Each month our Head of Cardiology, Lara Barron, reviews an ECG and discusses the interpretation and recommended treatment.
An eight-year-old female spayed Labrador Retriever presented for mild lethargy and diarrhoea. A possible arrhythmia was heard, and an ECG was performed. What is your rhythm diagnosis? What are some underlying causes for this rhythm and what diagnostics would you choose next? How is this rhythm treated?
Figure shows Six lead ECG. The top seven leads (I, II, III, aVR, aVL, aVF, V3) are all simultaneous recordings. The bottom, longer strip is a continuous recording of lead II. Paper speed: 25mm/sec, 10mm = 1mV.
This ECG is consistent with atrial fibrillation (AF), with an average heart rate of 187bpm. AF is a rapid, irregularly irregular rhythm (variable R-R interval), with no discernable P waves and a fibrillating baseline (f waves). It is typically rapid, with asynchronous pulses. It can often be identified on auscultation before it is confirmed on ECG, and is described as “shoes in a dryer”.
AF is one of the most common arrhythmias seen in dogs and often occurs secondary to atrial enlargement due to dilated cardiomyopathy or myxomatous mitral valve disease. Because AF requires enough atrial tissue to sustain this chaotic rhythm, it is seen more commonly in larger dogs than in smaller breeds or cats. Rarely, “lone AF” is diagnosed in giant breed dogs in the absence of underlying structural heart disease. AF can also occur due to increased vagal tone and is, therefore, less commonly seen due to systemic disease. Therefore, it is reasonable to start with a cardiac workup (thoracic radiographs, AFAST/TFAST, echocardiography, and blood pressure). Based on these results, tests for systemic disease (bloodwork, abdominal ultrasound) may also be indicated.
Two main methods for treating AF are available: rate control vs rhythm control.
Rate control consists of using medications to slow the ventricular response rate while the patient remains in atrial fibrillation.
Rhythm control consists of direct current electrical cardioversion (under general anaesthesia) with the goal of returning to a sinus rhythm.
When underlying heart disease is present, rate control is favoured due to the high likelihood of these dogs returning to AF following cardioversion.
Because AF rarely progresses to a fatal arrhythmia, emergency treatment with intravenous anti-arrhythmics is usually not necessary. However, the development of AF can drop cardiac output and is often the “straw that breaks the camel’s back”, pushing the patient into congestive heart failure (left, right or biventricular). If heart failure is present, treatment of failure is indicated first, as this will slow the HR and may affect the dose of anti-arrhythmics needed. Medication choice for rate control is dictated by underlying cardiac function and concurrent conditions (renal function, electrolyte levels, presence of ventricular ectopy, etc).