What are the five things you should know about companion animal cardiology?

1) Coughing does not equal congestive heart failure

Coughing is evoked by the stimulation of cough receptors located in the upper airways (larynx, trachea and bronchi). In contrast, irritation of the lower airways (bronchioles and alveoli) does not tend to elicit a cough (Widdicombe, 2001). This distinction is important to note when we consider patients with heart disease.

An assumption of CHF should not automatically be made when a patient with a heart murmur presents with coughing. A more reliable clinical parameter […] is an increased resting respiratory rate

For patients with significant cardiac remodelling, left atrial dilation with compression of the left mainstem bronchus is generally accepted as a probable cause of coughing. This may be even more apparent when concurrent airway disease exists (eg tracheal collapse) (Ferasin et al., 2013). However, left-sided congestive heart failure (CHF) or pulmonary oedema typically occurs in the lower airways where cough receptors are believed to be absent. Therefore, an assumption of CHF should not automatically be made when a patient with a heart murmur presents with coughing. A more reliable clinical parameter and one of the most sensitive non-radiographic markers for the presence of CHF is an increased resting respiratory rate (Schober et al., 2010).

2) The importance of left atrial size

Two of the most important acquired forms of small animal heart disease are hypertrophic cardiomyopathy (HCM) in cats and myxomatous mitral valve disease (MMVD) in dogs. Both conditions, when advanced, result in left atrial dilation and increased filling pressures in the heart.

Echocardiography is considered the gold-standard modality for assessing left atrial size (Figure 1). Proficiency with this technology is possible at first opinion level following a short period of training from an experienced operator (Dickson et al., 2022). The ability to identify left atrial enlargement offers significant diagnostic and prognostic value.

Increased left atrial size in dogs with MMVD is associated with an increased risk of developing CHF, atrial fibrillation (Guglielmini et al., 2020) and cardiac mortality (Sargent et al., 2015). For cats with HCM, increased left atrial size is associated with increased risk of CHF and sudden cardiac death (Payne et al., 2015).

At preclinical stages of heart disease, identification of increased left atrial size should prompt the initiation of medical therapy with pimobendan in dogs with MMVD

At preclinical stages of heart disease, identification of increased left atrial size alongside left ventricular dilation should prompt the initiation of medical therapy with pimobendan in dogs with MMVD, as studies have shown it extends the preclinical period of ACVIM stage B2 dogs by an average of 15 months (Keene et al., 2019; Boswood et al., 2016), whereas in cats with HCM, increased left atrial dimensions are associated with an increased risk of thromboembolism (Rush et al., 2002) and prophylactic treatment with an anti-platelet agent would be advised (Luis Fuentes et al., 2020).

3) When a puppy murmur is not innocent

Heart murmurs are common in puppies (Szatmari et al., 2015), and the presence of a heart murmur can indicate underlying congenital heart disease. But innocent murmurs, those which occur in the absence of structural heart disease, are also possible. While echocardiography is required for a definitive diagnosis, the unique characteristics of a heart murmur may help determine the likelihood of it being innocent in nature.

Murmurs of higher intensity (above grade 3) or those that are continuous or diastolic in timing are never innocent, so further investigation is required

Innocent heart murmurs are typically low grade (grade 2 or less) and may have a “musical” quality. They are always systolic in timing and often occur at the left heart base (Szatmari et al., 2015). Murmurs of higher intensity (above grade 3) or those that are continuous or diastolic in timing are never innocent, so further investigation is required.

The murmur associated with patent ductus arteriosus (PDA) is continuous and most audible on the left thorax, cranial to the heart base. You may need to extend the left forelimb cranially to auscultate this area appropriately. When identified and occluded promptly, PDA is a curable condition, so you do not want to miss or ignore this murmur.

4) Do not treat pericardial effusion with diuretics

A large volume or rapidly forming pericardial effusion can lead to cardiac tamponade, a condition that occurs when intrapericardial pressure exceeds that within the heart. As the right atrium is typically the lowest-pressure cardiac chamber, it is the first to collapse under increased intrapericardial pressure. This means blood flow from the cranial and caudal vena cava into the right atrium is impaired. Subsequent venous congestion results in symptoms of right-sided congestive heart failure (R-CHF), namely jugular distention, pleural effusion and ascites.

Diuretics can worsen symptoms of cardiac tamponade and should not be administered to patients with pericardial effusion

While the standard treatment of R-CHF involves diuretics, these are contraindicated in the case of pericardial effusion. Diuretics decrease the intravascular volume and therefore reduce intracardiac pressures. In the scenario of pericardial effusion, this could lead to an even greater pressure difference between the pericardial and cardiac pressures, worsening the cardiac tamponade.

The most effective treatment of cardiac tamponade is pericardiocentesis. Following this, right atrial pressures will rapidly return to normal levels, venous congestion will quickly resolve and cavity effusions will reabsorb.

5) Accelerated idioventricular rhythm is not v-tach

Accelerated idioventricular rhythm (AIVR) is a ventricular ectopic rhythm formed by four or more ventricular complexes. AIVR presents with a heart rate exceeding that of a typical ventricular escape rhythm (30bpm in dogs) but less than what is considered ventricular tachycardia (generally above 180bpm).

Typically, the rate of AIVR is 60 to 120bpm but can be up to 160bpm (Santilli et al., 2018). As the rate of AIVR is often similar to the underlying sinus rhythm, both rhythms compete to be the dominant pacemaker. Periods of both AIVR and sinus rhythm are therefore recognisable on electrocardiography (ECG) (Figure 2).

AIVR is recognised in hospitalised cats and dogs without underlying primary cardiac disease (Kittleson, 1998; Tilley, 1992). It has also been reported in patients with abdominal disorders (eg splenic disease and pancreatitis), following traumatic injury (eg road traffic accident), after abdominal surgery (eg for gastric dilatation-volvulus) and in those with neurological disease, likely as a consequence of myocardial ischaemic damage (Muir and Lipowitz, 1978; Macintire and Snider, 1984).

AIVR is well tolerated haemodynamically, and specific treatment is not required. This is in contrast to ventricular tachycardia, where an immediate attempt to convert to sinus rhythm (typically with intravenous lidocaine in the first instance) is required.