Cardiac function relies on a specialized excitation and conduction system of cardiac muscle and a coordinated contractile process. Alterations of the stimulating nodes, conduction pathway, and myocardial fibers can affect rhythmicity and efficacy of the pump system. Conditions often encountered in the critically ill animal (hypoxia, ischemia, electrolyte imbalances, neuromuscular disease, inflammation, toxemia, medications, etc.) can affect the conduction system, resulting in incomplete or abnormal conduction pathways, chaotic cardiac muscle contraction, and reduced cardiac output and tissue perfusion.

Electrocardiogram (ECG) interpretation is required to characterize a cardiac dysrhythmia, as an adjunct to determining cardiac enlargement, and as an indicator for certain electrolyte, systemic, and metabolic disorders. Treatment of dysrhythmias will be based on the ECG diagnosis and cardiovascular status of the patient.

An aberrant conduction of an electrical impulse through the heart (dysrhythmia, arrhythmia) can occur independently and myocardial disease is not required. Virtually any systemic illness, trauma, and situation with inadequate oxygen delivery can result in tachyarrhythmias. With the severest dysrhythmias, cardiac arrest can occur. Arrhythmias must be distinguished from 60-cycling activity and artifact from improperly placed leads, or patient movement. Good contact must be made between the leads and the patient's skin. Thick-haired dogs may need to have their hair clipped, and clean clips should be used with secure contacts. Electrode pads can be placed for continuous monitoring, or on the bottom of the digital pads. Contact is enhanced with electrode cream or alcohol. Alcohol should never be used when electrocautery use or defibrillation is anticipated. Arrhythmias are identified by their rate followed by the types of aberrant impulses and by their anatomic origin. A tachyarrhythmia is a rapid rate originating from any region of the conduction system (sinus tachycardia, supraventricular tachycardia), or from the atrial (atrial fibrillation) or ventricular (premature ventricular contraction, ventricular tachycardia, ventricular fibrillation) muscle. Other common rhythms encountered in the ER include accelerated idioventricular rhythm and electrical alternans.

Supraventricular Tachyarrhythmias (SVT)

Sinus Tachycardia

The heart rate is increased (in our hospital we consider this >120 bpm in the dog and >200 bpm in the cat). The most common causes of sinus tachycardia in the ER include hypovolemia and pain.

Atrial Tachycardia

Paroxysms of 3 or more atrial premature contractions constitute an atrial tachycardia, where the P wave and P-R interval are different than the normal beat. The R-R intervals of the burst are consistent. Causes include atrial enlargement, myocardial disease, systemic disease, trauma, hyperthyroidism, and drug­ induced.

Atrial Fibrillation

P waves are not normal, "f" wave predominate (fibrillation of the atria), with normal QRS complex. The R-R interval is irregular and unpredictable. The QRS rate is generally fast; however, slow a-fib can occur. The most common causes of atrial fibrillation in the ER include any disease associated with left atrial enlargement (e.g., cardiomyopathy, severe chronic mitral regurgitation) and severe systemic disease.

Ventricular Tachyarrhythmias (VT)

Ventricular Premature Contraction (VPC)

The primary rhythm is a sinus rhythm and intermittent VPCs occur. The abnormal contraction is wide and bizarre, and not associated with any P wave. There may be a pulse deficit associated with the contraction. When multiform VPCs exist, there is a greater chance for ventricular tachycardia and fibrillation. Any disease process resulting in ventricular myocardial hypoxemia can cause VPCs. Systemic causes are more common than primary myocardial disease processes in emergent patients. Occasionally they occur in normal hearts without clinical significance.

Ventricular Tachycardia

The primary rhythm is ventricular with rates >100 bpm. There are no associated P waves with the QRS complexes, and the QRS complexes are wide and bizarre. Systemic causes are more common than primary myocardial disease processes. Any disease process resulting in ventricular myocardial hypoxemia can cause ventricular tachycardia. Systemic causes are more common than primary myocardial disease processes in emergent patients.

Accelerated Idioventricular Rhythm (AIR)

An idioventricular rhythm under autonomic influence that breaks through a sinus rhythm at a slightly faster rate than the sinus rate. Typically, it emerges when a respiratory sinus arrhythmia slows down during inhalation and disappears when the rate increases again during exhalation. Rates of up to 180 bpm may be seen. There are no perfusion abnormalities associated with the rhythm and it requires no treatment.

Electrical Alternans

Alternating R wave amplitudes. This suggests excessive movement of the heart, and is associated with pericardial effusion.

Antiarrhythmic Treatment

Antiarrhythmic medications have the potential for inducing arrhythmias, or worsening arrhythmias. Treatment of arrhythmias is indicated to alleviate cardiovascular compromise, reduce the risk for potentially life-threatening arrhythmias, and to reduce the risk of myocardial damage. Oxygen is administered. Electrolyte imbalances, severe acidemia, hypoxemia, hypoglycemia and exposure to drugs/toxins that affect myocardial function are immediately addressed. It is not unusual for ventricular arrhythmias in the dog to be associated with abdominal organ dysfunction such as hepatic or splenic masses. Hyperthyroidism should be investigated for when an older cat presents with a tachycardia.

Supraventricular tachyarrhythmias may be manipulated by inducing a vagal maneuver (gently pressing in on the eyes or carotid artery). For acute therapy, IV calcium channel blockers and beta blockers may be used. Life-threatening refractory SVT may be treated with adenosine, digoxin, phenylephrine or edrophonium. Chronic therapy may include oral digoxin +/- beta­blocker or calcium channel blocker.

Ventricular tachyarrhythmias are generally not treated unless they are electrically unstable or promote hemodynamic instability. Examples include multiform complexes, R-on-T phenomenon, rapid tachycardia (>180–200 bpm). In acute situations lidocaine is administered IV. Additional medication may include IV procainamide, amiodarone, esmolol or sotalol. Cardioversion in some instances is necessary. Chronic therapy may include treatment with oral sotalol, atenolol, mexiletine, or procainamide.