Episode #272 Behind the Waveform: Critical Safety Implications of CO₂ Sensor Selection

Hello and welcome back to the Anesthesia Patient Safety Podcast. My name is Alli Bechtel, and I am your host. Thank you for joining us for another show. We have an exciting Rapid Response to Questions from our Readers episode today from the June 2025 APSF Newsletter. This rapid response focuses on the carbon dioxide sensors for mainstream CO₂ analysis for intubated and non-intubated patients developed by Nihon Kohden. There are two distinct models that include the cap-One TG 980P Quantitative sensor and the cap-ONE TG 920P semiquantitative sensor. There is a waveform display feature for both of these sensors that is compatible with all of their monitoring systems. These sensors have been primarily designed for monitoring respiration in non-operating room settings. The authors present two cases of patients receiving general anesthesia when the semiquantitative CO₂ sensor was used leading to unrecognized CO₂ rebreathing and respiratory acidosis. It is critical that anesthesia professionals understand that innovative technologies that are used in the operating room, the uses and limitations, in order to keep patient safe during anesthesia care.

Before we dive further into the episode today, we’d like to recognize Eagle, a major corporate supporter of APSF. Eagle has generously provided unrestricted support to further our vision that “no one shall be harmed by anesthesia care”. Thank you, Eagle – we wouldn’t be able to do all that we do without you!”

Our featured article today is “Beware of Semiquantitative Mainstream Carbon Dioxide Sensors in the Operating Room” by Amrutha Nagella and colleagues. To follow along with us, head over to APSF.org and click on the Newsletter heading. The first one down is the current newsletter. Then, scroll down until you get to our featured article today. I will include the link in the show notes as well.

To help kick off the show today, we are going to hear from one of the authors. Here she is now.

[Nagella] “ Hi, I’m Amrutha Nagella. I work as a research scientist at University of Buffalo.”

[Bechtel] I asked Nagella, what got her interested in this topic. Let’s take a listen to what she had to say.

[Nagella] “ During my anesthetic practice in India, we encountered an unusual case with a carbon dioxide sensor in the operating room. A 34-year-old ASA1patient undergoing anterior cervical discectomy developed respiratory acid acidosis despite standard anesthetic protocols.

Soon after, in the same operating room, a 26-year-old ASA1 patient undergoing septoplasty also exhibited a similar pattern. The reason for respiratory acidosis was not clear in both the cases. We observed that in both cases, the end carbon dioxide values fluctuated with changes in the fresh gas flow, something that typically shouldn’t happen.

This prompted a detailed troubleshooting process during which we identified the cost to be the carbon dioxide sensor, which was used in both cases. The CAP one, TG nine 20 P sensor. This miniaturized carbon dioxide sensor provides. Only a semi quantitative assessment of carbon dioxide and critically operates on the assumption that inspired carbon dioxide levels are zero, meaning it cannot detect carbon dioxide rebreathing an essential function During general anesthesia, we were unaware of the sensor’s limitations and had inadvertently used the sensor for intraoperative carbon dioxide monitoring.”

[Bechtel] Thank you so much to Nagella for helping to introduce this topic. Let’s go into a little more detail about the cases now.

The first case is a 34 year old ASA 1 patient who underwent anterior cervical discectomy under general anesthesia. The Datex Ohmeda 9100c NXT workstation was used with a Nihon Kohden Life Scope 3562 monitor with a cap-ONE TG 920p CO₂ analyzer. After IV induction and intubation, maintenance anesthesia involved 5% Desflurane with oxygen and air mixture 1:1 with a total fresh gas flow of 4L/min for the first 15 minutes which was decreased to 0.8L/min. The displayed CO₂ level on the monitor was 34mmHg. One hour later, the displayed CO₂ value during expiration had decreased to 8mmHg. Other hemodynamic parameters remained stable with normal respiratory values for ventilator parameters, airway pressures, and lung compliance. Increasing the fresh gas flow to 8L/min led to an immediate change in the displayed CO₂ value up to 33mmHg. Throughout the case, the displayed expired CO₂ values varied depending on the fresh gas flow. Check out Figure 1 for a visual representation of this. The next step involved arterial blood gas sampling which revealed a respiratory acidosis with pH = 7.18 and PaCO₂ 60mmHg. The anesthesia team discovered that the CO₂ absorber appeared exhausted and replaced it. Now, check out Figure 2 in the article which reveals that after changing the CO₂ absorber there were no longer any fluctuations in the displayed CO₂ values depending on the fresh gas flow rates.

The second case involves a 26 year old ASA 1 patient undergoing septoplasty in the same operating room with the same equipment as the first case. During general anesthesia, the same changes in displayed CO₂ levels occurred. Decreased expired CO₂ values corresponding to low fresh gas flows with increased CO₂ values when the fresh gas flows were increased. Once again, replacing the exhausted CO₂ absorbent corrected this variation.

Investigation by the anesthesia team into the CO₂ analyzer revealed that the cap-ONE mainstream sensor is a semi-quantitative CO₂ analyzer that cannot be calibrated since it only contains a single CO₂ sensor and assumes that the inspired air always has a zero level of CO₂. This semi-quantitative sensor is not designed to be used in the operating room during general anesthesia and a semi-closed circle system with a CO₂ absorber since there is no inspired CO₂ monitoring, which is critical to be able to detect rebreathing from a valve malfunction or absorbent depletion. Instead, these sensors are designed to be used in the ICU or PACU where rebreathing does not occur. At the author’s institution, these sensors were in fact moved from the ICU to the OR.

