- Capnometry describes the maximum value of carbon dioxide measured at the end of expiration or end-tidal carbon dioxide (ETCO2).
- Ventilation status, in the form of ETCO2, should ideally be monitored during general anesthesia.
- End-tidal carbon dioxide levels correlate fairly well with arterial carbon dioxide (PaCO2) in birds and mammals.
- Capnography should only be used to measure trends in reptiles as ETCO2 levels can be quite different from PaCO2 due to cardiac shunting of blood past the lungs.
- Both side-stream and mainstream capnographs can be used in exotic animal patients. Although side-stream units have a reduced effect on mechanical dead space, they are not reliable with small respiratory volumes.
- Normocapnia in mammals is associated with an ETCO2 of 35-45 mm Hg.
- Research suggests that an ETCO2 of 30-45 mm Hg is considered appropriate for the anesthetized grey parrot.
- Hypoventilation and hypercapnia are a potential problem in every species under general anesthesia, however the negative physiologic effects of hypoventilation develop much more rapidly in anesthetized birds. Therefore, assisted ventilation should begin immediately after anesthetic induction in birds to prevent hypercapnia and hypoxemia.
- This article is part of a series on anesthetic monitoring in exotic animal patients. Additional topics available include: blood pressure, electrocardiography, pulse oximetry, and monitoring vital signs.
How does capnometry work?
Capnometry measures the maximum value of carbon dioxide (CO2) obtained at the end of expiration (ETCO2), which reflects the level of CO2 present in alveolar gas.7,12,10,17,20 The most common method of measuring CO2 uses infrared spectroscopy.12 A beam of infrared radiation is sent through an air sample to a photometer. Carbon dioxide selectively absorbs specific wavelengths (4.26 µm) of light, which allows the CO2 level to be measured. The CO2 level is usually expressed as partial pressure in mm Hg, although some units display percentage CO2 or FCO2.12,16,18
The terms capnometry and capnography are often used interchangeably, but there is a difference (Table 1).
|Table 1. Definitions related to capnometry and capnography|
There are two types of capnographs: side-stream and mainstream (Table 2). A mainstream sensor analyzes expired gases as each breath passes through the airway adapter inserted between the anesthetic circuit and the endotracheal tube.5,12,23 Mainstream capnographs usually provide a more accurate and rapid measurement than side-stream analyzers.5,12 The side-stream capnograph continuously aspirates a small stream of expired air from the patient’s airway using a small tube and pump (Fig 1).5,12,23 The standard flow through the sampling line can be as high as 200 ml/min, however units with a low aspiration flow rate (50 ml/min) have also been developed (Microstream capnograph (NPB-75, Nellcor Puritan Bennett, Pleasanton, CA).1,5,10 Many monitors display both a number and a waveform.
|Table 2. Comparison of side-stream and mainstream capnographs 2,10,12,15,20|
|Sensor location||Away from the anesthetic breathing circuit||Directly on the hub of the endotracheal tube within the anesthetic breathing system|
|Gas sampling rate||50-200 ml/min in pediatric units||Requires no gas sampling|
|Response||Slower response time, delay of ~3 seconds between sample aspiration and display||More rapid response, near real time display
|Sensitivity||Not reliable with small respiratory volumes||Typically provide more accurate readings, functions better with smaller volumes|
|Dead space||Use of a side-stream endotracheal tube adaptor (as shown in Fig 1) decreases dead space but by a lesser degree when compared to mainstream capnographs||Mechanical dead space and resistance is a concern for small patients, however pediatric adaptors or connectors reduce dead space|
The challenges of capnography are primarily related to small patient size. In very small animals, the sampling rate of side-stream capnography can be similar to or higher than the tidal volume of the patient.20 This leads to dilution of the exhaled breath with fresh gas from the anesthetic breathing system, resulting in an underestimation of ETCO2.1,20 Adjustments in sampling flow rate or technique may be required.17 Small patient size can also have a significant impact on mechanical dead space and resistance when using a capnograph, however pediatric adaptors or connectors can help to reduce this dead space (Fig 2).1,13,10,12,15,20
Capnometry is a life-saving tool with a variety of potential uses (Table 3).4,20 There is good correlation between ETCO2 and arterial CO2 (PaCO2), which makes capnography an effective tool to evaluate the adequacy of ventilation.8,14,20,22 In small patients, capnography is often used to identify trends during anesthesia, especially when the patient is maintained on a mechanical ventilator.10
|Table 3. Potential uses of capnometry 10,20|
Analysis of the capnograph waveform allows the anesthetist to verify ETCO2 levels, while also gaining more detailed physiological information (Fig 3) (Video 1).7,10,20 During a standard capnograph, the first gas exhaled is anatomic dead space and is free of CO2. The partial pressure of carbon dioxide (PCO2) then rises, representing a mixture of dead space and alveolar gases. The plateau represents alveolar gases and it may (or may not) slope slightly upwards due to continued delivery of venous CO2 to the alveoli during exhalation. End-tidal CO2 is the highest PCO2 level before the next inspiration. The capnograph should return to zero during inspiration because inspired gases should not contain CO2.10,12
Video 1. Normal movement of a capnograph waveform. Note: If audio is enabled, the Doppler pulse can be heard in the background. Video credit: Katrina Lafferty, CVT, VTS.
