Cardiac dysrhythmias

12 Cardiac dysrhythmias


Cardiac dysrhythmias are a relatively common finding in emergency patients and are probably also underdiagnosed in general. Physical examination may generate the suspicion of a dysrhythmia (e.g. heart rate that is pathologically fast or slow, pulse deficits), but this can only be confirmed using electrocardiography. A wide variety of dysrhythmias exist and a full discussion is beyond the scope of this book. This chapter presents some of the most common dysrhythmias that the author sees, focusing on dogs, and summarizes the recommendations for their management.



Normal Cardiac Conduction


An impulse is normally generated at the sinoatrial (SA) node which is the dominant pacemaker. The impulse is conducted from there across the atria to the atrioventricular (AV) junction and into the AV node; it then passes down the bundle of His and bundle branches, on to the Purkinje fibres. The overall result is cardiac contraction. Impulse conduction is reflected as the normal sinus beat (P-QRS-T) and normal sinus rhythm results from impulses being generated at the SA node at a normal rate (Figure 12.1).


image

Figure 12.1 Normal sinus rhythm in a dog showing P-QRS-T complexes (lead II, 25 mm/s).


(Courtesy of Simon Dennis)



Clinical Tip



• Dysrhythmias can generally be classified into those that are due to primary cardiac disease, and those that are due to noncardiac systemic disease.

• As a general rule, dysrhythmias are not treated specifically with anti-dysrhythmic agents unless the rhythm disturbance itself is deemed to be clinically significant, i.e. associated with haemodynamic compromise. The emphasis is on treatment of the underlying disorder, and it must be remembered that anti-dysrhythmic agents are not benign and may have the potential to be pro-dysrhythmic.

• Haemodynamically significant dysrhythmias may need to be treated specifically prior to diagnosis of the underlying cause.


Tachydysrhythmias



Supraventricular tachycardia


Supraventricular tachycardia (SVT) may be atrial or junctional in origin and can be difficult to distinguish from ventricular tachycardia in some cases. On physical examination both may have rapid weak pulses with pulse deficits. Potential distinguishing features for SVT (Figure 12.2) on electrocardiography include:


• Tall and narrow QRS complexes in lead II

• Complexes typically uniform.

image

Figure 12.2 Supraventricular tachycardia in a dog at a rate of approximately 320 beats per minute (lead II, 25 mm/s). QRS complexes look relatively normal (tall, narrow, uniform) with a regular R-R interval.


(Courtesy of Simon Dennis)


It is noteworthy that physiological or appropriate sinus tachycardia, including in response to severe hypovolaemia, rarely exceeds a rate of 220–240 beats per minute in dogs at rest (300 beats per minute in cats); heart rate may exceed this in dogs such as greyhounds during intense exercise. The finding of a heart rate above this level should prompt electrocardiography.



Causes


Supraventricular tachycardias most commonly occur as a result of primary cardiac disorders that result in atrial dilation (e.g. mitral or tricuspid regurgitation). Some animals present in congestive heart failure.



Indications for treatment


Supraventricular tachycardia should be treated with specific anti-dysrhythmic therapy if treatment of congestive heart failure does not improve the dysrhythmia and it is deemed to be clinically significant.



Treatment


Although conversion to sinus rhythm is preferred, this is not always possible and the main aim of treatment is to slow the ventricular response rate such that the dysrhythmia no longer causes haemodynamic compromise. Oral digoxin and treatment for congestion are implemented in animals with congestive heart failure. The reader is referred to other sources for more information on the use of digoxin.


Vagal manoeuvres can be attempted and may be effective in some animals with SVT. This involves carotid sinus massage or applying pressure to the closed eyes. If vagal manoeuvres fail and treatment of congestion alone is ineffective, then additional medical therapy may be indicated. Diltiazem (a calcium channel blocker) is the most commonly used agent and may be given intravenously if available or orally. This agent must be used cautiously in animals with congestive heart failure if at all as it has negative inotropic effects that may cause clinical deterioration.



Atrial fibrillation


Atrial fibrillation (AF) (Figure 12.3) is characterized by a chaotic irregularly irregular heart rhythm on auscultation with marked pulse deficits. Electrocardiography shows:


• Absent P waves and irregular undulations of the baseline

• Normal-looking (tall and narrow) QRS complexes but an irregular R-R interval (this is an important diagnostic feature).

image

Figure 12.3 Atrial fibrillation in a dog at a rate of approximately 240 beats per min (lead II, 25 mm/s). QRS complexes look relatively normal (tall, narrow, uniform) but with an irregularly irregular R-R interval.


(Courtesy of Simon Dennis)



Causes


In emergency practice, AF is typically (although not always) seen in animals with severe primary structural heart disease, usually involving atrial dilation. The ventricular rate in such cases is typically fast (potentially over 200 beats per minute), and there is usually evidence of congestive heart failure (see Ch. 31). Lone AF is used to describe AF in dogs without evidence of structural heart disease (e.g. some Irish Wolfhounds). AF does occur in cats but less frequently than in dogs.



Indications for treatment


The approach to AF will depend on the rate, the underlying cause, and whether congestive heart failure is present. Stabilization of congestive heart failure is the priority, and treatment to control the ventricular response rate may then be included in subsequent therapy if still required.



Treatment


Treatment of AF is designed to slow the heart rate (typically to less than 160 beats per min in dogs) in order to improve cardiac output. However, as animals with severe heart failure may require some degree of heart rate elevation to maintain cardiac output, excessive reduction of ventricular response rate may not be desirable. Conversion to sinus rhythm is difficult and in fact not shown to be associated with any additional benefit.


Oral digoxin is the usual agent of choice for dogs with heart disease and the reader is referred to other sources for more information on the use of digoxin. Congestion should also be treated as required (see Ch. 31). Occasionally additional antidysrhythmic therapy (e.g. oral diltiazem) is indicated if treatment of congestion does not provide adequate ventricular rate control but care with dosing must be taken in animals with congestive heart failure.

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Cardiac dysrhythmias

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