This experience highlights the need for anesthesia professionals to be involved in resource allocation and availability for these sensors and monitors in different hospital locations. We need to be using the correct monitor for the clinical care setting. The current information from the manufacturer states that the TG 920P CO₂ sensor kit is not recommended for patients receiving oxygen by mask. But what about for use with anesthesia breathing circuits? A safety tag from the manufacturer could state the following: “Not suitable for use with rebreathing systems in the operating room during anesthesia.”

[Bechtel] Before we get to the response from the manufacturer, we are going to hear from the author again. I also asked Nagella when does she envision for the future with regards to this area. Here is her response.

[Nagella] “ Through this report, we wish to emphasize the need for clearer labeling and prominent safety warnings on, uh, the CAP one TG nine 20 P sensor. A clearly visible tag stating not for use in the operating room could help prevent accidental misuse of the sensor in the operating room where carbon dioxide monitoring becomes very crucial.

Additionally, we believe that the semi quantitative carbon dioxide analyzers should not display a waveform. This sensor is designed to display a waveform that is compatible with standard monitors. The presence of a waveform creates an illusion of real-time continuous carbon dioxide monitoring, both during inspiration and expiration, potentially misleading the Anesthesiologist.”

[Bechtel] Thank you so much to Nagella for contributing to the show today. And now its time to talk about the response from Nihon Kohden.

The CO₂ sensors available from Nihon Kohden include the TG-920P which was used in the 2 cases and the TG-980P which uses a different measurement method. The response from Nihon Kohden focuses on the differences between the measurement methods and the intended uses. Let’s start with the TG-920P series.

Here are some of the important considerations for this sensor.

• For measurement, mainstream capnography that uses a semi-quantitative method which measures the CO₂ partial pressure of expired air based on the assumption that the inspired air does not contain CO₂.
• No calibration necessary since it is based on the premise that there is no CO₂ in the inhaled air.
• Advantages include: quick and easy to use and in emergency situations, CO₂ measurement can begin immediately.
• Limitations of this sensor series include: In anesthesia circuits with depleted CO₂ absorbers or a facemask with insufficient fresh gas flow when the inspired gas contains CO₂, the displayed CO₂ value will be lower than the actual CO₂ concentration.
• There is an example of this in the Operator’s manual and in Figure 1 in the article.
• Additional considerations: Do not use the device when inspired air contains or may contain CO₂ Any inspired CO₂that might be present is not measured or reported. The device is applicable only for the inspired gas without CO₂ gas.

Check out Figure 2 in the article for the caution supplied by Nihon Kohden in the Operators manual. It states: Supply adequate oxygen when measuring CO₂ partial pressure of a patient connected to a Jackson Rees, Mapleson D, or any other respiration circuit where CO₂ gas may be present during inspiration. The semi-quantitative method measures CO₂ partial pressure based on the assumption of no CO₂ gas in the inspired air; it measures the CO₂ partial pressure of the expiration of every respiration. If the inspired air contains CO₂ gas, the measured CO₂ value may be lower than the actual value.

Now, let’s turn our attention to the TG-980P series.

• This sensor uses mainstream capnography with the quantitative measurement method with a single wave spectroscopic method.
• There is a required zero calibration of the CO₂ measurement manually on the patient monitor interface that takes about 5-6 seconds before measurement begins.
• It is recommended for use in situations where the inspired air may contain CO₂, but it can be used in situations where the inspired air does not contain CO₂ as well.
• It is designed to be used during anesthesia care, mechanical ventilation in the ICU, and non-invasive respiratory management.
• There is accurate measurement of any CO₂ partial pressure in the inspired air.

As you can see, there are important differences between these two sensors. They are not interchangeable and it is important to use the appropriate sensor that is designed for the specific clinical setting.

Did you know that Nihon Kohden CO₂ sensors use unique technology so that they can handle condensation with a heater? This means that they are smaller, lighter, and more robust than conventional mainstream sensors.

Here is the conclusion to the response from Nihon Kohden. They write, “Nihon Kohden is dedicated to improving the labelling of our devices to more clearly indicate the appropriate product in the presence of inspired air containing CO₂. As a medical device manufacturer, we will continue not only to improve our product technology on a day-to-day basis but also place a renewed emphasis on patient safety by working with anesthesia professionals on this matter in the future. Thank you for this valuable opportunity.”

Thank you to our anesthesia professionals and industry representatives for working together to bring us this rapid response article. This continued communication about the uses of medical devices and technology is important as we continue to work towards improved anesthesia patient safety.

If you have any questions or comments from today’s show, please email us at [email protected]. Please keep in mind that the information in this show is provided for informational purposes only and does not constitute medical or legal advice. We hope that you will visit APSF.org for detailed information and check out the show notes for links to all the topics we discussed today.

The October 2025 ASPF Newsletter is almost here. Before the new newsletter drops, we hope that you will catch up on any of the articles from the June newsletter that you may have missed. What are you waiting for? Head over to APSF.org and click on the Newsletter heading. The first one down is the current issue. You can also click on APSF Newsletter archives and right at the top is the June 2025 Newsletter. We hope that you will share the newsletter and this podcast with your colleagues and members of the perioperative team at your institution.

Until next time, stay vigilant so that no one shall be harmed by anesthesia care.

© 2025, The Anesthesia Patient Safety Foundation