Capnography (ETCO2) in veterinary anesthesia: how to read the capnogram waveform

Capnography measures the amount of carbon dioxide (CO2) in the air exhaled by the patient and displays it both as a number, the end-tidal CO2 concentration (ETCO2), and as a waveform over time, the capnogram. In veterinary anesthesia it is regarded as a safety monitor because it confirms, in real time, that the endotracheal tube is in the trachea and that the patient is ventilating, and it detects events such as hypoventilation, disconnection, obstruction or cardiorespiratory arrest before pulse oximetry shows desaturation. Recent small-animal anesthetic monitoring guidelines, including the ACVAA 2025 guidelines (Bailey and colleagues), recommend waveform capnography as part of the assessment of ventilation. This article is educational and does not replace the judgment of the veterinarian responsible for the case.

What capnography actually measures

CO2 is produced by cellular metabolism, carried by the blood to the lungs and eliminated on exhalation. Capnography samples airway gas and quantifies the partial pressure of CO2 on every breath. The ETCO2 value is an indirect reflection of arterial blood CO2 and therefore of the patient's ventilatory status: when ventilation falls, CO2 accumulates and ETCO2 rises; when ventilation increases, ETCO2 falls.

There are two common configurations. In a mainstream system the sensor sits directly in the circuit, at the tube, with a fast response; in a sidestream system a small pump aspirates a gas sample through thin tubing to the analyzer. In very small patients, attention to dead space and aspirated volume matters, as they can alter the reading. In either case, what makes capnography powerful is not the number alone: it is the number together with a waveform that can be inspected.

Why it is considered a safety standard in anesthesia

The first safety function is to confirm intubation. Right after intubation, a consistent CO2 waveform, breath after breath, indicates that the exhaled air is coming from the lungs and not the esophagus. Unrecognized esophageal intubation is a preventable cause of serious anesthetic events, and capnography is the most direct way to detect it immediately. In the human literature, waveform capnography is described as an indispensable safety monitor and is mandated by airway management guidelines for exactly this reason.

The second function is early warning. ETCO2 changes the moment ventilation or perfusion is disturbed, before stored oxygen is depleted and SpO2 falls. A circuit disconnection, an accidental extubation or an arrest produce immediate changes in the waveform, whereas desaturation appears only later, once the oxygen reserve is exhausted. This is why capnography acts as a sentinel: it buys the veterinarian time to act before hypoxemia. Reviews and studies in sedation show an association between capnography use and fewer hypoxemic episodes, although the strength of the evidence varies with the setting.

For these reasons, monitoring guidelines such as the ACVAA 2025 guidelines and the AAHA anesthesia and monitoring guidelines place capnography among the monitors recommended to assess ventilation during anesthesia, alongside pulse oximetry, ECG and blood pressure. No single monitor replaces continuous clinical observation and a person dedicated to watching the patient.

How to read the waveform: the four phases of a normal capnogram

A normal time-based capnogram has a recognizable rectangular shape, classically described in four phases. Phase I is the inspiratory baseline: it corresponds to the start of exhalation, when CO2-free gas from the airway dead space leaves and the trace stays near zero. Phase II is the rapid, near-vertical upstroke, as CO2-rich alveolar gas begins to mix and reaches the sensor.

Phase III is the expiratory plateau, slightly upward sloping, representing alveolar emptying; the highest point of that plateau, at the end of exhalation, is the ETCO2, the value the monitor reports. Phase IV is the rapid downstroke at the start of inspiration, when fresh CO2-free gas washes through the airway and the trace returns to baseline. In a stable, well-ventilated patient this pattern repeats regularly, with a well-defined plateau and complete return to zero between breaths.

Reading the waveform is, above all, recognizing this shape and asking: does the baseline return to zero? Is the plateau sharp and horizontal? Is the upstroke fast? Is the rate regular? Deviations in each of these questions point to different problems, as we will see next.

Abnormal shapes and what they suggest

An abrupt fall of ETCO2 to nearly zero, with complete loss of the waveform, is an alarm sign: it may mean extubation, circuit disconnection, complete tube obstruction or absence of effective circulation, as in a cardiorespiratory arrest. Interpretation depends on the immediate clinical context, but in every case it demands rapid verification and action. In cardiopulmonary resuscitation, ETCO2 is also used to verify intubation, judge the quality of chest compressions and recognize return of spontaneous circulation, which often announces itself with a sudden rise in CO2.

A sloping phase III plateau suggests obstruction to expiratory flow, such as bronchospasm, secretions or a partially obstructed or kinked tube: alveolar emptying becomes uneven and the plateau loses its horizontal shape. An elevated baseline, meaning CO2 that does not return to zero during inspiration, suggests CO2 rebreathing, from an exhausted soda-lime absorbent, an incompetent one-way valve or insufficient fresh gas flow in non-rebreathing systems.

The ETCO2 level is informative too. A high ETCO2 with a normally shaped waveform usually indicates hypoventilation, with CO2 accumulation, common under deep anesthesia; a low ETCO2 may reflect hyperventilation, but also a fall in perfusion, hypotension, hypothermia or embolism, situations in which less blood reaches the lungs to deliver CO2. A shark-fin shaped trace, with a slow, rounded upstroke, points to airway obstruction. And small clefts in the plateau, in a patient under controlled ventilation, can indicate returning spontaneous respiratory effort, a useful sign about anesthetic depth and neuromuscular blockade. These patterns guide reasoning, but the decision always rests with the veterinarian at the patient's side.

Why the waveform says more than the number alone

The ETCO2 number is a summary; the waveform is the story of how that number was generated. Two patients can display the same ETCO2 with very different waveforms: one with a clean plateau and a baseline at zero, ventilating well, and another with a sloping plateau and an elevated baseline, rebreathing and with obstructed flow. The number alone does not distinguish these situations; the shape of the wave does. It is the same logic as in our article on the oscillometry waveform in blood pressure: the number is only trustworthy when the wave that generated it can be inspected.

This combined reading of shape and number is what turns capnography from a simple readout into a safety tool. So it is worth treating the capnogram the way the ECG trace or the plethysmographic oximetry curve is treated: look at the wave before trusting the value. Integrating these signals is also what a multiparameter monitor such as INmonitor is designed to offer, bringing together ECG, oximetry, blood pressure and capnography in a continuous, connected reading available to the veterinarian during the procedure.

Frequently asked questions

Does capnography replace pulse oximetry? No. They measure different, complementary things: capnography assesses ventilation, that is, CO2 elimination, while oximetry assesses blood oxygenation. Monitoring guidelines recommend using both, together with ECG and blood pressure, because each warns of a different type of problem and at different moments.

Why is ETCO2 sometimes a little lower than arterial blood CO2? Because part of the lung is ventilated but poorly perfused, the so-called dead space, and that difference, the gradient, tends to widen when pulmonary perfusion falls. Following the ETCO2 trend over time is usually more informative than a single isolated value.

Does capnography work in a non-intubated patient? There are cannulas and adapters for sampling in non-intubated patients, especially during sedation, but the reading is less reliable because some exhaled gas is diluted in room air. The most robust waveform confirmation occurs with an instrumented airway. The choice of method and its interpretation always rest with the responsible veterinarian.