How I approach... Heart murmurs in kittens

Introduction

Young cats (< 1 year of age) frequently present to general practitioners for new patient exams, vaccinations, and breed screening evaluations, only for a heart murmur to be detected. In addition, there are occasions where a heart murmur can be found after clinical signs associated with heart disease are noted. It is important to know how to handle these situations in a confident and efficient manner in order to provide your patient with the best care and your client with the best service. Regardless of whether a heart murmur is present, if a patient is showing clinical signs associated with heart disease, referral to a cardiologist should always be considered.

A heart murmur is a sound wave created by turbulent blood flow moving through the heart or nearby vasculature. Heart murmurs are most notable when blood flows from a chamber of relatively high pressure to one with low pressure (i.e., ventricle to atrium). Movement of blood between two chambers of similar pressure may not create a murmur that is auscultable with a stethoscope, and it is important to note that not all congenital heart defects in cats will cause a murmur (e.g., reversed patent ductus arteriosus [PDA]), although nearly all do. In addition, murmurs frequently occur in cats without structural heart disease; these are termed benign or functional murmurs 1.

Regardless of why the cat is presenting to your exam room it is important to get a thorough history. If a heart murmur is noted on physical exam, specific details to ask the owner include: evidence of lethargy, exercise intolerance, weight of the patient in comparison to its littermates, any increased resting respiratory rate/effort, and any collapsing episodes. It is important to ask the owner about prophylactic deworming (specifically lungworm) and heartworm status. In addition, encourage the owner to ask the breeder if there were any cardiovascular concerns with the pet’s littermates or parents.

Physical examination

Murmur assessment is only a small part of a thorough cardiovascular examination. We personally choose to start a cardiovascular physical exam from the tail and work towards the head, as this is often less threatening for a nervous cat. Femoral pulses should ideally be palpated while listening to the heart to ensure the pulses can be palpated with each heartbeat. Femoral pulses should be assessed for synchronization with the heartbeat and the actual pulse quality. The pulses should be characterized as weak, normal, or increased in strength (also described as hyperdynamic or bounding). Animals with weak pulses have a low systolic pressure or a high diastolic pressure (as with pericardial effusion or dilated cardiomyopathy). Animals with bounding pulses have a low diastolic pressure or a high systolic pressure (as with patent ductus arteriosus or aortic insufficiency).

Heart rate and rhythm should be noted and documented. Often waiting a few moments for the kitten to become acquainted with the new surroundings of the exam room allows for the animal’s initial excitement-induced tachycardia to subside. If the rhythm is irregular, an electrocardiogram should be performed to definitively assess the cardiac rhythm. Mucous membrane color should and can be assessed at the gingival, vulvar, and nail bed areas. A normal, healthy cat should have pink mucous membranes with a capillary refill time of less than two seconds. Pale mucous membranes can occur in young cats with anemia. Cyanosis is caused by arterial hypoxemia due to severe respiratory or cardiac disease, and cyanotic mucous membranes can occur in cats with right to left intracardiac or great vessel shunts. Cyanosis is generalized with central venous admixture (i.e., with a Tetralogy of Fallot), or segmental/differential when right to left shunting occurs (i.e., with a reversed patent ductus arteriosus). Differential cyanosis is the term used to describe cyanosis of the lower extremities and vulva/prepuce while the upper extremities and oral mucous membranes appear well oxygenated (i.e., pink).

Respiratory rate and effort should be evaluated when the patient is calm. Unfortunately, auscultation of the lungs is not a very sensitive method of diagnosing pulmonary edema or pleural effusion in cats. Therefore if any clinical signs referable to the respiratory system are noted in your patient, three-view thoracic radiographs (both laterals and a ventrodorsal projection) should be considered ( Figure 1 ,  Figure 2 and Figure 3 ). The abdomen should be gently palpated for evidence of organomegaly or ascites which would be suggestive of right-sided heart failure. Likewise, generalized venous engorgement and/or jugular pulses are indicative of right heart disease. Table 1 lists the most common feline congenital cardiac defects.

How to evaluate a murmur

Cardiac auscultation is a skill acquired during veterinary school and honed with years of practice. Kittens in particular can be very challenging to auscultate, as they are frequently non-compliant. Options to calm the kitten include feeding the patient its favorite treat, cradling the patient in one hand and using the other hand to guide your stethoscope, or having an owner or technician cradle the patient. Cardiac auscultation should occur over the anatomic locations of the four cardiac valves ( Figure 4 ). The normal heart sounds (S1 and S2) are high frequency sounds and best heard with the diaphragm of the stethoscope. Gallop sounds occur during diastole (S3 and S4), are usually low frequency sounds, and these sounds are best heard with the bell of the stethoscope.

If a heart murmur is auscultated in a kitten, it should be characterized to aid in creating a differential list
(Table 2).

