7.5 Basic Anesthesia Monitoring and Reflex Assessment

Basic Anesthesia Monitoring and Reflex Assessment

Anesthesia monitoring is a vital duty in veterinary medicine that ensures patient safety and stability during surgical procedures. While the veterinarian and credentialed veterinary technician direct the anesthetic plan, the veterinary assistant plays a key role in supporting the monitoring process, measuring parameters, and maintaining the anesthesia log. This section covers vital signs monitoring (temperature, heart rate, pulse, respiration, mucous membrane color, capillary refill time) and the assessment of anesthetic depth through reflexes.

Vital Signs Monitoring Under Anesthesia

General anesthesia depresses a patient's cardiovascular, respiratory, and thermoregulatory systems. Continuous, systemic monitoring is required to detect early signs of compromise.

1. Body Temperature (T)

Hypothermia is the most common complication during veterinary anesthesia. It is caused by:

• Depressed Thermoregulation: Anesthetic drugs depress the hypothalamus, preventing the body's normal response to cold.
• Inability to Shiver: Muscle relaxants and general anesthetics inhibit the shivering response.
• Environmental Heat Loss: Patients are placed on cold metal tables, have hair shaved, are scrubbed with alcohol/antiseptics (evaporative cooling), and breathe cold, dry anesthetic gases.

Consequences of Hypothermia: Hypothermia causes bradycardia, slows drug metabolism (prolonging recovery), reduces oxygen delivery to tissues, and impairs wound healing and immune function.

• Monitoring: Temperature is monitored using a rectal probe or esophageal stethoscope probe.
• Target Temp: Maintain between 100°F and 102.5°F. A temp below 99°F is hypothermic; below 97°F is critical and requires active intervention.
• Warming Methods: Active warming includes circulating warm water blankets, forced-air warming units (e.g., Bair Hugger), warm IV fluid lines, and wrapping extremities in bubble wrap or baby socks. Electric heating pads are strictly contraindicated because anesthetized patients cannot move and have poor perfusion, making them highly susceptible to severe, deep tissue thermal burns.

2. Heart Rate (HR) and Pulse (P)

Monitoring cardiac activity ensures that tissues receive adequate oxygenated blood.

• Normal Anesthetized HR Ranges:
  • Large Dogs: 50 - 100 beats per minute (bpm)
  • Small Dogs: 70 - 120 bpm
  • Cats: 110 - 180 bpm
• Bradycardia: A slow heart rate (dogs <50-60 bpm, cats <100 bpm) can be caused by excessive anesthetic depth, hypothermia, or drugs like alpha-2 agonists (e.g., dexmedetomidine) and opioids.
• Tachycardia: A rapid heart rate can indicate a light plane of anesthesia, surgical pain/stimulation, hypercapnia (high CO2), hypoxia, or hypotension (low blood pressure).
• Pulse Evaluation: Assess pulse rate, rhythm, and strength. The pulse must be synchronous with the heart rate. Common pulse points include the femoral artery (medial thigh), dorsal pedal artery (dorsal metatarsus), lingual artery (under the tongue), and labial artery (inside the lip). A weak, thready pulse suggests poor perfusion or hypotension.

3. Respiratory Rate (RR) and Character

Anesthetic agents depress the respiratory drive, which can lead to hypoventilation (shallow breathing) or apnea (temporary cessation of breathing).

• Normal Anesthetized RR: 8 - 20 breaths per minute.
• Bradypnea: An abnormally slow respiratory rate (<6-8 breaths per minute) is common under anesthesia, especially with deep planes or opioid use.
• Character: Observe the depth of chest excursions (tidal volume). Shallow breathing (hypoventilation) can cause carbon dioxide buildup (hypercapnia) and low blood oxygen (hypoxia).
• Danger Signs:
  • Apnea: If the patient stops breathing for more than 30 seconds, alert the technician or veterinarian immediately. Assisted ventilation ("bagging" the patient by squeezing the reservoir bag on the anesthetic machine) may be required.
  • Agonal Breaths: These are gasping, spasmodic breaths that occur when a patient is in cardiac arrest or near death. They do not represent functional ventilation and must be reported immediately.

