Background
Hank is a 2 year-old, MC Beagle that presented to the cardiology service after a fainting episode that occurred two weeks ago while chasing a squirrel. His owners reported that he appeared weak right before fainting and urinated on himself during the episode, but quickly recovered and was back to normal shortly after. Hank was seen by his rDVM after the episode, where a loud heart murmur was auscultated. Because of a recent move, it was unclear if the last rDVM heard this heart murmur. The owner restricted exercise until Hank's appointment with the cardiology service, and reports that he is on no medication other than flea, tick, and heartworm preventative. Physical examination by the cardiology service was normal other than a grade V/VI basilar systolic heart murmur. Imaging modalities indicated pulmonary artery enlargement with turbulent blood flow, right ventricular enlargement, and diminished left ventricular dimensions. ECG showed an underlying sinus rhythm.
Problems
When Hank presented to the cardiology service, his only complaints were an episode of fainting, occasional coughing, and a grade V/VI heart murmur. After further clinical analysis, a few other problems were noted. I think Hank's most serious problems are the episode of fainting during exercise, the enlargement of his pulmonary artery, and his right ventricular enlargement, in that order. I believe that the most important problem is the episode of fainting, or syncope, because it indicates either short term loss of oxygen to the brain or interruption of cerebral blood flow (Davidow et. al., 1999). These are both serious problems because they can lead to cell death, and they indicate an underlying issue that would need to be addressed. Hank's enlarged pulmonary artery, seen by thoracic radiographs, is also a concern. This is not a normal physical finding and could be interrupting the typical flow of blood to Hank's organs by decreasing the amount of blood pumped through the pulmonic valve. Lastly, his right ventricular enlargement is also abnormal and suggestive of increased right ventricular pressure, which is not expected in a healthy dog. Higher pressures in the heart cause the muscles to work harder, which can cause enlargement in the heart (Shimoda et. al., 2013).
Differentials and Case Observation
There are two main differential diagnosis that could explain what is happening in Hank's case. The problems noted above give an indication that he may have a heart abnormality. However, it is also possible that Hank's cardiovascular symptoms are only secondary to a different type of disease process. One possible differential diagnosis is heartworm disease with secondary pulmonary hypertension. Heartworm disease is caused by the organism Dirofilaria immitis (Atkins). This picture shows the cycle of infection in dogs:
There are many clinical observations, along with the problems stated above, that support this differential diagnosis. First of all, syncope is a clinical sign of heartworm infection, as well as enlargement of both the right ventricle and the pulmonary artery. Syncope caused by heartworm disease is often associated with exertion due to decreased cardiac output (Davidow et. al, 1999). Obstruction of right ventricular outflow to the pulmonary artery would consequently result in less blood being delivered to the left side of the heart. During exercise, the body needs adequate cardiac output to meet the increased oxygen demand of the tissues, and syncope can occur due to decreased oxygen being delivered to the brain (Davidow et. al., 1999). The presence of worms in the heart can both thicken the vascular intima, due to the body's immune response, and obstruct blood flow, which may cause multi-system dysfunction (Grieve et. at., 1983). The worms are mostly found in the right side of the heart and have a tendency to decrease blood flow to the pulmonary artery. In active dogs, pulmonary hypertension is likely to develop due to the increased pulmonary artery resistance, which damages the endothelium and results in right ventricular hypertrophy (Atkins). It is known that structural alterations in the vascular wall is what contributes to pulmonary hypertension (Shimoda et. al., 2013). Pulmonary hypertension would also explain the septal flattening seen by the cardiologist in this case. The presence of the worms can also cause turbulent flow in the heart, which could explain Hank's grade V/VI left basilar systolic heart murmur. Clinical observations, including thoracic radiographs and an echocardiogram, indicated an enlargement of the pulmonary artery and a bulging area of on the right side of the heart. According the Merck Veterinary Manual, thoracic radiographs of a dog with heartworm disease would show enlarged pulmonary arteries, a "reverse D appearance" of the right heart on a VD view, and right ventricular enlargement. Hank's VD view thoracic radiograph show below indicates all of these signs of heartworm disease.