• The first characteristic is timing (i.e., during which portion of the cardiac cycle is the murmur occurring?). Murmurs that occur between S1 and S2 are systolic murmurs. Murmurs that occur between S2 and the next S1 are diastolic murmurs. Murmurs that occur throughout systole and diastole are continuous murmurs. Because of the rapid heart rate in many kittens, differentiating systolic from diastolic murmurs can be challenging. However, diastolic murmurs are uncommon in small animals.
   
• The second characteristic is location (i.e., where on the thorax is the point of maximal intensity (PMI) of the murmur? [left vs. right, and then apical vs. basilar vs. parasternal]). If a precordial thrill is palpable, it will be present at the PMI ( Figure 5 ).
   
• The third characteristic is grading the intensity of the murmur (i.e., how loud is the murmur?). Murmurs are graded from I-VI. The grade of a murmur essentially refers to how intense it is; E.g., grade I refers to a murmur so faint that it can be heard only with special effort, whereas a grade VI murmur is so loud that it is audible with the stethoscope just removed from contact with the chest wall. Murmurs that produce a precordial thrill, or are diastolic or continuous, are always pathologic and are best evaluated with echocardiography 3.

Diagnostics

Based on the character of the murmur and the patient’s clinical status, recommendations can be made for diagnostics and/or therapeutics. If the patient is clinically unstable or in respiratory distress, initial stabilization is recommended before diagnostics are performed, with the possible exception of thoracic radiographs. Definitive diagnosis of the underlying cause of the murmur requires a complete echocardiogram. Obtaining an echocardiogram enables the clinician to quickly understand the etiology of the murmur, determine if intervention is necessary, and develop a prognosis for the patient.

However, it is unrealistic to assume every kitten with a murmur will undergo a complete echocardiographic evaluation. If the murmur is < grade III/VI or is intermittent (i.e., it varies with heart rate and/or is not present at every examination), it is reasonable to recommend that the kitten be followed throughout its vaccine sequence for persistence of the murmur. Obtaining blood to run a packed cell volume can be a quick, inexpensive test to rule out anemia if the kitten has pale mucous membranes. If the patient is anemic, the cause of the anemia should be investigated and then corrected. When the packed cell volume is corrected the patient should be re-evaluated to assess whether the murmur still exists. A serum NT-proBNP test can be useful, particularly in cases where an echocardiogram is not an option. In a patient with a serum NT-proBNP of greater than 100 pmol/L, heart disease is more likely, whereas in a patient with a normal serum NT-proBNP (less than 100 pmol/L) heart disease is unlikely and the murmur is more likely to be benign 4.

As previously mentioned, thoracic radiographs should be considered if the patient is exhibiting abnormal respiratory signs. If an owner declines definitive diagnostics, they should be alerted to monitor for the development of signs consistent with progression of heart disease, i.e., signs of heart failure such as dyspnea or tachypnea.

Therapy and management

The possible therapeutic options, and therefore the discussion with the owner, are entirely dependent on the definitive diagnosis and echocardiographic findings. Medical management should be initiated in kittens with evidence of congestive heart failure unless the owners elect euthanasia. Other abnormalities that warrant medical management include tachy- or bradyarrhythmias, systolic anterior motion of the mitral valve, and severe pulmonary hypertension.

In kittens with congestive heart failure, furosemide and an angiotensin converting enzyme (ACE) inhibitor are regarded as the first line of treatment. The use of spironolactone and pimobendan should be considered in refractory cases or if the underlying disease is likely to benefit from these medications; for example, pimobendan is warranted in patients with evidence of systolic dysfunction. Typical doses are given in Table 3 . The use of sildenafil should be considered in kittens with evidence of severe pulmonary hypertension. Atenolol can be considered for control of severe dynamic outflow tract obstruction, and some arrhythmias ( Figure 6 ), but it should not be started in patients exhibiting signs of congestive heart failure. For specific antiarrhythmic therapy, the reader is referred to one of the various review articles or book chapters for a complete discussion on determining when therapy is warranted and how to choose the best drug.

Table 3. Common cardiac drugs and dosages. 

Some heart conditions may lend themselves to specific intervention. Catheter-based interventional procedures have been employed to effectively treat cats with patent ductus arteriosus (PDA) and pulmonic stenosis. However, surgical management via thoracotomy or thoracoscopy is more readily available for PDA ligations, vascular ring anomalies, and pericardial defects, and these approaches are as effective as the minimally invasive alternatives. Less common procedures, such as pulmonary arterial banding, have been effectively used to reduce shunting in cats with ventricular septal defects, and as cardiac bypass becomes more available in veterinary medicine, definitive surgical correction may become a reality for more of these patients.

It is not uncommon to detect a heart murmur in a kitten, and the clinician should have a systematic approach to such cases. A good history and thorough clinical examination are crucial in determining the next steps. Thoracic radiography can be useful for an initial assessment of a patient, but definitive diagnosis of the underlying cause of the murmur requires an echocardiogram, and this is recommended for murmurs that are grade 4 or above, or if there are clinical signs on examination. Therapy depends entirely on a definitive diagnosis.