February 2024

In this VETgirl online veterinary continuing education blog, Dr. Christopher Kennedy, DACVECC, DECVECC discusses how to assess the left atrium in cats on Focused Cardiac Ultrasound (FCU). In this blog, he will review how to image the left atrium from several angles, how to measure the left atrium, and how to identify overt left atrial enlargement in the cat.

By Dr. Christopher Kennedy, DACVECC, DECVECC

Assessing the Left Atrium in Cats on Focused Cardiac Ultrasound

The goals of Focused Cardiac Ultrasound (FCU) in feline left atrial assessment are to:

1. Image the left atrium from several angles.
2. Measure the left atrium.
3. Identify overt left atrial enlargement.

The views:

  • PLAX4
  • PSAX-base

The left atrium (LA)
We discussed previously that the LA has three roles: reservoir, conduit and pump.(1) During early diastole, the LA functions as a conduit for passive blood flow into the left ventricle (LV); in late diastole, the LA contracts and blood is pumped forward. The conduit function is dependent on LV diastolic function, i.e., the active relaxation and compliance of the LV.(1) Hypertrophic cardiomyopathy (HCM) is the most common feline cardiomyopathy and, along with restrictive cardiomyopathy, is characterized by diastolic dysfunction.(2) This means that the LV fills (or conversely, the LA empties) less efficiently in early diastole. It also means the atrial contraction in late diastole is less effective at transferring blood. Consequently, the volume of blood in the LA at end-diastole becomes progressively greater as diastolic function worsens.(3) In cats, end-diastolic LA size may be more useful than end-systolic size, which is what we typically measure in dogs.3 Additionally, LA enlargement in cats is associated with thrombus formation and risk of arterial thromboembolism (ATE).(2)

Cardiologists typically make measurements in right lateral recumbency: in non-dyspnoeic cats, LA:Ao and LAD measured by an ECC resident in right lateral and sternal recumbency were interchangeable.†

Left atrial-to-aortic ratio (LA:Ao)
In the PSAX-base view, the LA can be indexed to the Ao to provide an easier way of interpreting the LA size (See Figure 1a). The maximum LA:Ao is measured at end-systole, just after the aortic valve closes, when the LA is biggest.(3)  For cats, LA:Aomax is < 1.6 – 1.7.(3,4)  LA:Aomax > 1.9 is consistent with (not diagnostic of) congestive heart failure (CHF).(3) The LA:Aomin requires concurrent ECG and is measured just after the P-wave (atrial contraction) (See Figure 1b).(3) Values > 1.7 are consistent with CHF.

Figure 1: a) LA:Aomax recorded just after the closure of the aortic valve, when the left atrium is approximately at its fullest. b) LA:Aomin recorded after the P-wave (after atrial contraction) when the left atrium should be at its emptiest. Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Left atrial diameter (LAD)
LAD is the length of the minor axis of the LA. It is measured in the PLAX4 view (See Figure 2). Measurements are made just before the opening of the mitral valve, at the start of diastole, parallel to the mitral annulus. The LAD seems to be easier to obtain accurately than the LA:Ao and only requires one measurement; however, when performed by cardiologists, it was not as useful at differentiating HCM cats with and without CHF.3 Values up to 17 mm are found in normal cats, with most < 16 mm; values > 19 mm are consistent with CHF.(3,4)

Figure 2: PLAX4 view with LAD measurement. The measurement is made inner-edge-to-inner-edge, that is from the endothelium-blood interface of the atrial septum to the blood-endothelium interface of the atrial wall opposite. Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Left atrial volume (LAV)
LAV can be measured using PLAX4.(3)  The internal surface of the LA, at the blood-endothelial barrier, is traced and the echo machine uses a formula to calculate LAV – check to see if your machine has this function. The LAVmax is measured just before the opening of the mitral valve in early diastole (Figure 3a): < 0.6 ml/kg, CHF is unlikely; > 0.9 ml/Kg is consistent with CHF.(3) The LAVmin is measured at the just after the P-wave on the ECG: Values > 0.5 ml/Kg are consistent with CHF (See Figure 3b).(3)