Another possible differential diagnosis, which was suggested by the cardiologist viewing this case, is pulmonic stenosis. Pulmonic stenosis is a congenital heart condition that causes narrowing of the pulmonary artery, which results in the pulmonary valve being too small for the load required of it (Durham). According to a genetic study, beagles are one of the breeds that are at a higher risk of having this congenital defect (Francis et. al., 2011). The picture below compares a normal heart to one with pulmonary stenosis:
As you can see in the image, pulmonary stenosis also causes right ventricular hypertrophy because of the high ventricular pressure needed to expel blood through the narrowed pulmonary valve ("Pulmonary"). Similarly to heartworm, pulmonary stenosis could be the cause of Hank's syncope due to less blood reaching the pulmonary circulation, resulting in less blood entering the left ventricle and being pumped to the systemic tissues. There are also other clinical observations that make pulmonary stenosis a possible differential diagnosis. Typically, echocardiographic features of pulmonary stenosis are thickening of right ventricle, tethered pulmonic cusps, decrease in size of the left ventricle, and widening of the pulmonary artery past the site of stenosis (Francis et. al., 2011). Hank's images reveal findings consistent with those previously mentioned; his pulmonic valve leaflets are elongated and tethered, he has right ventricle hypertrophy, and the left ventricle has reduced dimensions. The reduced size of the left ventricle in indicative of the amount of pressure in the right ventricle. One last observation that could be caused by pulmonary stenosis is Hank's grade V/VI left basilar systolic murmur. Murmurs are often associated with pulmonary stenosis because of the turbulent blood flow in the pulmonary artery (Francis et. al., 2011). However, one observation that could eliminate pulmonary stenosis as a possible differential diagnosis is the fact that Hank's primary care veterinarian did not say he previously had a heart murmur. This is an area of lacking information in the case, but it is an important observation to note. If Hank did not always have a heart murmur, then pulmonary stenosis would be a less likely diagnosis since it is a congenital condition and would have been present from birth.
Understanding
The bulging of the pulmonary artery can easily be seen on Hank's radiographs. To illustrate this, you can compare Hank's heart to an anatomically normal heart labeled by Dr. Fries:
As stated previously, two possible differentials for the bulging of Hank's pulmonary artery are heartworm disease and pulmonary stenosis. In heartworm disease, the worms would create the bulge, but in the case of pulmonary stenosis, the bulge would be created due to the high pressure in the artery (Francis et. al., 2011). Hank also had a grade V/VI left basilar systolic heart murmur. This means that he has turbulent blood flow during systole, or the contractile phase of the heart, that can be heard with just the rim of the stethoscope on the chest. The most common differential for this type of heart murmur is pulmonary stenosis, since it is often due to obstruction of one of the great vessels at the base of the heart (Durham). Hank's murmur and the results of the imaging done in this case revealed some pathophysiological changes in his heart. In general terms, if they could be reversed, I think that it would be most beneficial to Hank's wellbeing if he did not have an obstruction of his pulmonary valve and his left ventricle was of normal size. This would hopefully increase the amount of blood able to be oxygenated and pumped to his systemic tissues, and allow him to live a full life.
There are many basic science principles that are important in this case and are related to Hank's problems. First, the flow of blood is a basic concept that must be understood in order to grasp what is happening with Hank. Typically, deoxygenated blood from the systemic tissues flows into the right atrium, where it is pumped into the right ventricle and out to the lungs to be oxygenated through the pulmonary artery. Oxygenated blood from the lungs returns to the heart and into the left atrium through the pulmonary vein, and it then travels into the left ventricle where it is pumped to the body via the aorta. Although Hank's flow of blood is the same, the changes in his heart cause this system to not run as smoothly as it should. For example, his pulmonary artery is enlarged due to a type of obstruction, and his right ventricle is enlarged due to the change in pressure caused by this. The obstruction of his pulmonary artery is resulting in less blood being pumped into the lungs to be oxygenated. This, combined with his smaller-than-normal left ventricle, results in less oxygenated blood being pumped to the systemic tissues. This disruption of blood flow helps to explain how Hank fainted during exercise due to a lack of oxygen to his brain: his heart was unable to keep up with his tissue's increased demand for oxygen during exercise because of its insufficient cardiac output. Another science principle important to this case is the pressures in the heart during diastole and systole. This is important because you must first understand normal before understanding abnormal. Typically, the left ventricle is the thickest area of muscle in the heart because it has the highest pressure to successfully pump blood to the systemic tissues. The high pressure is necessary to overcome the aortic pressure and allow the blood to flow out of the ventricle, since fluid wants to flow from areas of higher pressure to areas of lower pressure. Pressures in the atria and right ventricle are relatively low in comparison, because they do not need to overcome areas of extremely high pressure. However, in Hank, the pressures in his heart are not as they should be, which resulted in changes of his heart structure. His right ventricle is enlarged because of abnormally high pressure in the ventricle caused by obstruction of flow out of the pulmonary artery. The pressure in his right ventricle must increase to higher than normal levels to overcome the high pressure in the pulmonary artery and allow blood to flow to the lungs. Consequently, the higher pressure in Hank's right ventricle caused his flattened interventricular septum, as well as the changes observed in the left ventricle.
