Capnography for Vets: A Help or a Hindrance?

Mainstream Capnography on a dog with the VM2500 Mainstream Capnograph
"We're in the middle of surgery, and the capnograph is all over the place," proclaimed the slightly panicked voice on the other end of the phone. "Okay, let's work through this step by step. First, please check on your patient and tell me what is happening. Are they breathing?"

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Capnography can give you an amazingly complete picture of your patient’s condition under anaesthesia, but there is a lot to understand before a clear image can emerge. Without context, the waveforms produced can begin to look quite scary.

We systematically assessed the patient’s vital signs. The airway was secure with an ET tube in place, and oxygen was administered at 4 litres per minute. The patient’s respiration was slow, steady, and rhythmical at 12 breaths per minute, with the chest rising and falling. The peripheral pulse remained steady at 120 beats per minute. Analgesia appeared effective, there had been no response to surgery, and the surgeon had completed the procedure.

Understanding how your monitor works

“Alright, let’s move to the monitor. What is it telling us? First, check the oxygen saturation (SpO2).”

The SpO2 read 99%, the pulse rate was 120 beats per minute, blood pressure measured 110/75, the temperature was 37.2 degrees Celsius, and the ECG trace indicated a normal heart rate of 120 beats per minute. However, the capnograph was displaying erratic readings, with values fluctuating from 34 to 10, and the respiration rate indicating 60 at times, along with intermittent rebreathing, showing an FiCO2 of 5.

“Is it a side or mainstream capnograph?”

The reply came, “I don’t know, it’s a capnograph.”

I received a picture and then a video of the waveform on my phone. (see below)

“Alright, let’s examine the shape of the waveform. By the way, I suspect it’s a mainstream capnograph, but we’ll address that shortly.”

capno waveform  - Capnography for Vets: A Help or a Hindrance?
An image of the capnograph which was sent to us during the call in question

Understanding the Capnography Waveforms

The waveform rose smoothly from a baseline of zero as the patient exhaled. This indicated a lack of rebreathing, suggesting a balance between fresh gas flow, airway resistance, and dead space. The plateau in the waveform indicated the start of a steady exhalation, interrupted by a series of jagged, rhythmical upsweeps. The ETCO2 number fluctuated from 38 to 29 to 19 to 10 as it tracked and measured each peak before returning to zero. These waveform spikes appeared to correspond to the pulse ox waveform and it was reported closely resembled the ECG rhythm. 

The capnograph clearly exhibited cardiogenic oscillations, and showed that the patient was in a better condition than the nurse at that moment. Cardiogenic oscillations occur when the patient is very relaxed and the exhaled breath is being driven by their heart beating, which results in characteristic peaks shown in the image in the above paragraph.

Furthermore, the cardiogenic oscillations clearly showed that the device used was a mainstream capnograph – the peaks displayed when cardiogenic oscillations occur on a sidestream capnograph are more rounded, a softening effect which is caused by mixing of the CO2 sample because of the additional distance that the breath sample has to travel before being measured by the sensor. 

The Take-Home Lessons

1.     Always keep a manual check on the patient, focusing on airway, breathing, and circulation. Listen to heart and lung sounds and palpate the peripheral pulse.

2.     Compare the patient’s data with the electronic monitor. While they may not always match perfectly, they should be close. Monitors are generally reliable but not infallible and can occasionally provide inaccurate data.

3.     Record the data on the anaesthetic record chart. This not only helps you track the patient’s progress but also serves as a crucial reference if things don’t go as planned or if you need to hand over the case to a colleague.

4.     Learn how to interpret the monitor’s waveforms. They provide valuable insights into the patient’s condition, from the sharp spikes in the SpO2 waveform indicating high blood pressure during dynamic ventricular contractions to rounded flattened waves indicating low pulse pressure/BP. Capnography has even more diverse and informative waveform shapes and patterns, giving a real insight into far more than just the respiration rate of the patient, provided you know how to interpret them.

5.     Consider acquiring the “Capnography Field Guide.” This pocket-sized, indestructible notebook offers guidance on interpreting more than 30 different waveform patterns and covers the effects of various breathing circuits. It will help you understand why the case described above involved a mainstream monitor and the key differences between side stream and mainstream capnography.

Please note that the picture at the top of this article is a still photograph taken from an actual video of a real, relaxed, and healthy dog recovering from a simple surgery, which you can find on YouTube here:

The rest of this case study is a fictional account based on several phone calls. The author receives numerous similar calls each week, which inspired him to write the book.

You can obtain your copy today below:

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