Cardiac murmurs, innocent or pathological, are often detected in young dogs and cats with outcome depending on the murmur source. However, little is known about the prevalence and characteristics of innocent murmurs in the wider canine and feline population.
What is a heart murmur?
Both innocent and pathological murmurs share many physiological characteristics. First, they are both the result of turbulent blood flow, which happens when laminar flow ceases. The blood flow follows Darcy’s law: the flow equals the difference in pressure along a tubular structure, divided by its resistance. Initially, the flow can increase linearly with an increase in anatomic pressure; however, after a certain threshold, the flow no longer follows a linear trajectory but increases at the square root of anatomic pressure. Therefore, after certain levels of pressure gradient, the flow becomes turbulent as explained in Figure 1.
To determine whether a fluid follows a laminar (organised) or turbulent (disorganised) flow, we can use the Reynold’s number. This is a dimensionless number used in fluid mechanics to indicate whether fluid flow in a tubular structure is steady or turbulent. If the Reynold’s number is over 2,000Re the flow is likely turbulent, and vice versa. Turbulence is worsened by high velocities, large vessel diameter, high fluid density and low viscosity.
What is an innocent heart murmur?
At the beginning of the 20th century, Dr George Frederic Still was the first person to document innocent murmurs in paediatric patients, identifying that patients with no cardiac disease could have murmurs and gallops. Indeed, pathological heart murmurs are often associated with congenital heart disease and additional risk factors such as a family history of heart disease, chromosome abnormalities, exposure to toxins and uterine infections in the perinatal period or pre-term delivery (Frank and Jacobe, 2011). Further, murmurs can be systolic or diastolic, but the majority of innocent heart murmurs are systolic and soft (ie intensity of less than 3/6) (Figure 2).
How to identify an innocent murmur
There are four types of innocent murmurs found in people: Still’s vibratory murmur, pulmonary flow murmur, supraclavicular systemic flow murmur and venous hums.
Still’s vibratory murmurs
Still’s murmurs are described as brief, vibratory sounds with intensity between 1/6 and 3/6, midsystolic with a low-pitch sound. They have been compared to the sound of an Aeolian Island harp. They tend to be louder when supine due to an increased venous return. The cause is unclear but there are various hypotheses, such as the presence of fibrous bands in the left ventricle, a small aortic size, an insertion of the tricuspid valve in the right outflow tract, increased cardiac output with bradycardia, vibrations of cardiac structures and lower aortic elastance when the ventricular contractility increases.
Pulmonary flow murmurs
Pulmonary flow murmurs have similar characteristics to Still’s murmurs, but they are harsher with a high-pitched sound. Their intensity increases with increased venous return; therefore, the murmurs are more obvious in inspiration and with a supine position.
Supraclavicular systemic flow murmurs
Supraclavicular systemic flow murmurs are harsh, have a crescendo-decrescendo pattern and, as all the other innocent murmurs, have an intensity lower than 3/6. They represent the normal flow from the aorta into the arteries of the neck and head.
Venous hums are continuous low-pitched murmurs audible at the thoracic inlet (lower neck) and correspond to the venous return of the superior vena cava. They usually disappear in standing position due to a reduced venous return from the upper body. Innocent murmurs are very common in children and virtually all children will have a murmur during their childhood. However, less than 1 percent of auscultated murmurs are pathological.
For this reason, guidelines in human medicine have been proposed in order to avoid unnecessary investigations and expenses. In a recent review from 2014, transthoracic echocardiography for investigations of presumed innocent murmurs was considered “rarely appropriate” (Campbell et al., 2014).
Diagnosing innocent murmurs
Innocent murmurs in pets have similar characteristics to those described in people; however, there is less information available and they are not universally classified by their aetiology. Innocent murmurs are usually soft, graded 1/6 or 2/6, systolic and usually reported to disappear at roughly six months of age. Further, they may be dynamic and change in intensity during or after exercise or excitement.
Innocent murmurs in pets have similar characteristics to those described in people; however, there is less information available and they are not universally classified by their aetiology
Possible causes of innocent murmurs in pets are various and include increased cardiac output and a narrower aorta. However, many causes are not associated with cardiac disease, such as pregnancy, fever or anaemia.
