August 2024

In this VETgirl online veterinary continuing education blog, Dr. Natasha Yeh and Dr. Christopher Kennedy, DACVECC, DECVECC discuss how focused cardiac ultrasonography (FCU) can be used to increase your index of suspicion for patients with left-sided congestive heart failure (LCHF) secondary to myxomatous mitral valve disease (MMVC).

Scenario: A 14-yo-male castrated Shih Tzu is presented for respiratory distress. Lung ultrasound shows B-lines. It must be left-sided congestive heart failure (LCHF), right? Not always.

The goals of FCU in MMVD and L-CHF

  • Identify the left and the right sides of the heart
  • Evaluate for left atrial enlargement
  • Identify a thickened mitral valve
  • Peak at the pulmonary vein

The views

  • PLAX4
  • PSAX-apex
  • PSAX-base

History and physical
When used properly, FCU can be a valuable tool in the assessment and management of patients. Its effectiveness relies on integration with the patient’s historical and physical examination findings (see Dr. Kennedy’s 1st blog post on Focused Cardiac Ultrasonography Examination HERE).

MMVD is a frequent cause of LCHF in older, small breed dogs. However, this population is also susceptible to other cardiopulmonary diseases. One notable differential is pulmonary hypertension secondary to chronic respiratory disease (class III pulmonary hypertension). Additionally, other respiratory diseases can cause B-lines.

How can we tease apart our Shih Tzu with respiratory distress and B-lines?

Historical information can help us prioritize or de-prioritize LCHF:

  • Patients with previously diagnosed MMVD are logically at risk of LCHF.
  • Patients receiving loop diuretics presumably (but not always) have experienced heart failure in the past. They are classified as ACVIM Stage C.
  • A history of chronic coughing is suggestive of respiratory disease and possible pulmonary hypertension.
  • Syncope is more common with pulmonary hypertension.

Physical exam findings can be helpful too:

  • Cyanosis is more common with respiratory diseases and pulmonary hypertension.
  • A left, apical, systolic heart murmur is expected with MMVD due to turbulent blood flow (mitral regurgitation). Its absence makes LCHF secondary to MMVD less likely.

FCU Questions
When performing FCU in these patients, we ask four questions:

  • Is the right heart big?
  • Is the LA enlarged?
  • Is the mitral valve thickened?
  • Is the pulmonary vein enlarged?

Is the right heart big?
A future blog will be dedicated to identifying right heart disease. For now, we start with the PLAX4 and subjectively compare the sizes of the right and left heart. The right heart should be < 1/3 the size of the left heart; in FCU, we use < ½ as it is easier to subjectively assess (video 1). If the right heart is bigger than the left, the right heart is a problem and LCHF is less likely (video 2, video 3). (For FCU examination, see this blog).

Video 1. PLAX4 of a normal heart. The right heart is located in the near field and the left heart in the far field. Note the left ventricle is clearly bigger than the right.

Video 2. PLAX4 of showing right sided cardiomegaly. You can see that the right ventricle is bigger than the left ventricle; the interventricular septum is also being pushed leftward.

Video 3. PSAX-apex showing right sided cardiomegaly. The right ventricle is the near-field chamber located above the “mushroom” which is the left ventricle. You can see that the right ventricle is bigger than the left ventricle; the interventricular septum is also bowing towards the left ventricle.

Is the LA enlarged?
This blog was dedicated to assessing the left atrium (LA). For MMVD to cause LCHF, with few exceptions, the left atrium should be enlarged. This is due to chronic volume overload secondary to mitral regurgitation. Things we can look at include LA:Ao (ratio of left atrium to aorta size), LAD (left atrial diameter) and LAV (left atrial volume). In practice, we can use the “absolutely massive” scale – is the LA subjectively massive? Yes – there might be LCHF (video 4). No – LCHF isn’t likely, look for something else. If you are unsure, look for other evidence. We use the absolutely massive scale because image acquisition error and measurement error can lead to misdiagnosis.

Video 4. PLAX4 showing a massive left atrium.

Figure 1 Still image of video 4 with left atrium (LA) labelled.

Is the mitral valve thickened?
MMVD is a degenerative valve disease that leads to thickening and irregularity of the valve. If the valve is thickened, MMVD (but not necessarily LCHF) is present (video 5). If the valve is not thickened, MMVD is not present (video 6).

Video 5. PLAX4 showing a thickened mitral valve.

Figure 2. Still image of video 5 with the mitral valve (MV) labelled

Video 6. PLAX4 showing a normal thickness mitral valve.

Is the pulmonary vein enlarged?
The pulmonary veins return blood from the lungs to the left atrium. In LCHF, the pulmonary venous system is congested, so we expect the pulmonary veins to be dilated.1 With a slight slide cranially and a little shuffling of the probe (usually tilting the probe toward the heart base and sometimes a small twist), a pulmonary vein can be brought into view (figure 3). Normally, the pulmonary vein and the pulmonary artery are the same size. If the vein is much bigger than the artery, this supports pulmonary venous congestion (video 7); if they are the same size, LCHF is less likely; if the artery is bigger than the vein, prioritize pulmonary hypertension (figure 4).2

Figure 3. PLAX4 showing the pulmonary vein (PV, pink lines) and the pulmonary artery (PA, blue lines).

Figure 4. PLAX4 showing an enlarged pulmonary artery. Note how the artery is obviously bigger than the pulmonary vein.

Video 7. PLAX4 showing an enlarged pulmonary vein. Note how the vein is obviously bigger than the pulmonary artery.

Summary
MMVD is common in older, small breed dogs. However, respiratory diseases and pulmonary hypertension can present similarly and do not respond to loop diuretics – but are responsive to oxygen. Responding to oxygen can complicate the diagnosis: if our 14 yo Shih Tzu received both furosemide and oxygen on presentation, it can be unclear which therapy was effective. Using FCU, we can increase our index of suspicion for MMVD causing LCHF by asking the four questions discussed above.

References and further reading

  1. Merveille, A‐et al. (2015) ‘Pulmonary vein‐to‐pulmonary artery ratio is an echocardiographic index of congestive heart failure in dogs with degenerative mitral valve disease’, Journal of Veterinary Internal Medicine, 29(6), pp. 1502–1509. doi:10.1111/jvim.13634.
  2. Roels, E.et al. (2019) ‘Diagnostic value of the pulmonary vein-to-right pulmonary artery ratio in dogs with pulmonary hypertension of precapillary origin’, Journal of Veterinary Cardiology, 24, pp. 85–94. doi:10.1016/j.jvc.2019.06.001.

Please note that the opinions in this blog are expressed by the speaker(s)/sponsor(s), and not directly endorsed by VETgirl.

  1. Jade Rivera-Martinez says:

    Very very helpful article. I wish I would have read this when dealing with my 13 y/o Pug who possibly had CHD and hypertension. He passed of mostly unknown causes.

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