Capnography: What does it tell us?

Capnography is the measurement of respiratory CO2.

Respiration involves  cellular metabolism, O2 & CO2 transportation (circulation) and ventilation. It results in the production and clearance of CO2 from the body.

During respiration CO2 is expired from the body while O2 is inspired. Capnography measures CO2 levels as they rise and fall in the patient’s breath.  There are two methods of measuring respiratory CO2.

Side-stream collection (collecting a small amount of respired air through a small tube off the main ET / anaesthetic connection). This form of measurement can be a little delayed and is much more challenging in small patients. It does allow CO2 measurement in sedated patients.

Mainstream collection (the sensor is part of the anaesthetic connection and directly measures CO2 levels entering or leaving the patient).

The capnograph trace is broken into four phases:

Phase 1: Dead space expiration. This is the beginning of expiration where CO2 levels are usually low (represents air that is sitting in the upper airways and not lungs).

Phase 2: The rising phase of the trace where alveolar CO2 levels leave the lungs during expiration.

Phase 3: CO2 levels plateau as CO2 rich alveolar air is expelled. This reaches a peak at the end tidal phase where expiration ceases then changes into inspiration.

Phase 4: Inspiration begins and CO2 levels rapidly fall.

CO2 is the by-product of cellular metabolism. It is transferred from the cell to the blood where it is transported to the lungs. CO2 then diffuses out of the blood into the alveolar space and is expelled in the breath. This is a complex process and part of the acid – base regulatory system.



During normal respiration O2 and CO2 move in and out of the body in a regulated fashion. Oxygen saturation is maintained somewhere between 95 – 100% while CO2 reaches a maximum of 35 – 45mm Hg during peak expiration. Anything that interferes with the patient’s metabolism, CO2 or O2 transportation (circulation & pulmonary perfusion) or respiration will impact CO2 levels. Capnography provides a window into the patients respiratory and circulatory system.

CO2 levels can go up or down. Common causes include:

Rising CO2 levels: Reduced ventilation, increased metabolism or elevating body temperature.

Falling CO2 levels: Increased ventilation, decreased metabolism and falling body temperature.

Step wise elevation of the baseline CO2 levels may indicate rebreathing problems with the anaesthetic machine.

Significant changes in the capnograph trace may indicate incorrect placement of the ET tube (within the oesophagus), or incorrect seal between ET tube & trachea.

It is important to consider factors that affect patient metabolism (hypothermia / hyperthermia), circulation (cardiac disease) or respiratory rate / function (CNS disease or injury / respiratory disease / over perfusion) when observing changes to the capnograph trace. The other consideration is the anaesthetic machine and its circuitry. Exhausted soda lime, sticky pop off valves, poorly positioned or loosely fitting ET tubes can all affect CO2 levels.

Capnography has resulted in much safer human anaesthesia. It has the potential to do the same for veterinary patients. Capnography provides a much more sensitive picture of the patient’s physiology than pulse oximetry. The two challenges for veterinary practices are:

  1. Cost. Capnography monitors are expensive.
  2. Education. Staff need to have a good understanding just what changes to the capnograph trace represent and how to address them.

The following links provide a very good source of information on capnography and respiratory physiology.


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