‘Eye-balling’ Echoes

A 2D venture into Echocardiography for General Practitioners

Introduction

Echocardiography is the gold standard for the assessment of cardiac function in small animals. However, the high level of training and expertise required can be intimidating to the general practitioner. In particular the cardiac calculations, doppler functions and M-mode graphs can be difficult to acquire and interpret. Competency in these functions is crucial at referral level to quantify the degree and progression of heart disease and to detect mild or less obvious pathology. Nevertheless, in a first opinion setting and without the option of referral, the general practitioner can obtain valuable information with the ability to visualise the heart using three or four standard views, and by having confidence in what are normal versus abnormal findings.

The purpose of this article is to illustrate what useful diagnostic information can be obtained with a simple 2D assessment of the heart using standard echocardiographic views. For more information on how to obtain basic right-sided echocardiographic views in small animals, please visit our IMV Academy e-learning platform >> 

LV – Left ventricle

LA – left atrium

RV – Right ventricle

RA – Right atrium

IVS – Interventricular septum

PC – Pericardium

LA:Ao – left atrial to aortic ratio

HCM – Hypertrophic cardiomyopathy

DCM – Dilated hypertrophic cardiomyopathy

PCE – Pericardial effusion

RPSA – Right parasternal short axis

RPLA – Right parasternal long axis

Ultrasonographic changes visualised during echocardiography of acquired cardiac disease

Thickening of the Mitral Valves

Mitral valve disease is the most commonly acquired heart disease in dogs, affecting mainly smaller breeds. Clinical evidence of mitral valve disease has been detected in 30% of dogs aged 13 years and older (1). It is possible to visualise nodular thickenings on the mitral valve leaflets that can progress to coalescing nodules and eventually leaflet retraction (1). The affected valve will eventually become incompetent, leading to mitral regurgitation. Colour doppler is useful to assess for the presence of turbulent regurgitation of blood from the left ventricle into the left atrium during systole. As the disease progresses, the left ventricle can become enlarged. Assessment of this in the RPSA view is shown in Figure 1.  In more severe cases the mitral valve may be visualised prolapsing backward into the left atrium during systole which is a poor prognostic indicator.

Eyeball

Enlarged Left Atrium

Compare Figure 2, a right parasternal short axis view imaged at the level of the aortic valve in a normal dog, with Figure 3, a RPSA view imaged at the level of the aortic valve exhibiting severe left atrial enlargement.

A simple measurement can be obtained from this view to objectively assess left atrial enlargement. The left atrium/aortic ratio (LA:Ao) is measured in early diastole when the aortic valve first closes, illustrated in Fig 2. The aortic measurement is made first, along the junction of the non-coronary cusp and left coronary cusp of the aortic valve. The left atrium is measured by extending the line along the same trajectory, measuring from inner edge to inner edge of the left atrium.  Although the left atrial measurement should be maximised, take care not to include the pulmonary vein.

If the LA:Ao is ≥ 1.6 in canine patients, this in an indication that left atrial enlargement and cardiomegaly is present.

The EPIC study illustrated that the LA:Ao ratio has practical application in determining when an animal would benefit from the initiation of pimobendan treatment (2). A canine patient with a LA:Ao ≥1.6 will benefit from treatment with pimobendan with administration of pimobendan extending the asymptomatic period by around 15 months in dogs with cardiomegaly related to Mitral Valve Disease (MVD).

eyeball

Eccentric hypertrophy

Eccentric hypertrophy is characterised by dilation of the left ventricle (Figure 4) and is typically seen in association with conditions such as dilated cardiomyopathy (DCM).  Dilated cardiomyopathy is the second most common acquired cardiac disease in dogs, mostly affecting large breeds (1). During the progression of disease, the heart becomes dilated due to chronic volume overload, eventually resulting in systolic dysfunction. The thickness of the left ventricular myocardial wall decreases, the heart becomes more spherical in shape and dilation of both the left atrium and left ventricle occurs. Further progression of the disease process may result in right heart enlargement (3). When ‘eye-balling’ a heart with advanced DCM, it appears rounded, grossly enlarged and poorly contractile with thin myocardial walls. Incompetence of the mitral valve can occur secondary to the changes observed in the left ventricle and left atrium, which cause stretching of the mitral annulus.

eyeball

Concentric hypertrophy

Concentric hypertrophy is characterised by an increase in overall thickness of the left ventricular myocardium, involving both the left ventricular free wall and interventricular septum. The overall size of the heart remains relatively unchanged therefore as the myocardium increases in size, the internal size of the left ventricle is reduced.

Concentric hypertrophy is most commonly observed in cases of feline hypertrophic cardiomyopathy (HCM). A left ventricular free wall or interventricular septum with a width of >6mm at end-diastole in feline patients should be interpreted as evidence of HCM (4), (5).

