There are a wide range of potential arrhythmias, or heart rhythm disturbances, that are seen in small animal practice. These arrhythmias can occur for many different reasons, and while some have little clinical significance, some can be life threatening. Arrhythmias may be suspected from a patient's history or physical examination, or may occur as a result of anaesthesia, surgery, trauma, or as a result of systemic disease or metabolic problem, such as an electrolyte imbalance. Among the numerous arrhythmias, there are a small number of immediately life threatening ones, which are typically either very fast or very slow.
Cardiac conduction
Before analysis of arrhythmias can begin, it is important to understand what a normal electrocardiogram (ECG) trace looks like, and just as importantly, why it looks that way (Figure 1). The normal, or sinus complex (Figure 2), can be broken in to different components, as each part of the conduction system is activated:
The QRS complex should be viewed as a whole entity, but the state of the heart at each step of the depolarisation and repolarisation process should be appreciated. This sinus complex is repeated, and is known as a sinus rhythm. Further ECG interpretation can be guided by following six basic questions (Box 1). This logical step-by-step guide, should help the veterinary nurse decide if a rhythm is normal or not, and help answer what is abnormal about it.
Normal heart rhythms
Normal sinus rhythm
Sinus rhythms are regular rhythms, all with a P-QRS wave. Frequently patients exhibit different heart rates for example, the difference between resting and excited. Exact heart rates of patients can vary, but the following rates can be taken as an average (Table 1), to determine whether a rate is bradycardic, normal or tachycardic. Cardiologists have recently been challenging these previous size and heart rate differentiations (Ferasin et al, 2010; Lamb et al, 2010), but these rates suffice as a general approach to heart rate determination. Figure 3 shows an example of a normal sinus rhythm on an ECG trace.
| Adult dog | 70-160 beats/minute |
| Giant breeds | 60-140 beats/minute |
| Toy breeds | 70-180 beats/minute |
| Puppies | 70-220 beats/minute |
| Adult cats | 120-240 beats/minute |
Sinus arrhythmia
Sinus arrhythmia is a regularly irregular rhythm, because it always has a P wave (although this may vary in size, referred to as a ‘wandering pacemaker’), followed by a normal QRS complex, but the heart rate can vary. It is associated with vagal tone and often corresponds with the respiratory cycle, particularly in dogs. One of the most common times a veterinary nurse might see or hear this rhythm is when a patient is anaesthetised, but it can be a normal finding in healthy fit dogs (Dennis, 2010a). It is rarely seen in cats presenting to the clinic, but occasionally can be seen in patients with non-cardiac disease causing elevated vagal tone, such as upper respiratory tract obstruction. An example of sinus arrhythmia is shown in Figure 4.
The above section has looked at rate and rhythm, and so the next step is that the nurse analyses the ECG trace to see if there are P waves for every QRS complex, QRS complexes for every P wave, and if they are consistently and reasonably related. If there are any normal sinus complexes they should be examined to determine what a normal complex should look like for that patient. If there is anything that does not look like a normal sinus complex, the nurse should consider in what way it is abnormal. If there is no P wave with a QRS complex, what does the QRS look like? If it is narrow and upright then it will be predominantly atrial or supraventricular in origin, i.e. arising from above the ventricles — either in the atria, the atrioventricular junction or the bundle of His. If the complex is wide and bizarre, then it will have originated from the ventricles.
