Listen to the sound: what is normal?

Cardiac auscultation is an essential part of our daily work of assessing clinical patients. A heart murmur can be the sole finding of structural heart disease and auscultation can provide important, even life-saving information. Crucial information about a potential cardiac disease is gained from the localisation, timing and grade of a heart murmur (Sisson 1999). While loud heart murmurs are usually obvious and indicate severe cardiac pathology, a low-grade murmur can be difficult to detect and raises the question of its significance (Jenni et al. 2009, Ljungvall et al. 2014). The problem for primary care physicians is to distinguish murmurs related to an underlying heart defect from murmurs created by the normal flow of blood within a structurally sound cardiovascular system. This differentiation is important for the clinician and the owners, as it either relieves owners from their concerns or might have consequences for further management of the animal, and might involve a referral to a specialist for further investigations. Clues are obtained from the history, clinical presentation and physical examination (Sisson 1999, Frank and Jacobe 2011). However, little is known about the prevalence of physiological heart murmurs in dogs. The relevance of a heart murmur in a dog without clinical signs is therefore difficult to assess. A heart murmur arises from turbulent blood flow, which can be caused by abnormal communication between cardiac chambers, stenotic or insufficient valves and changes in blood viscosity or vessel diameter (Reynolds number; Sisson 1999). As clinicians, we are aware of physiological murmurs in puppies, which should disappear by six months of age, and low-grade heart murmurs in athletic breeds and in some breeds, such as boxers that might have a slightly different cardiac anatomy than other dogs (Koplitz et al. 2006). Furthermore, conditions influencing cardiac output or consistency of the blood (i.e. decreased blood viscosity) are known to result in heart murmurs (Sisson 1999). But what about the group of dogs that are no longer puppies, that do not belong to one of the breeds more likely to have a heart murmur and do not have a systemic disease associated with a heart murmur? This question, common during daily practice, is addressed by the study from Drut et al. (2014) published in the current issue. The authors included 95 healthy dogs, between one and five years of age, and performed a physical examination, including cardiac auscultation, which was carried out by three different investigators (one experienced, two less experienced). Additional investigations to exclude systemic diseases, which might be a cause of a heart murmur (blood investigation, urine analysis, blood pressure) were obtained and echocardiography was performed if a heart murmur was identified. The authors detected a prevalence of 6–12% for physiological heart murmurs. Indeed, 27–50% of dogs with a heart murmur did not have cardiac abnormalities, but rather had a physiological heart murmur. The authors reveal with this study that physiological heart murmurs can be found not only in puppies and breeds more likely to have heart murmurs but in the wider and older dog population. This study also demonstrates that 50–73% of supposedly healthy dogs with heart murmurs had cardiac disease. However, these were mild and caused either by mitral regurgitation (9/22; 41%) or a mild aortic or pulmonic stenosis (3–7/22; 14–32%). This confirms that a low-grade heart murmur in a dog without clinical signs suggests minor cardiac disease (Jenni et al. 2009, Ljungvall et al. 2014). The detected pathologies might have been caused by the breeds included in the study, consisting mainly of small breeds, Yorkshire terries, German shepherd dogs, golden and Labrador retrievers as the more frequent breeds that might present more commonly with valvular disease or aortic stenosis (O'Grady et al. 1989, Borgarelli et al. 2004, Serfass et al. 2006, Stern et al. 2012). The range of prevalence in the study from Drut et al. (2014) is caused by limited agreement in veterinary cardiology about what is normal in our patients. For aortic velocities, different groups consider different cut-off values as normal, varying between <1.8 and <2.5 m/s (Bussadori et al. 2000, Jenni et al. 2009, Stern et al. 2012). Potential breed differences have been discussed, with boxers potentially having smaller left ventricular outflow tracts (Koplitz et al. 2006) causing higher ‘normal’ aortic flow velocities. Another question is the clinical relevance of a mildly increased flow velocity, as we know that a mild stenosis might not influence long-term survival. However, on the other hand a smaller vessel or an abnormal valve causing stenosis of an outflow tract should not be accepted as normal and applying strict criteria might reduce the risk of a higher prevalence of cardiac disease in a predisposed breed in the future. Most physiological murmurs arise from turbulence at the origin of the great vessels, and should be low grade (I–III/VI), early or midsystolic and loudest at the heart base (Saunders 1995, Sisson 1999, Frank & Jacobe 2011), which was the case in the study from Drut et al. (2014). Interestingly, these murmurs should not radiate. Applying the strict criteria of that study for diagnosis, the physiological heart murmurs did not radiate. Using flexible criteria, three of the