Heart Rate and Autonomic Tone in Anesthetized Patients

Vocabulary terms covering the physiological mechanisms, reflexes, and pharmacological influences on heart rate in veterinary patients, specifically under anesthesia.

Pacemaker Cells

Cells capable of spontaneous automaticity and pacing the heart, found in the Sinoatrial (SA) Node, Atrioventricular (AV) Node, and His-Purkinje system.

Healthy Adult Canine Heart Rate Range

$40$ to $260\,bpm$, with a rate between $50$ and $120\,bpm$ occurring $85\%$ of the time.

Healthy Adult Feline Heart Rate Range

$120$ to $220\,bpm$ with an average rate of $160\,bpm$.

Sympathetic Tone

A component of the autonomic nervous system that increases heart rate through adrenergic receptors ($\text{Alpha 1 \& 2, Beta 1 \& 2}$) and is associated with 'Fight or Flight' responses.

Parasympathetic Tone

A component of the autonomic nervous system that decreases heart rate, often associated with 'Rest \& Digest' functions.

Vagal Tone

A term used interchangeably with parasympathetic tone because $75\%-80\%$ of all parasympathetic stimulation comes from branches of the vagus nerve.

Hypervagotonia

Chronically elevated parasympathetic tone commonly caused by breed characteristics (Brachycephalic Airway Syndrome), gastrointestinal diseases, or chronic respiratory diseases.

Vasovagal Response

Acute parasympathetic stimulation that may result in bradycardia, hypotension, temporary sinus arrest, and syncope.

Oculocardiac Reflex (OCR)

A reflex triggered by digital compression of the eyeballs or traction on extraocular muscles that can cause bradycardia.

Bezold-Jarisch Reflex

A reflex where intracardiac C fibers sharply increase parasympathetic tone in response to vigorous contraction of under-filled ventricles, resulting in reflex bradycardia.

Arterial Baroreceptors

Stretch receptors located in the aortic arch and carotid sinuses that adjust heart rate and vascular tone in response to changes in blood pressure.

Bainbridge Reflex

Sympathetic stimulation sent to the SA node to increase heart rate in response to the distention of the vena cava, pulmonary veins, or atrial chambers.

Cushing's Reflex

A sequence where increased intracranial pressure exceeding mean arterial pressure triggers a sudden rise in blood pressure and subsequent reflex bradycardia.

Positive Chronotropic Effect

An influence that increases the heart rate.

Negative Chronotropic Effect

An influence that decreases the heart rate.

Hyperkalemia Metabolic Effect

A metabolic state defined by potassium levels greater than $6.5\,mmol/L$ that leads to a decreased heart rate.

Opioids Chronotropic Effect

Anesthetic drugs that exert a negative chronotropic effect (decrease heart rate).

Ketamine Chronotropic Effect

An anesthetic drug that exerts a positive chronotropic effect (increases heart rate), except in painful patients.

Alpha2 Agonists Chronotropic Effect

Drugs like xylazine and dexmedetomidine that cause vasoconstriction and hypertension, leading to a negative chronotropic effect (reflex bradycardia).

Zenalpha®

A combination of medetomidine (alpha2 agonist) and vatinoxan (alpha2 antagonist) that results in a higher average lowest heart rate ($66\,bpm$) compared to medetomidine alone ($43\,bpm$).

Induction Agents (Propofol \& Alfaxalone)

Drugs that indirectly increase heart rate as a response to hypotension caused by vasodilation and decreased cardiac contractility.

Anticholinergics

Drugs such as atropine and glycopyrrolate that increase heart rate by decreasing parasympathetic tone.

Beta-Blockers

Drugs including atenolol and propranolol that exert a negative chronotropic effect.

Bronchodilators (Terbutaline \& Albuterol)

Medications that exert a positive chronotropic effect, increasing heart rate.