If the capnograph trace fails to return to zero, this usually indicates rebreathing of exhaled gas.10,15,23 To remove carbon dioxide from the non-rebreathing system, higher oxygen flows are required, leading to dilution of ETCO2. These lowered ETCO2 values can be exacerbated by the sampling size required when using a side-stream capnometer. The anesthetist must also rule out other causes of rebreathing, such as machine malfunction or dead space.
Analysis of capnograph waveforms can also be used to confirm airway patency and correct placement of the endotracheal tube (Fig 4).10,20 A normal capnogram for six breaths suggests tracheal intubation. False negatives can occur with large gas leaks, cardiac arrest, and severe bronchospasm, in which no trace appears on the capnogram.20
Exotic animal patients are often intubated with uncuffed endotracheal tubes and there is a high likelihood some gas will leak around the tube, resulting in an altered, less accurate capnograph waveform. The true ETCO2 of the patient will be higher than the capnograph reading. In patients with a slow respiratory rate, the waveform may appear more rounded. In patients with an elevated respiratory rate or particularly small tidal volume the waveform appears more peaked (Fig 5). Not all uncuffed tubes are associated with leakage of gas as the tube can form a good seal within the trachea (Fig 6).
|Table 4. Interpretation of changes or trends in end-tidal carbon dioxide (ETCO2) levels 10,11,12|
|Gradual rise||Indicates progressive hypercapnia and corrective action should be taken.
Failure of the fresh gas supply
Exhaustion of the soda lime in a closed system anesthesia
Problems with the anesthetic breathing system
Excessive depth of anesthesia
|Gradual fall||Increased ventilation
Decreased cardiac output
Pending cardiac arrest
Ventilation-perfusion (VQ) mismatch
Light plane of anesthesia
|Sudden fall or absence||Airway obstruction
Disconnection/obstruction of the breathing system
Disconnection/obstruction of the sample aspiration tube
Apnea or respiratory arrest
Exotic companion mammals
Capnography has been shown to provide valuable estimations of PaCO2 in rabbits (Oryctolagus cuniculus) and ferrets (Mustela putorius furo).5,9,19 Normocapnia in mammals is associated with an ETCO2 of 35-45 mm Hg.1 Patients with an ETCO2 (or PaCO2) less then 35 mm Hg are considered hypocapnic. Values for hypercapnia range from 65-75 mm Hg. 1 End-tidal CO2 usually underestimates PaCO2 by 2-5 mm Hg in mammals. 18,20
The normal range for ETCO2 in mammals is 35-45 mm Hg.
All birds hypoventilate under anesthesia.15 The negative physiologic effects of hypoventilation or respiratory arrest develop much more rapidly in anesthetized birds than in mammals, and assisted ventilation, beginning immediately after anesthetic induction is strongly recommended to prevent hypercapnia and hypoxemia.5,8 As in mammals, there is a strong correlation between ETCO2 and PaCO2 in birds over a wide range of partial pressures. In isoflurane anesthetized grey parrots (Psittacus erithacus) receiving intermittent positive pressure ventilation, this correlation was greatest between 20-60 mm Hg.8 Side-stream capnography provided a good estimate of PaCO2 in isoflurane-anesthetized raptors weighing more than 400 grams. The birds of prey received manual positive ventilation with a Bain circuit.5,15
An ETCO2 of 30-45 mm Hg was considered appropriate for anesthetized grey parrots.8
Airflow through the avian parabronchial lung is unidirectional, which means that all exhaled gases pass through the cranial air sacs and trachea without having a dilutional effect on ETCO2.8,17 This explains why ETCO2 generally exceeds PaCO2 by approximately 5 mm Hg in birds, the reverse of what is seen in mammals.8,15,24
All reptiles require intermittent positive pressure ventilation at a surgical plane of anesthesia. Although capnography can provide useful trends in reptiles (Fig 7), ETCO2 can be quite different from PaCO2 because of cardiac shunting of blood past the lungs.3,6 Values are rarely greater than 25 mm Hg in the reptile and it can be normal for values to suddenly drop to zero because of this cardiac shunting.
Capnometry measures the maximum value of carbon dioxide (CO2) obtained at the end of expiration or end-tidal carbon dioxide (ETCO2). There is good correlation between ETCO2 and arterial CO2 (PaCO2) in birds and mammals and capnography can be used as a reliable tool to evaluate the adequacy of ventilation in these species. Normocapnia in mammals is associated with an ETCO2 of 35-45 mm Hg. End-tidal CO2 usually underestimates PaCO2 by 2-5 mm Hg in mammals. Airflow through the avian parabronchial lung is unidirectional, which means all exhaled gases pass through the cranial air sacs and trachea without having a dilutional effect on end-tidal carbon dioxide. Therefore, ETCO2 generally exceeds PaCO2 by approximately 5 mm Hg in birds. An ETCO2 of 30-45 mm Hg was considered appropriate for anesthetized grey parrots. Capnography should only be used to identify trends in reptiles. End-tidal CO2 can be quite different from PaCO2 because of cardiac shunting of blood past the reptilian lungs.