Normal ETCO2 and SpO2 values in dogs and cats under anesthesia: what each parameter detects

Under anesthesia, end-tidal carbon dioxide (ETCO2) is typically 35 to 45 mmHg in dogs and 28 to 32 mmHg in cats, while oxygen saturation by pulse oximetry (SpO2) should stay above 95% in patients breathing supplemental oxygen. Hypoxemia is generally defined as an SpO2 at or below 90%, corresponding to a PaO2 of about 60 mmHg. Capnography watches ventilation and perfusion, and oximetry watches oxygenation; together they anticipate problems before they become clinical.

Why monitor ventilation and oxygenation during anesthesia

Anesthetics depress the respiratory center and relax the musculature, so anesthetized dogs and cats tend to hypoventilate and to depend on supplemental oxygen. The anesthesia monitoring guidelines of the American College of Veterinary Anesthesia and Analgesia (ACVAA) recommend continuous tracking of circulation, oxygenation and ventilation throughout the procedure and recovery.

Capnography and oximetry are complementary and not interchangeable. Oximetry answers how much oxygen the arterial blood is carrying; capnography answers how well the patient is eliminating CO2, which reflects ventilation and, indirectly, cardiac output. Monitoring only one of the two leaves a blind spot.

ETCO2: normal ranges and alert thresholds

Capnography measures CO2 at the end of expiration, which under stable anesthesia approximates PaCO2. Normal range by species: dog 35 to 45 mmHg · cat 28 to 32 mmHg. In many protocols a broad 35 to 45 mmHg range is used as a practical reference during anesthesia.

Common alert thresholds: hypocapnia when ETCO2 is below 35 mmHg (hyperventilation, hypothermia, falling cardiac output or pulmonary perfusion) · hypercapnia when above 45 mmHg (hypoventilation). Mild-to-moderate hypercapnia, in the 50 to 55 mmHg range, is often tolerated in stable patients; values above 60 mmHg generally indicate considering assisted ventilation. These numbers are monitoring references, not directives: the decision always rests with the attending veterinarian, according to patient and context.

What capnography detects early: apnea, circuit disconnection and accidental extubation (abrupt loss of the waveform), esophageal intubation (CO2 absent or near zero), airway obstruction (shark-fin waveform), CO2 rebreathing (baseline that does not return to zero) and a sudden drop in cardiac output or cardiopulmonary arrest (abrupt fall in ETCO2). This is why the waveform often alerts within a few respiratory cycles, before desaturation.

SpO2: normal ranges and the hypoxemia threshold

Pulse oximetry estimates hemoglobin oxygen saturation (SpO2). In anesthetized patients breathing 100% oxygen, the expected reading is 95 to 100% (often 98 to 100%). Hypoxemia is considered an SpO2 at or below 90%, a point that corresponds to a PaO2 of approximately 60 mmHg on the oxyhemoglobin dissociation curve.

Note a critical point: in patients breathing a high oxygen fraction, SpO2 may stay near 100% even when lung function is already impaired, because the dissociation curve is flat in that region. In other words, oximetry detects hypoxemia with some delay in these cases; it is capnography that tends to alert first. In recovery, with the patient already on room air, the threshold is more sensitive: a veterinary study proposed SpO2 below 95% as the cutoff for hypoxemia (PaO2 below 80 mmHg).

What oximetry detects: hypoxemia from severe hypoventilation, airway or gas-exchange problems, failure of oxygen delivery and low peripheral perfusion (which also degrades the signal). Unstable readings may reflect hypothermia, vasoconstriction, movement or pigmentation, and require clinical correlation.

Reference summary: dog and cat

The ranges below are general monitoring references anchored in the ACVAA guidelines and veterinary literature; interpretation and management rest with the attending veterinarian, considering species, FiO2, patient status and the procedure.

Connected multiparameter monitoring

Because ETCO2 and SpO2 tell different stories at different times, safe anesthesia monitoring combines capnography, oximetry, electrocardiogram, blood pressure and temperature on a single panel. A multiparameter monitor such as the <b>IN</b>monitor centralizes these signals and integrates them into the <b>IN</b>pulse platform, with history on <b>IN</b>cloud for procedure recording and audit, in line with the ACVAA recommendation for detailed documentation.