Arrhythmias
A multitude of reasons can cause arrhythmias, some are related to underlying cardiac disease, but many are not. To explain why arrhythmias can be life threatening is to understand what the conduction system does. The P-QRS represents the spread of electrical activity through the heart. Normal spread of electrical activity through the heart results in the heart contracting in the most efficient way to maximise stroke volume. The volume of blood pumped out from the ventricles is known as cardiac output (CO). CO is dependent on the heart rate (HR) and stroke volume (SV). To describe this system of haemodynamic stability, a simple equation is used:
It follows that if the heart rate becomes excessively fast, cardiac output is compromised, because the ventricles do not have the time to relax and refill with blood as the heart beats faster, the heart does not have sufficient pressure for coronary perfusion, and so this can promote further arrhythmias. Conversely, very slow heart rates can also affect the body's ability to maintain cardiac output (Ware, 2007). Therefore, as well as haemodynamic stability, another concern is electrical stability. Unstable arrhythmias can result in sudden cardiac death (Dennis, 2010b). Dennis also discusses a number of arrhythmias that are both electrically and haemodynamically unstable:
However, for a veterinary nurse to distinguish which are clinically relevant, and which are not, can be rather daunting. Early identification of arrhythmias or rhythm changes mean that prompt action can be taken if necessary, but a veterinary surgeon should always be consulted if changes occur. Box 2 provides a list of possible clinical signs associated with poor cardiac output (not all will be present in every patient). The obvious advantage of a nurse being able to identify life-threatening arrhythmias is that equipment such as intravenous catheters, antiarrhythmic drugs, and supplemental oxygen can be prepared immediately problems are identified.
As previously stated, not all arrhythmias are life threatening, and may occur as a result of the depth of anaesthesia, poor oxygenation (e.g. a blocked endotracheal tube) or as a response to pain. In these circumstances, it is often the nurse who recognises these changes first, and under direction of the veterinary surgeon, can make decisions to treat the arrhythmia.
Bradyarrhythmias
Sinus bradycardia
This is a regular rhythm with a P wave for every QRS complex. This rhythm is usually slower than the rates listed in Table 1. It is often seen in resting or sleeping dogs, but also in athletic dogs.
Atrioventricular (AV) block
Also known as heart block, AV block is a condition where the electrical impulse is not conducted normally through the AV node. This can result in slowed conduction (first degree AV block), occasional failure of conduction (second degree AV block), or persistent failure of conduction (third degree AV block). First degree AV block has the appearance of a sinus rhythm, but the P–R interval is prolonged. Because there is always a P wave, first degree AV block can be a subtle finding. Usually, second degree AV block is where the SA node initiates P waves, but some fail to be conducted through the AV node, and therefore do not produce a corresponding QRS complex (Figure 5). Second degree AV block can often be seen in veterinary practice, and is caused by some of the following reasons:
Depending on the intrinsic heart rate, second degree AV block may or may not be clinically significant (Dennis, 2010a). If it is very slow, haemodynamic compromise can occur, and a patient may present with signs of lethargy, exercise intolerance or syncope.
Third degree AV block (also known as complete AV block) is seen when no P waves are conducted through the AV node. The ECG therefore shows a number of unconducted P waves, with a spontaneous ventricular or junctional (AV node) rhythm. These are ‘escape’ complexes and are essential to maintain cardiac output. In Figure 6 the wide and bizarre complexes are an example of an escape rhythm. Patients that present in third degree AV block can have clinical signs such as syncope, weakness or even sudden death. In some patients there are no clinical signs — this is commonly seen in cats. Pacemaker implantation is often considered in patients with third degree AV block, because the ventricular escape mechanism can fail, making it an electrically unstable arrhythmia, resulting in sudden death (Schrope, 2006). Kellum and Stepien's (2006) study on cats with third degree AV block, found that cats generally have a higher ventricular escape rhythm, making the clinical signs less severe.
For diagnostic and treatment purposes it is important to distinguish between first, second and third degree AV block.
Atrial standstill
As the name suggests, this is where the atria are not active at all, and so no P waves are evident on the ECG trace. The SA node may create an electrical impulse, but the atrial muscle fails to depolarise (Figure 7). To keep the patient alive, there are junctional (AV node) or ventricular complexes. Clinical signs in the patient include syncope, lethargy and weakness. This rhythm is often caused by hyperkalaemia.
Tachyarrhythmias
Sinus tachycardia
A regular rhythm with a P wave for every QRS complex. This rhythm is faster than the rates listed in Table 1. It is often seen in excitable or stressed patients, and is very common in cats.
