Veterinary Echocardiography Newsletter 7: Cardiac screening

 

Not signed up to our monthly newsletter? You can sign up for free here.

 

Echocardiography is a skill that takes a lot of time and practice, but you don’t have to dedicate hundreds of hours to it in order to rule serious cardiac pathology in or out; nor do you necessarily need a cardiac-grade machine or a phased array transducer. In the article below, I discuss ways you can screen for major causes of heart murmurs, breathlessness or syncope without the use of spectral Doppler. All of these techniques can be performed with a microconvex probe.

Murmurs

One of the most common indications for a cardiac scan is the discovery of a murmur. In dogs, the most common cause is mitral valve disease; in cats, either obstruction or regurgitation as a result of hypertrophic cardiomyopathy (HCM).

Cats

Hearing a heart murmur in a feline patient can strike fear into any vet working the animal up for even a routine operation, such as a spay. The big worry is, of course, HCM, where a change in loading conditions under anaesthetic could lead to new or worsening outflow tract obstruction, mitral regurgitation, or ischaemia. Severe HCM and associated diastolic dysfunction and dilated left atrium can result in thrombus formation, particularly in the left atrial auricle.

Above: Example of a thrombus in the left atrial auricle (or appendage), seen with intraoperative transosophageal echocardiography using 2D biplane, and then 3D.

While Doppler assessment of diastolic function and myocardial velocities are an integral part of the cardiologist’s repertoire, these tools are not necessary for a screening protocol designed to pick up animals with more advanced (and therefore overt) disease.

 

2D signs of HCM

  • Increased wall and/or papillary muscle thickness. Using the right parasternal long or short axis views, it is important to measure wall thickness not only at the basal level, but also the mid-cavity (McLea, Boon and Bright, 2013), because hypertrophy may not necessarily be evenly distributed. By tilting your probe even further, you can also image right to the apex to rule out apically distributed HCM. If you do not have an ECG, you can identify diastole as simply the point at which the ventricle is at its largest, and systole as the point at which it is at its smallest. If you have not taken many measurements of cavity size and wall thickness before, you may find it easier to do this from the long axis (video example below), where you can easily see where along the length of the ventricle you are. If you would like more help in performing these measurements, please just ask or reply to your newsletter email. I am happy to make you a step-by-step tutorial video of this process.

Above: Right parasternal long-axis view, demonstrating increased wall thickness which can be appreciated by eye.

 

Above: Examples of measuring wall thickness and diastolic cavity diameter from the short axis in a dog.

 

Severe HCM is defined as wall thickness >9mm (Payne et al., 2013). There is disagreement over whether a 5mm or 6mm wall thickness at end diastole should be the cut-off for a positive diagnosis of HCM (Silva et al., 2013). If you are simply performing a screening test, you may decide that you do not require such high sensitivity and feel it is acceptable to accept the higher value of 6mm. After all, you are probably not looking to ‘diagnose’ HCM, but simply to gauge anaesthetic risk.

More recent studies (Schober, Savino and Yildez, 2017; Häggström et al., 2016) have recommended adjusting for body weight using allometric scaling. It is worth discussing the current literature and deciding, as a practice, where you wish to set your tolerances. Also bear in mind that the distribution of hypertrophy is rarely purely concentric, with the septum often the thickest part. The biggest pitfall when measuring septal thickness is to accidentally include parts of the right ventricular chordae, papillary muscles or moderator band.

 

Above: The true septal width is marked in blue. Notice how chords in the right ventricle run along the top of it, and could easily be erroneously included in the septal measurement.

 

  • Dilated left atrium. A measurement from the right parasternal view >10mm and a LA/Ao ratio of over 1.5 is indicative of dilatation (Silva et al., 2013). However, these set measurements could potentially over-diagnose HCM in smaller cats, and under-diagnose it in larger ones, so it is again best to adjust for body weight using allometric scaling (Häggström et al., 2016). With experience, left atrial dilatation becomes visually obvious, and there will be an example shown later in this article.

 

Above: Two examples of LA:Ao ratio measured in a dog. This should be measured when the aortic valve leaflets are closed in both dogs and cats.

