Anesthetic mortality in veterinary patients has decreased from approximately 1 in 100 to roughly 1 in 1,000-2,000 over the past three decades, largely due to improved monitoring practices by veterinary technicians. The anesthetist—typically the vet tech—is the single most important safety factor during any anesthetic event. The Anesthesia & Analgesia Reference provides quick access to drug protocols and reversal agents.
Effective anesthesia monitoring integrates multiple parameters simultaneously. No single value tells the complete story; it is the trend across parameters that guides clinical decisions.
Heart rate (HR): Monitored via ECG, pulse oximeter, or esophageal stethoscope. Tachycardia may indicate inadequate anesthetic depth, pain, or hypotension. Bradycardia can signal excessive vagal tone, hypothermia, or opioid effects. Canine normal under anesthesia is typically 60-120 BPM; feline 100-180 BPM.
Respiratory rate (RR): Assessed visually, via capnograph, or by reservoir bag movement. Normal anesthetized respiratory rate ranges from 8-20 breaths/min. Rates below 8 suggest excessive depth or respiratory depression; rates above 20 may indicate light planes or pain.
Pulse oximetry (SpO2): Measures functional hemoglobin saturation. The probe is placed on the tongue, ear pinna, toe web, prepuce, or vulva. Target is >95%. Values below 90% constitute hypoxemia and require immediate intervention (increase FiO2, check airway, manual ventilation).
Capnography (ETCO2): Measures end-tidal carbon dioxide, the gold standard for confirming endotracheal tube placement and assessing ventilation. Normal ETCO2 is 35-45 mmHg. Values above 60 indicate hypoventilation requiring assisted ventilation; values below 20 suggest hyperventilation, circuit disconnect, or cardiac arrest.
Blood pressure (BP): Measured by Doppler or oscillometric methods. Mean arterial pressure (MAP) should be maintained above 60 mmHg to ensure adequate organ perfusion. Systolic pressure below 80 mmHg or MAP below 60 mmHg requires treatment—reduce inhalant concentration, increase IV fluid rate, or administer vasopressors.
Temperature: Hypothermia is the most common anesthetic complication. Core temperature is monitored via esophageal or rectal probe. Target: 37-39°C (98.6-102.2°F) for dogs and cats. Active warming (forced-air warming blankets, circulating warm water pads) should be initiated proactively, not reactively.
Physical reflexes provide critical information about anesthetic depth that electronic monitors cannot replace.
Jaw tone: Progressive loss of jaw tone indicates deepening anesthesia. At a surgical plane, the jaw should be relaxed but not completely flaccid.
Palpebral reflex: A light touch to the medial canthus of the eyelid. Present (blink) suggests a light plane; absent suggests adequate surgical depth. Complete absence with all other reflexes gone suggests the patient may be too deep.
Eye position: In dogs, a ventromedial rotation of the globe indicates a moderate surgical plane. Central and fixed pupils suggest either a very light or very deep plane—correlation with other parameters is essential. In cats, eye position is less reliable; pupil size is more useful (miotic = deep, mydriatic = light or very deep).
| Parameter | Light Plane | Surgical Plane | Too Deep |
|---|---|---|---|
| Jaw Tone | Moderate to strong | Relaxed | Flaccid |
| Palpebral Reflex | Brisk | Sluggish to absent | Absent |
| Eye Position (Dog) | Central, active | Ventromedial rotation | Central, fixed, dilated |
| Heart Rate | Elevated or normal | Stable, normal | Bradycardia |
| Respiratory Rate | Elevated | Regular, 8-15/min | Slow, shallow, or apneic |
| Blood Pressure | Normal to elevated | Normal | Hypotension |
All parameters should be recorded at a minimum of every 5 minutes on the anesthesia record. During critical periods (induction, position changes, complication management, recovery), increase documentation frequency to every 2-3 minutes. The anesthesia record is a legal medical document that must include: patient identification, anesthetic protocol (premedication, induction, maintenance agents and concentrations), all drugs administered with time and dose, IV fluid type and rate, all monitored parameters at regular intervals, complications and interventions, and recovery notes.
Hypotension (MAP < 60 mmHg): First, verify the reading (reposition cuff or Doppler crystal). If confirmed: reduce inhalant by 0.5% increments, increase crystalloid fluid rate (10-20 mL/kg bolus in dogs, 5-10 mL/kg in cats), and consider vasopressor support (dopamine 2-10 mcg/kg/min CRI or ephedrine 0.05-0.1 mg/kg IV bolus).
Hypothermia (< 36.5°C): Prevention is far easier than treatment. Use forced-air warming blankets (Bair Hugger), warm IV fluids, insulate the patient from the surgical table with towels, minimize surgical prep time, and keep ambient room temperature above 21°C.
Apnea/Hypoventilation (RR < 6 or ETCO2 > 60): Provide manual (hand) ventilation at 10-15 cmH2O peak inspiratory pressure, 10-15 breaths/min. Check inhalant concentration and reduce if excessive. Ensure the endotracheal tube is patent and properly positioned.
Bradycardia: If HR drops below 60 BPM in dogs or 100 BPM in cats, assess depth (may be too deep). Administer atropine 0.02-0.04 mg/kg IV or glycopyrrolate 0.005-0.01 mg/kg IV for vagally mediated bradycardia. If alpha-2 agonist related, consider atipamezole reversal.
Warning: A sudden loss of ETCO2 waveform with no change in SpO2 is highly suspicious for circuit disconnect or esophageal intubation. Check tube placement immediately with direct visualization and auscultation. Loss of both ETCO2 and SpO2 simultaneously suggests cardiac arrest—begin CPR immediately.
The Vital Signs Reference provides species-specific normal ranges to help calibrate your monitoring thresholds. Always know your patient's baseline values before induction.
- Monitor continuously, record every 5 minutes — HR, RR, SpO2, ETCO2, BP, temperature, reflexes, and eye position.
- SpO2 > 95%, ETCO2 35-45, MAP > 60 — these are your critical thresholds for intervention.
- Hypothermia is the most common complication — use active warming proactively from induction onward.
- Physical assessment of depth complements monitors — jaw tone, palpebral reflex, and eye position remain essential.
- The anesthesia record is a legal document — complete and accurate documentation protects the patient and the practice.