ISSN 2398-2950      

Anesthetic monitoring: respiratory system (capnograph)


John Dodam

Keith Simpson

Synonym(s): Alveolar air analysis, End-tidal carbon dioxide measurement



  • Carried out by an infrared analyzer. Gases whose molecules contain two dissimilar atoms absorb radiation in the infrared region of the spectrum.
  • Respiratory gases are analyzed by two possible methods - Sidestream or mainstream analysis.
  • Sidestream analysis. With this method a sample of the respiratory gas is withdrawn continuously throughout the respiratory cycle and sampled in an analyzer that is contained within the main unit. These units typically have a 1.0-2.0m small bore pipe leading from the ET tube to the main unit.
  • Mainstream analysis: The gas is analyzed as it passes through the sensor located at the end of the ET tube. With these units the sensor is connected to the main unit by a cable. Carbon dioxide concentration is assessed as the patient breathes through the sensor.
    The different sampling mechanisms have differing implications for their use:


  • Because a finite sample is being withdrawn from the patient airway there is a lower limit to the size of patient that can be monitored.
  • In its simplest terms if the sample volume starts to approach the tidal volume of the patient then dilution of the sampled gas will occur and the reported CO2 concentration will fall.
  • It is for this reason that it is desirable to have a Sidestream analyzer with a low sampling flow rate.

Typically, medical units intended for the human market have withdrawal rates of around 200ml per minute. These rates are generally too high for animals weighing under 5kg and sample rates of 100 or even 50ml per minute are preferred.

  • The sample must be withdrawn down a sampling tube and this inevitably introduces a time delay between the patient exhaling and a rising CO2 concentration in the analyzer. This is generally of about a second but it should be borne in mind that for this reason rising and falling waveforms will not be synchronized with exhalation and inspiration of the patient.
  • The sample can in practical terms be taken from anywhere. Thus with a sidestream unit the patient does not have to be intubated.
  • Gas can be sampled directly from a nostril or from inside a face mask or even from an intra-tracheal catheter.


  • There is no finite sample volume being withdrawn with a mainstream unit so in theory this type of unit should be able to monitor patients down to a very small size without compromise.

In practice this is not so.

  • The actual mainstream sensor has a fixed internal volume that must be filled with gas for the result to be accurate.
  • If a patient's breath is diluted in filling that volume then a reduced reading will result, so there are still limitations on its lower limit. Additionally the sensor unit has to be placed at the end of the ET tube, which imposes physical restrictions on its use.
  • More modern mainstream units are reducing in size thus reducing the importance of this fact. The requirement for intubation and connection to an ET tube means that Mainstream units are less flexible in their use compared to sidestream systems.
  • The highest carbon dioxide concentration is typically found at the end of expiration and is called the 'end-tidal' carbon dioxide.
  • The measured carbon dioxide concentration can be displayed in a number of ways:
    • Capnometerdisplays the end-tidal value as a number only. This can be in percentage terms or in millimeters of mercury pressure. Atmospheric pressure is 760mmHg, so a 5% carbon dioxide reading equates to 5 x 760/100 mmHg = 38mmHg.
    • Capnographdisplays the carbon dioxide measured as a graphical display with height being directly proportional to carbon dioxide concentration. The trace is referred to as a capnogram  Capnography - typical capnogram  .
  • In a healthy animal end-tidal carbon dioxide concentration is an accurate reflection of alveolar carbon dioxide concentration and that in turn is an accurate reflection of pulmonary venous carbon dioxide concentration, i.e. the carbon dioxide concentration of oxygenated blood returning from the lungs to the left ventricle. Therefore end-tidal carbon dioxide values directly reflect arterial carbon dioxide concentrations. Generally the end-tidal value will be less than the arterial value because there are a number of diffusion gradients to overcome   Capnography - diffusion pathways  .
  • Useful to monitor adequacy of gas flow in non-rebreathing anesthetic systems and efficacy of soda-lime in rebreathing systems. Carbon dioxide should fall to zero during inspiration. If higher, it indicates inadequate removal of carbon dioxide from inhaled carrier gases.
  • Anything that alters the pathway between the release of carbon dioxide from the pulmonary circulation in the lungs and the exhalation of gas from the trachea will affect the end-tidal CO2 value.
  • Circulatory disturbances affecting the pulmonary arterial flow will be reflected in the end-tidal CO2 value.
  • Reduced pulmonary flow for a variety of reasons (embolism, arterial occlusion, increased intra-thoracic pressure) will lead to a sharp fall in end-tidal CO2 readings.
  • Ventilation/Perfusion (V/Q) mismatches   Ventilation perfusion mismatching  will also affect end-tidal CO2 values. If Ventilation is good but Perfusion poor (high V/Q) then a reduced end-tidal CO2 will result. Such effects may be seen with the conditions outlined above for reduced pulmonary flow. If Perfusion is good but Ventilation poor (low V/Q) then an elevated end-tidal CO2 value will result. Such effects may be seen with:
    • Restrictive bronchial disease
    • Foreign bodies
    • Emphysema.
  • Capnography provides a non-invasive method of assessing arterial carbon dioxide concentration by displaying the exhaled and inhaled CO2 concentrations in real-time.