Figure 3: a) PLAX4 view showing LAVmax recorded just before the opening of the mitral valve in early diastole (5 Kg, 0.6 ml/Kg). b) LAVmin recorded after the P-wave (after atrial contraction) in late diastole (5 Kg, 0.2 ml/Kg). Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

The “absolutely massive” scale
Try not to get bogged down in the little numbers: an LA:Ao of 1.7 or LAD of 17 mm are ambiguous, whereas an LA:Ao > 2.0 or LAD > 20 mm are not.(5)  Bigger values are likely to be clinically relevant – integrate the FCU information with your clinical examination. We can use the “absolutely massive” scale, where we do not need to measure the LA, as it is absolutely massive (See Videos 1 & 2).

Video 1: PLAX4 view of a massive left atrium

Video 2: PSAX-base view of a massive left atrium

False pericardial effusion (FaPE)
FaPE occurs with an absolutely massive LA. In short axis, the LA can be seen around the left ventricle in PSAX-pap. If you move to PSAX-base you will see this is the LA, not pericardial effusion (See video 3). Similarly, if your sector size is set too small, in PLAX4 you might mistake an absolutely massive LA for pericardial effusion – increase your sector size (see “The FCU exam” post).

Video 3: PSAX-pap view with false pericardial effusion. In the bottom right of the sector, a large cavity with a hypoechoic interior and hyperechoic walls is seen. This is not pericardial effusion; it is the left atrium!

LA thrombus
A catastrophic consequence of LA enlargement in cats is ATE.(2) In the PLAX4 and PSAX-base views, you can fan the ultrasound beam through the LA to identify a thrombus (See Video 4, Figure 4) or “smoke” (videos 5 & 6), which is thought to be the precursor to thrombus formation. Anticoagulation is recommended.

Spontaneous echocontrast, or “smoke”, typically has a swirling or streaming nature. Ensure you have set your gain level appropriately, as over-gaining can mimic smoke, though the swirling/ streaming nature would not be present.

Video 4: PSAX-base view showing a large, hyperechoic and rounded structure in the left auricle. This is a thrombus.


Figure 4: still image of video 4. The thrombus can be seen clearly in the left auricle. It measures approximately 2 cm x 2.5 cm, which is big! Ao, aorta, LA, left atrium, PV, pulmonic valve. Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC

Video 5: PLAX4 view. Spontaneous echocontrast, or “smoke”, is seen in the left atrium.

Video 6: PSAX-base view. Spontaneous echocontrast, or “smoke”, is seen in the left atrium.

References and further reading
1. Thomas L, Marwick TH, Bogdan AP, et al. Left atrial structure and function, and left ventricular diastolic dysfunction. J Am Coll Cardiol 2019;73(15):1961-1977
2. Kittleson MD, Côté E. The Feline Cardiomyopathies: 1. General concepts. J Feline Med Surg. 2021 Nov;23(11):1009-1027. doi: 10.1177/1098612X211021819. PMID: 34693806.
3. Duler L, Scollan KF, LeBlanc NL. Left atrial size and volume in cats with primary cardiomyopathy with and without congestive heart failure. J Vet Cardiol. 2019 Aug;24:36-47. doi: 10.1016/j.jvc.2019.04.003. Epub 2019 May 7.
4. Ware WA and Bonagura JD, editors. Cardiovascular disease in companion animals, 2nd edition. Boca Raton, FL, USA, CRC Press, Taylor & Francis Group, 2022.
5. Ward JL, DeFrancesco TC. The Role of Point-of-Care Ultrasound in Managing Cardiac Emergencies. Vet Clin North Am Small Anim Pract. 2023 Jul 7:S0195-5616(23)00090-6. doi: 10.1016/j.cvsm.2023.05.017.

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