Comprehensive Veterinary Guide to Heart Rate Management Under Anesthesia

Normal Heart Rate Thresholds Under Anesthesia

• Dog Tachycardia Threshold: Generally characterized by a heart rate greater than $140\,\text{beats per minute (bpm)}$.
• Cat Tachycardia Threshold: Generally characterized by a heart rate greater than $200\,\text{bpm}$.
• Patient Status Context: These thresholds apply specifically to patients under anesthesia. A patient's heart rate while anesthetized should ideally run lower than when they are awake and active in a kennel environment. While specific hospital standards or references may vary slightly, any dog exceeding $140\,\text{bpm}$ indicates an opportunity for intervention to lower the rate.

Causes of Tachycardia Under Anesthesia

• Light Anesthesia: This is considered the most common cause. It frequently occurs during the transition from the preparation area to the surgery suite. Maintaining a sufficiently deep plane of anesthesia is necessary to prevent this.
• Pain or Surgical Manipulation: The heart rate may increase in response to painful stimuli or the physical manipulation of internal organs. Specific examples include:
  • The surgeon picking up the intestines.
  • The removal of ovaries and the uterus during a spay procedure.
• Drug Side Effects: Certain medications used in the anesthetic protocol can drive the heart rate up:
  • Atropine: Often administered as a pre-medication; it is designed to keep the heart rate elevated.
  • Ketamine: Known to result in heart rates higher than those seen with other anesthetic agents.
• Hypovolemia: A very common cause often related to the patient's pre-surgical condition. For example, if a patient is undergoing surgery for a foreign body and has been vomiting for several days, they may be volume-depleted (hypovolemic) before the procedure starts.
• Anemia: A decrease in circulating red blood cells (RBCs) requires the body to increase the heart rate to ensure all cells are properly perfused. This is common in:
  • Splenectomy procedures.
  • C-sections.
  • Cases with high intra-operative blood loss.
• Hypoxia: Inadequate oxygenation will cause a compensatory increase in heart rate. Potential scenarios include:
  • Thoracic trauma in trauma patients.
  • Pneumonia.
  • Physical restriction of ventilation, such as a surgeon leaning on the patient's chest.
  • Obesity, where the weight of the animal interferes with breathing while laying on their back.
  • Equipment failures, including issues with the endotracheal tube or the anesthesia machine itself.

Assessing and Treating Tachycardia

• The "Big Picture" Approach: Anesthetists should never treat a high heart rate in isolation. The specific cause must be identified before a treatment plan is implemented.
• Blood Pressure Correlation: Blood pressure and heart rate must be interpreted together:
  • High Heart Rate + Low Blood Pressure: This likely indicates hypovolemia rather than pain. Treatments may include a fluid bolus.
  • High Heart Rate + High Blood Pressure: This indicates the patient may be too light or experiencing pain. Treatments may include increasing the anesthetic gas concentration.
• Physical Assessment:
  • Mucous Membranes (MM): Check the color of the gums to ensure they are pink.
  • Capillary Refill Time (CRT): Evaluate perfusion by pressing on the gums; if the patient is under a surgical drape, the anesthetist must physically crawl under it to assess this.
• Treatment Options: Depending on the diagnosis, interventions may include:
  • Increasing anesthetic gas concentration.
  • Administering a fluid bolus.
  • Providing additional or different pain medications.
  • Starting a Constant Rate Infusion (CRI) for pain management.

Bradycardia Thresholds and Risks

• Dog Bradycardia Threshold: Generally considered less than $60\,\text{bpm}$. An anesthetist might allow a dog to remain at $56\,\text{bpm}$ if all other vital signs are stable, but closer monitoring is required.
• Cat Bradycardia Threshold: Cats are never allowed to drop below $100\,\text{bpm}$. Cats have very limited physiological reserves to manage slow heart rates, and a rate below this threshold often leads to a continuous, dangerous decline—referred to as "the bad place."

Causes of Bradycardia

• Anesthetic Depth: The patient may simply be too deep under anesthesia.
• Hypothermia: A very common cause, particularly in cats and small dogs. Heat escapes rapidly during long procedures, especially those where the abdomen is open.
• Drug Side Effects: Multiple pre-medications and intra-operative drugs cause heart rate depression, including:
  • Opioids.
  • Dexmedetomidine.
  • Acepromazine.
• Severe Hypotension or Hypoxia: While these initially cause tachycardia, if the condition is not addressed and the patient slips deeper into a critical state, the heart rate will eventually crash.

Assessing and Treating Bradycardia

• Comprehensive Vital Sign Check: Evaluate blood pressure, gum color, and body temperature before choosing a treatment.
• Thermal Management: To combat bradycardia caused by hypothermia:
  • Cover the patient's head, feet, and chest (areas not involved in the surgery).
  • Warm the inhaled oxygen. Note that the oxygen provided by the machine is naturally "ice cold."
• Adjusting Anesthetic Depth: If the patient is too deep, lowering the gas concentration is the ideal first step before reaching for drugs.
• Pharmacological Interventions (Anticholinergics):
  • Atropine: Increases heart rate very quickly but has a short duration of action.
  • Glycopyrrolate: Takes longer to achieve its effect but lasts longer in the system.
• Reversal Agents: If the bradycardia is a reaction to an overdose or sensitivity to opioids, Naloxone can be used as a reversal agent.

Final Clinical Advice

• Rule of Thumb: "Treat the patient, not the machine."
• The monitor provides the data (e.g., that the patient is tachycardic or bradycardic), but the anesthetist must look at the big picture—blood pressure, mucous membranes, gum color, and body temperature—to determine the underlying cause and the correct treatment.