Whilst an actual reading or numerical value is helpful much more can be learned from the capnogram itself  Capnography - capnogram components  .

  • Phase II represents the mixing of alveolar gas with dead space gas.
  • Phase III represents the emptying of the alveoli of gas and typically has a positive slope for two main reasons:
    • CO2 is continuously being delivered to the lungs during expiration.
    • Reducing lung volume and increasing CO2 delivery leads to a rise in end-tidal CO2.
  • The alpha angle is the angle between phase II and phase III. This angle is important as it indicates the Ventilation/Perfusion ratio of the lungs. If the V/Q ratio is normal there will be a sharp transition from phase II to phase III. In real terms this means that emission of CO2 from the lungs is efficient and CO2 is rapidly eliminated. The normal slope is discernible as slightly positive. If the alpha angle is large (marked slope to phase III) then this represents late emptying of alveoli with a low V/Q (high CO2
    concentrations) and is indicative of pathology   Capnography - pathology capnogram  .
  • The capnogram is a summation of all the different processes leading to the elimination of carbon dioxide. The individual processes cannot be separated or determined by looking at the capnogram but the changes in the capnogram can be used to indicate which areas are involved.
    To put this into perspective take the situation where the end-tidal CO2 reading is high at 6.0%. If the alpha angle is small and the slope of phase III normal then there is no late emptying problem and the high end-tidal CO2 value is most likely due to hypoventilation. If the alpha angle is large and phase III has a marked slope then late emptying is occurring and there is a V/Q mismatch. Areas of the lung may be under-ventilated due to posture, or there may be restrictive airway disease. The same end-tidal reading has resulted from two different causes but can be discriminated by the capnogram.


  • Blood gas concentrations are basic indicators of the state of body function, so any information relating to these parameters during anesthesia is valuable in assessing the animal's well-being.
  • Indicates incorrect endotracheal tube placement   Endotracheal intubation  or if the endotracheal tube is blocked or kinked. With misplacement or occlusion the usual fluctuations in end-tidal carbon dioxide which occur in normal respiration are absent or much reduced.
  • Abrupt reduction in end-tidal carbon dioxide concentration is a sign of pulmonary embolism, as perfusion of the lung tissue is dramatically reduced.
  • Reflects changes in cardiac output; again, reduction of pulmonary perfusion will lead to reduction in end-tidal carbon dioxide concentrations.
  • Allows remote monitoring of respiratory function in cases where close access to the animal is limited, without disturbing surgical drapes.
  • Useful aid in monitoring the adequacy of controlled respiration as in intermittent positive pressure ventilation for intrathoracic surgery and when neuromuscular blocking drugs   Anesthesia: non-depolarizing neuromuscular blockade  are employed.
  • A normal capnogram provides an indication that the anesthetic circuit is functioning properly. Increases in inspired carbon dioxide tension could indicate inadequate oxygen flow rate (rebreathing circuit), or incompetent valves, soda-lime absorption, or hose disconnection.
  • Indicates adequate oxygen flow rate in non-rebreathing circuits and normal soda-lime and circuit function in rebreathing systems.


  • Equipment required is expensive.
  • Occasionally, cross-checking the readings from the capnograph with those obtained from arterial blood gas analysis is preferable, particularly during long procedures. However, the equipment for such analysis is also expensive and time limitations on the acceptable interval before processing can mean the use of outside laboratories is not an option in many cases.
  • With sidestream sampling, the sampled gas must be returned to the circuit or scavenged as it contains anesthetic agents.


  • Only comparable alternative is arterial blood gas analysis but the caveats given above apply.
  • Monitoring standard respiratory system parameters and also those indicating a healthy cardiovascular system with adequate tissue perfusion, is often the only practical alternative.


  • Collect all necessary equipment.


  • Infrared gas analyzer.
  • Adaptor with a small side arm to connect the endotracheal tube adaptor to the breathing circuit. Fine-gauge tubing from the side arm carries the continuously withdrawn gas samples to the analyzer.
  • Another sampling system places the detector at the end of the endotracheal tube. This configuration is called main-stream sampling.
  • Both these types of units are sold to veterinarians. In US, JD Medical distributes a Nonin capnometer that utilizes main-stream sampling.

To provide a more accurate estimation of PaCO2 and, especially in small patients, the gas sampled by a side-stream capnograph should be sampled from deep within the endotracheal tube. This can be done by affixing a polypropylene urinary catheter to the end of the capnograph sampling line.

  • Gas from a capnograph will contain anesthetic gas from the circuit and ideally should be routed back into the anesthetic circuit.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Wagner A E, Gaynor J S, Dunlop C I et al (1998) Monitoring adequacy of ventilation by capnometry during thoracotomy in dogsJAVMA 212 (3), 337-379 PubMed.
  • Wright P S & Hellyer P W (1996) Respiratory monitoring during anesthesia: pulse oximetry and capnography. Comp Cont Ed Pract Vet 18 (10), 1083-1097 VetMedResource.

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