Atrial fibrillation (AF)
AF is the most common persistent arrhythmia seen in small animals (Pariaut, 2010). The QRS complexes have a normal morphology, but AF is typified by the absence of P waves (Figure 8). It is also notable by its irregularity, and usually has a fast rate. The absence of atrial contraction (which contributes to 20–30% of ventricular filling) and the rapid ventricular rate (when present) lead to reduced cardiac output. It is frequently caused by atrial enlargement and is often seen as a result of underlying structural heart disease. AF is often accompanied with heart failure, if the ventricular rate is uncontrolled (Ware, 2007). While this rhythm is not life threatening, the underlying problems caused by reduced cardiac output and congestive heart failure, means that patients require prompt veterinary attention.
Ventricular arrhythmias
Ventricular premature complexes (VPC)
VPCs are a common finding in dogs and cats (Martin, 2007) and can be caused by a variety of cardiac and non-cardiac causes. VPCs can be seen in patients that have undergone major surgery (Ware, 2007), in healthy animals (Meurs et al, 2001), in breeds that are predisposed to cardiomyopathy such as Dobermans (Calvert et al, 2000) and Boxers (Stern, 2010; Meurs, 1999), as a result of cardiac disease, non-cardiac diseases such as metabolic, respiratory or haematological disease, and central nervous system disease (Luis Fuentes, 1998; Eastwood and Elwood, 2003). A VPC is caused by a focus or foci within the ventricular myocardium, where spontaneous depolarisation arises. Because the depolarisation spreads slowly across the myocardium, and not rapidly through the normal conduction channels, it is prolonged, which makes the QRS wider on the ECG and bizarre in shape. VPCs are not only characteristic by appearing notably different from a QRS complex of supraventricular origin (e.g. sinus QRS complex), but also by the lack of an associated preceding P wave. When VPCs are present occasionally in a patient with sinus rhythm, an irregularity will be seen when a VPC occurs (Figure 9). The VPC occurs earlier than a normal complex would be expected, thus giving it a premature appearance on the ECG trace.
Vntricular tachycardia
There is general agreement among cardiologists that ventricular tachycardia is classified as three or more consecutive VPCs, which can either be sustained (>30 seconds) or non sustained (<30 seconds) (Martin, 2007). Ventricular tachycardia is usually associated with more severe cardiac disease or systemic disease, in contrast to single VPCs (Dennis, 2010a).
Rapid ventricular tachycardia will affect cardiac output, and thus patients may present collapsed, lethargic or exercise intolerant. Ventricular tachycardia can commonly be seen in Boxers and Dobermans with cardiomyopathy, as stated above, but can also be seen as secondary to conditions such as splenic masses, electrolyte disturbances or as a consequence of some drugs. An example of ventricular tachycardia is seen in Figure 10.
Due to the effect that it can have on cardiac output, rapid ventricular tachycardia can be life threatening if left untreated, because it can develop into ventricular fibrillation and sudden death. A veterinary surgeon must be informed immediately. Nursing care involves placing an intravenous catheter, administration of emergency drugs such as lidocaine, and attaching an ECG.
Cardiac arrest rhythms
Ventricular fibrillation (VF) is known best by its terminal nature. As the name suggests, it is ventricular in origin, but it arises from different foci within the ventricle, and produces very little cardiac output. There are no normal waveforms and the complexes vary in size and shape, and it is usually seen at a fast rate (Figure 11). It can often lead on from ventricular tachycardia, and would need immediate cardiopulmonary cerebral resuscitation (CPCR) and electrical defibrillation. A note of caution however, is that surgical spirit is flammable, so ensure that a non-flammable conductive gel is used. The reasons for VF can be numerous and so the underlying medical condition needs to be known.
Other arrest rhythms include:
All three of these rhythms, require immediate veterinary attention.
Conclusion
Some arrhythmias are very common in small animal practice, and it is important for the veterinary nurse to understand what is happening to the patient and what affect it is having on cardiac output. Treatment options vary dependent on the underlying disease, and whether or not the rhythm disturbance is haemodynamically or electrically unstable. If the patient is exhibiting signs of collapse, lethargy or exercise intolerance, a veterinary surgeon should be notified as a matter of urgency, and an ECG trace recorded for further assessment.