Self-Reflection
One major learning issue I had with this case was that there were some gaps in the information presented, most notably in Hank's history. For example, a presenting complaint when Hank was brought to the cardiology service was that he was coughing. However, it never mentioned anything else about it. In this case, it would be important to know if Hank had been coughing or showing any other clinical signs before his fainting episode occurred. It was also unclear to me if the Hank's murmur had ever been auscultated prior to his fainting episode. This would be helpful information in determining differential diagnoses because it could indicate if Hank's problems are caused by a congenital heart defect or not. The possibility that Hank's heart murmur may have only recently showed up is what led me to a possible differential diagnosis of heartworm disease. However, it stated in the history that Hank has been on heartworm preventative medication. If it were possible, it would have been interesting to know if Hank had ever been tested for heartworm before or after being put on his preventative medication, and also how often and for how long the owners had been giving it. Another learning personal learning issue I encountered was my inability to differentiate between what structures I was looking at on the echocardiograms. This is important because I would like to be able to see what is happening in the heart for myself and not have to rely so heavily on what the cardiologist says is happening. Previous teachings of cardiac imaging have allowed me to understand thoracic radiographs, but I still struggle with the basics of echocardiograms. Echocardiograms are an important imaging modality because it allows the heart and great vessels to be viewed in more detail than radiographs alone.
References
Atkins, C. Overview of Heartworm Disease. Merck Veterinary Manual. http://www.merckvetmanual.com/circulatory-system/heartworm-disease/overview-of-heartworm-disease
Davidow, E. B., Proulx, J., Woodfield, J. A. (1999). Pathophysiology and differential diagnosis of neurocardiogenic syncope. The American Journal of Cardiology: 23(7). https://www.researchgate.net/profile/Blair_Grubb/publication/12731787_Pathophysiology_and_differential_diagnosis_of_neurocardiogenic_syncope/links/00b7d514c542c68e6e000000.pdf
Durham Jr., E. H. What’s up with this heart? A review of congenital heart disease. 2009. http://www.vin.com/members/cms/project/defaultadv1.aspx?id=3995819&pid=11287
Francis, A. J., Johnson, M. J. S., Culshaw, G. C., Corcoran, B. M., Martin, M. W. S., French, A.T. (2011). Outcome in 55 dogs with pulmonic stenosis that did not undergo balloon valvuloplasty or surgery. Journal of Small Animal Practice. DOI: 10.1111/j.1748-5827.2011.01059 https://www.researchgate.net/profile/Mike_Martin2/publication/51179503_Outcome_in_55_dogs_with_pulmonic_stenosis_that_did_not_undergo_balloon_valvuloplasty_or_surgery/links/544fa60e0cf2bca5ce92ac25.pdf
Grieve, R. B., Lok, J. B., Glickman, L. T. (1983). Epidemiology of canine heartworm infection. Epidemiologic reviews: 5. http://www.sunyjcc.edu/sites/default/files/epidemiology_of_canine_heartworm.pdf
Johnson, L., Boon, J., Orton, C. E. (1999). Clinical characteristics of 53 dogs with Doppler-derived evidence of pulmonary hypertension. J Vet Intern Med, 13:440-447 http://onlinelibrary.wiley.com/doi/10.1111/j.1939-1676.1999.tb01461.x/epdf
Pulmonic Stenosis in Dogs. Cornell University College of Veterinary Medicine. Retrieved February 18, 2017 http://www.vet.cornell.edu/hospital/Services/Companion/Cardiology/conditions/pulmonic-stenosis.cfm
Shimoda, L. A., and S. Laurie. (2013). Vascular remodeling in pulmonary hypertension. J Mol Med: 91(3): 297-309. doi: 10.1007/s00109-013-0998-0 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3584237/