Abnormal heart rate (HR) and arrhythmias are commonly observed during general anesthesia. Monitoring for arrhythmias via electrocardiography is crucial during the perianesthetic period, and in combination with direct auscultation, SpO2, and audible Doppler, comprehensive cardiovascular monitoring is provided.3
Bradycardia generally refers to heart rates <50 bpm in large dogs, <70 bpm in small dogs, and <100 bpm in cats.2,4,6-8 Bradycardias are generally a result of administration of vagotonic drugs (eg, α-2 adrenergic agonists or opioids), stimulation of vagally mediated reflexes (eg, oculocardiac reflex, intubation), hyperkalemia, or consequences of anesthesia, including excessive depth, hypothermia, and hypoxia.4,6-8 When the cause of bradycardia is increased vagal tone, an anticholinergic should be administered (eg, atropine, glycopyrrolate). Bradycardia is an expected reflex following α-2 administration and need not be treated unless hypotension and/or reduced peripheral perfusion become present. In that case, an α-2 antagonist should be administered first and given time to take effect before administering an anticholinergic. If the bradycardia is caused by excessive anesthetic depth, hypothermia, or hyperkalemia, the underlying cause should be corrected.
Tachycardia and tachyarrhythmias generally refer to HRs >200 bpm in cats and >160 bpm in dogs.2,4,6-8 The concern with significant increases in HR during general anesthesia is a reduction in filling time and stroke volume, impairing cardiovascular function.2,7
Related Article: Bradyarrhythmia & Tachyarrhythmia
Anesthetic-related causes of sinus tachycardia are drugs (eg, ketamine, anticholinergics), inadequate depth of anesthesia, pain, or response to surgery, hypoxia, hypercapnia, or hypotension. Additional patient factors to consider include hyperthyroidism, heart disease, anemia, shock, pheochromocytoma, and anaphylaxis. Treatment of sinus tachycardia should focus on the cause. For example, in an animal that is painful, additional analgesia should be provided.
Intermittent and isolated ventricular premature complexes can be normal and harmless in healthy dogs and cats undergoing anesthesia. Intervention is generally not required unless irregular heart beats become more frequent (>20 irregular bpm), cause a reduction in BP, or transition to ventricular tachycardia. Again, treatment should focus on the source, but in the face of ventricular tachycardia, lidocaine is the primary therapy, followed by procainamide for refractory cases.2,7 Care should be taken to always dose reduce lidocaine in cats (see Table) given higher susceptibility to toxicity.