physiological heart murmurs did radiate to the thoracic inlet. This suggests that the detection of a radiating heart murmur might be consistent with cardiac pathology; in case of the thoracic inlet, most likely increased aortic velocities (Sisson 1999). Four murmurs were excluded as they were heard only over the thoracic inlet. In human medicine different localisations of physiological/innocent heart murmurs in children are known (Asprey 1998, Saunders 1995). Little is known about localisations of physiological heart murmurs, other than the heart base, in our dogs and further investigations might be required. Furthermore, it is reported that physiological heart murmurs can change with body position and heart rate (Saunders 1995). Heart rate was taken into account by Drut et al. (2014), but body condition and possibly stress of the individual dog might have contributed to the low interobserver agreement. All observers did auscultate the dogs within 6 hours, but a different body position and stress level is possible. The order of investigators and the body position of the dogs were not reported by the authors, and might have played a role. A poor interobserver agreement in cardiac auscultation has been reported in dogs and humans previously (Pedersen et al. 1999, Hoglund et al. 2004, Vukanovic-Criley et al. 2010). Better agreement exists if a loud heart murmur is present, which is reassuring as these are commonly associated with severe cardiac diseases, whereas quieter heart murmurs are more challenging to detect, but are also usually less concerning, as long as the dog shows no clinical signs (Frank & Jacobe 2011, Ljungvall et al. 2014). Auscultation is a subjective assessment, even if the widely used 6 grade system is applied (Sisson 1999). Experience seems to play a major role; the more familiar we become with cardiac auscultation the more reliable is our assessment, which is shown by the current and previous studies (Pedersen et al. 1999, Hoglund et al. 2004, Vukanovic-Criley et al. 2010). Next to observer experience, individual differences in grading a heart murmur, for example into a grade II/VI or a III/VI, will have a subjective component, even if the same grading criteria are applied. There are several areas in veterinary cardiology, where subjective differences in the assessment of cardiac diseases exist, and the discussion whether a four grade grading system of heart murmurs is preferable to our current six grade grading system is only one of these (Ljungvall et al. 2014). Furthermore, the choice of stethoscope appears to influence whether or not a heart murmur is detected. Previous studies in dogs have shown that electronic stethoscopes might be superior in particular in the detection of low-grade heart murmurs (Voros et al. 2012), however practice and experience might be more important than the device (Hoyte et al. 2005). In the current study (Drut et al. 2014) the choice of stethoscope will have been of minor relevance only, as all investigators used conventional acoustic stethoscopes, and no difference in the detection of heart murmurs was present between the observers using the same and a different stethoscope. Variable heart murmurs were present and some heart murmurs might have been missed. This might have also been the reason why some heart murmurs were detected by the less experienced but not the experienced investigator (Drut et al. 2014). A reinvestigation of these dogs by the experienced investigator could have confirmed the presence or absence of a heart murmur, but was not reported. Furthermore, only dogs with heart murmurs underwent echocardiography and cardiac diseases not resulting in a heart murmur might have been missed. However, the presence of a heart disease in a dog without a heart murmur and clinical signs is unlikely, and the aim of the study was to detect the prevalence of physiological heart murmurs. How do we know now whether a heart murmur is physiological? We should not ignore the crucial information, which can be gained by a physical examination and cardiac auscultation, despite its limitations. If a heart murmur, which does not radiate, is detected in a dog, which does not show any clinical signs, has no history of cardiac diseases and does not belong to a breed known to be predisposed to cardiac diseases, a physiological heart murmur or minor pathology, unlikely to be of concern for the individual animal, is likely to be present. The study also shows the importance of experience for our clinical skills, in this case cardiac auscultation, and we should be encouraged to continuously practice this basic and easily carried out daily procedure. We still know only little about what is normal in our patients. Studies, like the one from Durt et al. (2014) contribute to this knowledge, which will allow us to feel more confident in the assessment of our small animal cases. Sonja Fonfara is Senior Lecturer in Small Animal Cardiorespiratory Medicine at the University of Bristol. She did her internship and residency in veterinary cardiology at the University of Liverpool, which was followed by a lectureship. She holds the certificate in veterinary cardiology and the diploma of the European College of Veterinary Internal Medicine in Cardiology, has a doctoral thesis in veterinary pathology, and a PhD investigating cardiac remodelling, which is one of her main research interests.

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