 

  • Outflow tract obstruction. Left ventricular outflow tract obstruction can occur due to septal hypertrophy blocking the passage of blood during part of systole, or due to systolic anterior motion (SAM) of the mitral valve. Any type of obstruction causes turbulence on colour Doppler, but can again be detected in 2D black and white imaging only. This is one example where M-mode actually can be useful; in looking at whether the anterior leaflet makes contact with the interventricular septum during systole. Contact during diastole is not a concern, because it does not cause any obstruction to blood leaving the heart.

 

Above: Example of M-mode.

 

Other echocardiographic signs of more advanced HCM include impaired systolic function and pulmonary hypertension, but for a feline screening test, it is sufficient to become proficient at using wall thickness and left atrial size to rule significant disease in or out.

 

Dogs

The most common cause of a murmur in dogs is mitral valve disease. This is usually primary (i.e. a disease of the valve itself), but in dogs with dilated cardiomyopathy (DCM), mitral regurgitation (MR) can be secondary to ventricular dilatation.

Above: In this dog with DCM, the ventricle measures 6.3cm in diastole.

 

If your machine has high quality colour Doppler, you can screen for MR very quickly. If your colour frame rates are too low or the colour is prone to ‘bleeding’ and not giving you a lot of useful information, do not despair; there are plenty of 2D signs you can also look out for.

Above: Severe mitral regurgitation is obvious with high quality colour Doppler, as seen here with the Siui Apogee 2300.

 

 

Above: Even from 2D imaging, it is clear that the two leaflets do not meet in systole and there is a visible gap. Also notice how obviously the left atrium is dilated.

 

  • Dilated left atrium. This is often visually obvious (as in the case above), but you can also perform a simple linear measurement in the right parasternal long axis view, and take an LA:Ao ratio in the short axis, as discussed above.

 

  • In severe, long-standing mitral valve disease, the right ventricle will also be dilated due to pulmonary hypertension. There will likely be significant tricuspid regurgitation accompanying it. The right ventricle should always be around 1/3 of the size of the left ventricle in the right parasternal view.

 

 

Above: Dilated right ventricle in a cat (on the left of the screen), which dwarfs the left ventricle.

Breathlessness & syncope

Besides preoperative screening in the presence of murmurs, the other major indication for a cardiac screen are acute admissions. Classic cardiac symptoms include breathlessness and syncope, and you may wish to be able to quickly rule a major cardiac cause in or out right away. Valve disease is another common cause of these symptoms, but a more acute cause can be pericardial effusion. A quick screen can be potentially life-saving in the case of impending cardiac tamponade, and we have dedicated an entire article to pericardial vrs pleural effusions here.

 

Can this really all be done with a microconvex probe?

Don’t believe that this can all be done with a lower end machine? Check out cardiac screening with the Scan Pad – a £2800 machine!

 

References

Bonagura JD. 2000. Feline echocardiography. J Feline Med Surg 2, 147-151.

Häggström, J., Andersson, Å., Falk, T. et al. (2016). Effect of Body Weight on Echocardiographic Measurements in 19,866 Pure‐Bred Cats with or without Heart Disease. J Vet Intern Med. 2016 Sep-Oct; 30(5): 1601–1611.

MacLea, H., Boon, J., Bright, J. 2013. Doppler echocardiographic evaluation of midventricular obstruction in cats with hypertrophic cardiomyopathy. J Vet Intern Med. Nov-Dec;27(6):1416-20.

Payne, J., Borgeat, D., Connolly, A., et al. 2013. Prognostic Indicators in Cats with Hypertrophic Cardiomyopathy. J Vet Intern Med;27:1427–1436.

Schober, K., Savino, S., Vedat, Y. 2017. Reference intervals and allometric scaling of two-dimensional echocardiographic measurements in 150 healthy cats. J Vet Med Sci. 79(11): 1764–1771.

Silva, A., Muzzi, R., Oberlender, G., et al. 2013. Feline hypertrophic cardiomyopathy: an echocardiographic approach. Arch Med Vet 45, 1-6.