ISSN 2398-2942      

Anesthetic monitoring: respiratory system (capnograph)


John Dodam

Keith Simpson

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



  • Carried out by an infrared analyser. Gases whose molecules contain two dissimilar atoms absorb radiation in the infrared region of spectrum.
  • Respiratory gases analysed by two possible methods:
    • Sidestream.
    • Mainstream
  • Sidestream analysis: sample of respiratory gas withdrawn continuously throughout respiratory cycle and sampled in an analyzer contained within main unit. Units typically have a 1.0-2.0 m small bore pipe leading from ET tube to main unit.
  • Mainstream analysis: gas is analysed as it passes through sensor at end of ET tube. Sensor connected to main unit by a cable. CO2 concentration assessed as patient breathes through sensor.
  • Different sampling mechanisms have differing implications for their use:


  • Because a finite sample is being withdrawn from patient's airway there is a lower limit to size of patient that can be monitored.
  • If sample volume starts to approach the tidal volume of patient then dilution of sampled gas will occur and reported CO2 concentration will fall.
    Thus desirable to have a sidestream analyser with a low sampling flow rate.
  • Typically, medical units intended for human market have withdrawal rates of around 200 ml/min.
    Too high for animals weighing under 5kg and sample rates of 100 or even 50 ml per minute preferred.
  • Sample must be withdrawn down a sampling tube and which introduces time delay between patient exhaling and a rising CO2 concentration in the analyser. Lasts approx 1 sec but because of this rising and falling waveforms will not be synchronized with exhalation and inspiration of patient.
  • Sample can be taken from anywhere. Thus no intubation necessary.
  • Gas can be sampled directly from nostril, inside face mask or even from intra-tracheal catheter.


  • No finite sample volume withdrawn with mainstream unit so should be able to monitor small patients without compromise.
    In practice this is not so.
  • Mainstream sensor has fixed internal volume that must be filled with gas for an accurate result.
  • If patient's breath is diluted in filling that volume then results in reduced reading, so still limitations on its lower limit. Additionally, sensor unit has to be placed at end of ET tube, which imposes physical restrictions on use. Less important now modern mainstream units are smaller.
  • Mainstream units are less flexible in use compared to sidestream due to requirement for intubation and connection to ET tube Anesthetic equipment endotracheal tube Anesthetic equipment endotracheal tube connectors.
  • Samples of respired gas are withdrawn continually from the anesthetic system, as near as possible to trachea.
  • The CO2 concentration is displayed as a percentage on a continuous paper trace, a dial with a swinging needle indicator or as an electronic display.
  • The highest CO2 concentration is typically found at end of expiration and called the 'end-tidal' carbon dioxide.
  • Measured CO2 concentration displayed in a number of ways:
    • Capnometer displays end-tidal value as a number only. Can be in percentage terms or in millimeters of mercury pressure. Atmospheric pressure is 760mmHg, so 5% carbon dioxide reading equates to 5 x 760/100 mmHg = 38mmHg.
    • Capnograph displays CO2 measured as a graphical display with height being directly proportional to CO2 concentration. Trace referred to as a capnogram Capnography - typical capnogram.
  • In a healthy animal, end-tidal CO2 concentration is an accurate reflection of alveolar CO2 concentration and that, in turn, is an accurate reflection of pulmonary venous CO2 concentration ie CO2 concentration of oxygenated blood returning from lungs to left ventricle. Therefore end-tidal CO2 values directly reflect arterial CO2 concentrations. Generally end-tidal value will be less than arterial value because there are a number of diffusion gradients to overcome Capnography - diffusion pathway.
  • Useful to monitor adequacy of gas flow in non-rebreathing anesthetic systems and efficacy of soda-lime in rebreathing systems. CO2 should fall to zero during inspiration. If higher, indicates inadequate removal of CO2 from inhaled carrier gases.
  • Anything that alters pathway between release of CO2 from pulmonary circulation in lungs and exhalation of gas from trachea will affect end-tidal CO2 value. Circulatory disturbances affecting pulmonary arterial flow reflected in end-tidal CO2 value. Sharp fall in end-tidal CO2 readings Capnography - output change in CO2 caused by reduced pulmonary flow due to:
  • Ventilation/perfusion (V/Q) mismatches Ventilation-perfusion mismatching also affect end-tidal CO2 values. If ventilation is good but perfusion poor (high V/Q) then results in reduced end-tidal CO2. Such effects may be seen with conditions outlined above for reduced pulmonary flow. If perfusion is good but ventilation poor (low V/Q) then elevated end-tidal CO2 value will result. Such effects seen with:
  • Capnography provides a non-invasive method of assessing arterial CO2 concentration by displaying 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 alveoli of gas and typically has a positive slope for two main reasons:
    • CO2 is continuously being delivered to lungs during expiration.
    • Reducing lung volume and increasing CO2 delivery leads to a rise in end-tidal CO2.
  • Alpha angle is angle between phase II and phase III and indicates ventilation/perfusion ratio of lungs, therefore important. If V/Q ratio normal there will be a sharp transition from phase II to phase III. This means that emission of CO2 from lungs is efficient and CO2 is rapidly eliminated. Normal slope is discernible as slightly positive. If alpha angle is large (marked slope to phase III) this represents late emptying of alveoli with a low V/Q (high CO2 concentrations) and is indicative of pathology Capnography - pathology capnogram.
  • Capnogram is a summation of all the different processes leading to elimination of CO2. Individual processes cannot be separated or determined by looking at capnogram but changes in capnogram indicate which areas are involved ie situation where end-tidal CO2 reading is high at 6.0%. If alpha angle is small and 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 alpha angle is large and phase III has a marked slope then late emptying is occurring and there is V/Q mismatch. Areas of lung may be under-ventilated due to posture or there may be restrictive airway disease. Same end-tidal reading has resulted from 2 different causes but can be discriminated by the capnogram.


