End-tidal carbon dioxide (ETCO2) monitoring through capnography often is called the "anesthesia disaster early-warning system." Vitally important, it is
the only parameter that thoroughly reflects a patient's ventilatory status, and it can signal problems within two breaths.
Either expired CO2 or ETCO2 will measure the CO2 produced in the cells, a function of metabolism. CO2 transported from the cells to the lungs is a function of circulation; CO2 is eliminated by the lungs.
Capnography gives a graphic and a numerical readout of the CO2 concentration in a patient's exhaled gases. It provides a means to assess ventilation, integrity of the airway, and the breathing
circuit, as well as cardiopulmonary function.
A capnogram is the graphic portrayal of the changing concentration of exhaled CO2 during the respiratory cycle. P, Q, R and S are letters used to refer to different portions of the waveform. A normal waveform
(Figure 3a) should have a baseline of zero during inspiration (i.e., inspiratory baseline). This is followed by an expiratory
upstroke (P and Q) that contains little or no CO2 and moves the curve upward until it levels out at a plateau (Q and R). CO2 concentration continues to increase until it reaches its maximum at point R just before the onset of inhalation (i.e., inspiratory
Figure 3a: A normal waveform has a baseline of zero during inspiration.
The height, frequency, shape, rhythm and baseline position of the waveform are monitored during anesthesia. CO2 concentration in the sample is reflected by the wave height. Changes in the standard waveform should alert the veterinarian
to a problem with the patient, the airway or the anesthetic circuit. Normal readings are in the range of 35 to 45 mm Hg (Figure
Figure 3b: Normal readings are in the range of 35 to 45 mm Hg.
Note that monitoring CO2 concentrations also allows the clinician to monitor respiratory rate. Thus, most multiparameter monitors will have a built-in
Figure 3c: Capnography results of hypoventilation in a patient.
Increased CO2 readings may be a sign of faulty check valves, an exhausted soda lime or mild to moderate patient airway obstruction, including
hypoventilation (Figure 3c). Decreased CO2 readings may be a sign of hyperventilation (Figure 3d), esophageal intubation, extubation, disconnection from the breathing
circuit or obstruction of the endotracheal tube.
Figure 3d: Capnography results of hyperentilation in a patient.
CO2 format is a key consideration in choosing equipment. If selecting a mainstream device, ensure that the probe has no moving
parts inside so it endures the rigorous environment of a busy veterinary practice. When using a sidestream device, pay close
attention to the sample rate; sample rates of 50mm/min or less are recommended by the Food and Drug Administration for monitoring
neonates, so this is a good goal if your practice treats small dogs and cats. If CO2 is not in your practice's budget, ensure that the monitor you purchase allows you to upgrade to CO2 later, because "plug-and-play" CO2 (Figure 4) currently is considered state of the art.
Figure 4: "Plug-and-play" CO2 currently is considered state of the art.