The pattern of hypertrophy in feline HCM can be very variable; symmetric or asymmetric, focal or generalized (5). If symmetrical hypertrophy occurs, with an increase in size of the left ventricular free wall and interventricular septum, the ventricles can appear to be ‘ice-cream’ coned in shape when viewed in the right parasternal long axis view or 4-chamber view of the heart.  Enlargement and rounding of the left atrial chamber is seen in contrast with the relative size reduction and narrowing observed of the left ventricular chamber.

eyeball

Pericardial Effusion

Pericaridial effusion is easily visualised as anechoic fluid between the heart and the hyperechoic line representing the pericardium. Equalisation of pressure in the pericardial space with mean diastolic pressure in the cardiac chambers results in impaired cardiac filling and cardiac tamponade. This can be seen during echocardiography as collapsing or inversion of the free wall of the right atrium. In dogs, 31 – 57% of pericardial effusions have neoplastic origin and so careful examination during echocardiography for the presence of cardiac neoplasia in cases with pericardial effusion is important (6). If the patient is stable, ultrasonographic evaluation prior to drainage is recommended to maximise the likelihood of visualization, because the presence of fluid aids the visualization of masses.

eyeball

Cardiac Tumours

In dogs, the most common type of cardiac tumour is haemangiosarcoma, most commonly presenting as a mass involving the right atrium and the right atrial appendage, (7),(8).

Thoracic radiography has poor sensitivity for the detection of heart base masses, as low as 40 - 63% illustrated in a single study with experienced operators (9). When pericardial effusion is present, echocardiography has been shown to have a high specificity (100%) and sensitivity (82%) for detection and characterization of masses (10). With up to 60% of pericardial effusions in dogs seen as a result of cardiac or pericardial masses (11), the value of ultrasound should not be underestimated in the detection of cardiac tumours.

Larger masses may be fairly straightforward to visualise during echocardiography, however smaller masses can be more challenging to detect especially for the less experienced operator. Diagnostic differentials can include atrial thrombus or pericardial cysts (7). A hemangiosarcoma is more likely to exhibit irregular margins and invasion of the myocardium. A thrombus is more likely to exhibit smooth margins and a central area appearing trabeculated or more hypoechoic than the periphery (7). Careful examination of the origin of the mass can assist with differentiation.

eyeball

The ability to obtain basic right-sided echocardiographic views is invaluable when assessing patients with suspect cardiac disease. Subjective assessment by a relatively inexperienced operator can still provide a wealth of useful information. B-mode imaging with a microconvex probe provides an excellent starting place for any general first opinion practitioner.  

References

  1. Smith Jr, F.W.K.. et al. (2008) Manual of Canine and Feline Cardiology. 5th edn., Saunders.
  2. Boswood, A.,. et al. (2016). Effect of pimobendan in dogs with preclinical myxomatous mitral valve disease and cardiomegaly: the EPIC Study—a randomized clinical trial. Journal of Veterinary Internal Medicine, 30:1765–1779.
  3. Chetboul, V. (2015) Dilated Cardiomyopathy and other cardiomyopathies in dog. In: Clinical Echocardiography of the Dog and Cat, Ed: Chetboul V., Elselvier, St Louis Missouri, pp. 207–228.
  4. Fox, P.R. et al; (1995) Echocardiographic assessment of spontaneously occurring feline hypertrophic cardiomyopathy. An animal model of human disease. Circulation, 92(9): 2645-2651.
  5. Madron, É. (2015) Evaluation of Feline Cardiomyopathies. In: Clinical Echocardiography of the Dog and Cat, 1st edn., Ed: Chetboul V., Elselvier, St Louis Missouri,pp. 207–228.
  6. Madron, E. (2015) Pericardial Diseases. In: Clinical Echocardiography of the Dog and Cat, 1st edn., Ed: Chetboul V., Elselvier, St Louis Missouri, pp. 259 - 267.
  7. Bussadori, C. (2015) Cardiac Tumours. In: Clinical Echocardiography of the Dog and Cat, 1st edn., Ed: Chetboul V., Elselvier, St Louis Missouri pp. 271–280.
  8. Treggiari, E. et al. (2017) A descriptive review of cardiac tumours in dogs and cats. Veterinary and Comparative Oncology, 15, 2: 273–288.
  9. Guglielmini, C. et al. (2016) Sensitivity, specificity, and interobserver variability of survey thoracic radiography for the detection of heart base masses in dogs. Journal of the American Veterinary Medical Association, 248(12): 1391–1398.
  10. MacDonald, K.A., et al. (2009) Echocardiographic and clinicopathologic characterization of pericardial effusion in dogs: 107 cases (1985–2006). Journal of the American Veterinary Medical Association, 235: 1456–1461.
  11. Berg, R.J. and Wingfield, W. (1984) Pericardial-effusion in the dog - a review of 42 cases. Journal of the American Animal Hospital Association, 20: 721–730.

 

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