  • Blood gas concentrations are basic indicators of the state of body function Acid base imbalance , so any information relating to these parameters during anesthesia is valuable in assessing animal's well-being.
  • Indicates incorrect endotracheal tube placement Endotracheal intubation or if endotracheal tube is blocked or kinked. Usual fluctuations in end-tidal CO2 which occur in normal respiration are absent or much reduced.
  • Abrupt reduction in end-tidal CO2 concentration is a sign of pulmonary embolism as perfusion of lung tissue is dramatically reduced.
  • Reflects changes in cardiac output; again, reduction of pulmonary perfusion leads to reduction in end-tidal CO2 concentrations.
  • Allows remote monitoring of respiratory function in cases where close access to 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: monitoring of neuromuscular blockade are employed.
  • Normal capnogram provides shows anesthetic circuit is functioning properly. Increases in inspired carbon dioxide tension could indicate inadequate oxygen flow rate (non-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 is expensive.
  • Occasionally, cross-checking readings from capnograph with those obtained from arterial blood gas analysis Arterial blood gas sampling is preferable, particularly during long procedures. However, equipment for such analyses 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, sampled gas must be returned to the circuit or scavenged as it contains anesthetic agents.


  • Only comparable alternative is arterial blood gas analysis Arterial blood gas sampling 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 only practical alternative.


  • Collect all necessary equipment.


  • Infrared gas analyser.
  • Adaptor with a small side arm to connect endotracheal tube adaptor to breathing circuit. Fine-gauge tubing from side arm carries continuously withdrawn gas samples to the analyser.
  • Another sampling system places detector at end of endotracheal tube (mainstream sampling).
  • Both types of units are sold to veterinarians. In US, JD Medical distributes a Nonin capnometer that utilizes mainstream sampling.
    To provide a more accurate estimation of PaCO2 and, especially in small patients, gas sampled by a side-stream capnograph should be sampled from deep within endotracheal tube. Done by affixing polypropylene urinary catheter to end of capnograph sampling line.
  • Gas from capnograph will contain anesthetic gas from circuit and ideally should be routed back into 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 dogs. JAVMA 212 (3), 377-379 PubMed.
  • Wright B & 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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