4. Mucous Membrane (MM) Color and Capillary Refill Time (CRT)

These parameters provide a quick, non-invasive assessment of tissue perfusion and oxygenation.

• Capillary Refill Time (CRT): Press on the gums to blanch them, then release and count the seconds it takes for the pink color to return. A normal CRT is less than 2 seconds. A prolonged CRT (>2 seconds) indicates poor peripheral tissue perfusion, often due to low blood pressure (hypotension) or vasoconstriction.

Anesthetic Depth and Reflex Assessment

Anesthetic depth is divided into stages and planes. The veterinary assistant must evaluate several reflexes to determine if the patient is at a safe, surgical plane of anesthesia.

The Stages of Anesthesia

• Stage I (Voluntary Movement): The patient is conscious but becomes disoriented.
• Stage II (Involuntary Excitement/Delirium): The patient is unconscious but may vocalize, struggle, or show rapid breathing. This stage is bypassed quickly with rapid-acting induction agents.
• Stage III (Surgical Anesthesia): Divided into three planes:
  • Plane 1 (Light): Eyelid reflexes are active, muscle tone is strong. Too light for surgery.
  • Plane 2 (Medium/Surgical): Relaxed muscle tone, sluggish or absent reflexes, stable vitals. This is the target plane for surgical procedures.
  • Plane 3 (Deep): Flaccid muscles, severely depressed respiration, central dilated pupils. Dangerously deep.
• Stage IV (Anesthetic Overdose): Cardiovascular and respiratory collapse. Pupils are fully dilated and unresponsive. Immediate resuscitation is required.

Key Reflexes and Indicators

The assistant evaluates the following markers of depth:

1. Palpebral Reflex (Eyelid Blink): Gently tap the skin near the inner or outer canthus of the eye. In a light plane of anesthesia, the patient will blink. In the ideal surgical plane (Stage III, Plane 2), this reflex is sluggish or completely absent.
2. Pedal Reflex (Toe Pinch): Firmly pinch the webbing between the toes. If the patient withdraws the limb, the anesthetic plane is too light. This reflex must be absent before surgery begins.
3. Jaw Tone: Assess muscle relaxation by gently opening the patient's mouth. Marked resistance indicates a light plane of anesthesia. A relaxed but present resistance indicates the ideal surgical plane. A completely flaccid, loose jaw suggests the patient is too deep.
4. Eye Position and Pupil Size:

• Light Anesthesia: The eye is central (looking straight at you) with a constricted or normal-sized pupil, and an active palpebral reflex.
• Surgical Anesthesia (Ideal): The eye rotates in a ventromedial direction (pointing down and inward). The pupil is moderately relaxed.
• Dangerously Deep Anesthesia: The eye returns to a central position, but the pupil is widely dilated (mydriasis) and unresponsive to light. This is a critical warning sign of anesthetic overdose.

Reporting Vitals and Documentation

• Frequency: Vital signs and anesthetic parameters must be monitored and written on the anesthesia log every 5 minutes.
• Identifying Trends: Rather than looking at single values, the assistant must watch for trends (e.g., a heart rate that drops from 100 to 80 to 60 over 15 minutes, or a body temperature that steadily declines).
• Immediate Verbal Reporting: The assistant must immediately alert the veterinary technician or veterinarian if any of the following occur:
  • Bradycardia: Dog HR <60 bpm, Cat HR <100 bpm.
  • Bradypnea or Apnea: RR <6-8 breaths per minute or cessation of breathing for >30 seconds.
  • Abnormal MM/CRT: Pale/cyanotic gums or CRT >2 seconds.
  • Temperature Drop: Temp <98°F.
  • Change in Reflexes: Loss of all jaw tone with central, widely dilated pupils (indicating overdose) or sudden active blinking/movement (indicating patient is waking up).