Pregnancy is associated with an increased circulating volume that results in an increased cardiac output (Mishra et al., 1992). Thermal stress (ie heat) increases cardiac output but stroke volume is decreased or maintained, therefore, considering cardiac output is the product of heart rate and stroke volume, the heart rate increases (Wilson and Crandall, 2011). Athletes have bigger stroke volume and this has been demonstrated in dogs too (O’Brien and Rogers, 1999; Stepien et al., 1998; Wilson and Crandall, 2011). Finally, anaemia leads to reduced blood viscosity, leading to an increase in Reynold’s number and turbulent flow (Mesihović-Dinarević et al., 2005).
Studies in Boxers and Whippets
An issue in the diagnosis of innocent murmurs is interobserver variability. In a study on 27 Boxers, a breed frequently diagnosed with murmurs, the agreement was good at rest but reduced markedly after exercise (Höglund et al., 2004). This raises the problem of how reliable auscultation can be in determining whether a murmur is innocent or pathological. However, it is reasonable to believe that the agreement increases with increasing intensity.
An issue in the diagnosis of innocent murmurs is s interobserver variability [and] the problem of how reliable auscultation can be in determining whether a murmur is innocent or pathological
A second prospective study in Boxers involved puppies without structural heart disease and followed them for 40 months (Höglund et al., 2011). The study demonstrated that the aortic and pulmonic velocities did not change significantly over time and the intensity of murmurs varied with age and stress levels. In this study, 2/19 dogs had a 3/6 murmur after exercise despite the absence of structural heart disease (Höglund et al., 2011).
In an Italian population of 500 Boxers over one year of age, cardiac disease was found in 17.8 percent of dogs and consisted of subaortic stenosis or pulmonic stenosis. None of the dogs with a heart murmur graded 1/6 or 2/6 had congenital heart disease, leading to 35 percent of that population having innocent murmurs (Bussadori et al., 2001).
Whippets are also frequently diagnosed with innocent murmurs and a study showed that dogs with normal hearts were more likely to have an innocent murmur if they had higher aortic and pulmonic velocities alongside a high cardiac output. Instead, values of haematocrit and haemoglobin did not show a significant relationship with the presence of murmurs (Bavegems et al., 2011).
Studies in young adult dogs
Another study reported innocent murmurs in young adult dogs and although this was performed on only 95 dogs, no breed predispositions were noted. In the study group, the prevalence of innocent murmurs varied between 6 and 12 percent depending on which echocardiographic criteria were used (Drut et al., 2015).
The role of biomarkers
Lastly, a study on 186 healthy dogs investigated the role of NT-proBNP in differentiating innocent and pathological heart murmurs. The biomarker was not able to rule out a congenital heart disease but murmurs that were longer than 80 percent of systole were most likely pathological (Marinus et al., 2017). In order to assess the length of a murmur, a phonocardiogram is required (ie from a digital stethoscope) and this is rarely available in practice.
Studies in different settings
The prevalence of innocent murmurs in a very large population of shelter dogs (76,301) and cats (57,025) was reported to be 0.1 percent and 0.16 percent respectively. Similar values were found for the prevalence of congenital heart disease in dogs (0.13 percent) and cats (0.14 percent) (Schrope, 2015). In a referral setting, the prevalence of congenital heart disease was higher for obvious reasons, with dogs at 17 percent and cats at 5 percent (MacDonald, 2006).
In summary, innocent murmurs aren’t frequent; however, early differentiation from pathological murmurs can change the course of action. There are murmur characteristics that can help us in understanding whether a murmur is more likely to be innocent or due to underlying cardiac disease. Innocent murmurs are systolic, soft (less than 3/6) and generally disappear with time.
The gold standard to rule out underlying cardiac disease, especially in predisposed breeds, remains echocardiography – a non-invasive and widely available test
The gold standard to rule out underlying cardiac disease, especially in predisposed breeds, remains echocardiography – a non-invasive and widely available test.