www.stressmechanism.com

We use TCOMM to trend and to decrease ABG/CBG and blood draws if they correlate.

My experience has been with the Sentec TcCO2 applied to either the forehead or ear lobe. Burns have not been an issue as long as the site is changed per manufactures recommendations. Generally TcCO2 values trend approximately 3 torr less than PaCO2.

Thanks dear @Lewis Coleman for your second enlightening comment!

Actually the HFOV is a wonderful life saver in our PICU and NICU practices, and I describe it as a more "gentle" ventilation for the severe respiratory failure / ARDS .

Following.We are keen on buying a capnograph in order to diagnose HVS when performing a free run test in assessment of astma in children.

Any good suggestion for that?

The PeriMed company is in the process of introducing new monitoring technology in their transcutaneous O2/CO2 machines. You might wish to investigate the new technology.

Thanks for this info - I will check on this !