January 2024

In this VETgirl online veterinary continuing education blog, Dr. Christopher Kennedy, DACVECC, DECVECC discusses how to assess the left atrium in dogs 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.

By Dr. Christopher Kennedy, DACVECC, DECVECC

Assessing the Left Atrium in Dogs on Focused Cardiac Ultrasound

The goals of Focused Cardiac Ultrasound (FCU) in canine 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 for assessing the left atrium on FCU are:

  • PLAX4
  • PSAX-base

The left atrium (LA)
The LA is upstream of the left ventricle (LV) and downstream of the pulmonary circuit. The pressure in the LA is the filling pressure for the left ventricle and the exit pressure of the pulmonary circuit.(1) Increases in LA pressure can occur because of LV diastolic dysfunction (which necessitates higher LV filling pressure), mitral regurgitation or fluid overload (which both volume overload the LA). Increased LA pressure increases the exit pressure of the pulmonary circuit and leads to pulmonary edema.(1)

The LA has three functional roles: reservoir, conduit and pump.(2,3) In systole, the LA is a reservoir for blood emptying from the pulmonary circuit, storing it until the mitral valve opens (early diastole), when the LA becomes a conduit for blood flowing into the LV to provide the next stroke volume. Just after the P-wave, the LA contracts (late diastole) and blood is pumped into the LV – this function is lost on atrial fibrillation, explaining the decrease in cardiac output.

In response to chronic volume or pressure elevations, the LA dilates.(2) The degree of dilation is related to the chronicity and severity. An enlarged LA suggests increased LA pressure and a normal LA suggests normal LA pressure: This is not always true. Finding LA enlargement via FCU indicates that a) the left heart might be fluid intolerant and b) if pulmonary edema is present, pressure-volume overload of the LA is potentially responsible. All findings must be integrated with the clinical examination.

Left atrial-to-aortic ratio (LA:Ao)
The LA can be indexed to the Ao to provide an easier way of interpreting the LA size. It is measured using the PSAX-base view (See Video 1, Figure 1). Normal values for dogs are < 1.6.(4) The LA:Ao is commonly used and commonly misused. Generally, we measure it in early diastole and use the closure of the aortic valve to indicate this timing. We lack consensus on how and when in the cardiac cycle to make measurements (See Figure 2). Moreover, it is subject to acquisition and measurement error, making it unreliable (See Figure 3). Even with images obtained by a cardiologist, the LA:Ao has poor inter-overserved agreement.(5)

Video 1: PSAX-base view.

Figure 1: a) PSAX-base showing the LA:Ao. LA, left atrium, Ao, aorta, PV, pulmonic valve, TV, tricuspid valve, Vein, pulmonary vein. The video has been frozen immediately after the closure of the aortic valve. The left atrium is largest in early diastole, just before the mitral valve opens to empty blood into the left ventricle. b) The left atrium is highlighted and a measurement is proposed. With such a prominent pulmonary vein it can be difficult to know exactly where to measure the LA. Importantly, the LA:Ao reported is small (0.92), which in FCU means this LA is definitely not enlarged. Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC.

Figure 3: a) and b) show two different ways that I measured this LA. c) shows a slightly different view and the measurement the cardiologist ultimately made. The measurement in a) was made using an imaginary line to accommodate for the presence of the pulmonary vein and b) was taken to the visible atrial endocardium. Neither were consistent with the cardiologist’s measurement: when reviewing other views (i.e., PLAX4) the LA did not seem particularly enlarged, which is one reason the cardiologist favored the lower measurement and highlights that one potentially mis-measured view should not be overinterpreted. 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 Fgure 4). Measurements are made just before the opening of the mitral valve, at the start of diastole, parallel to the mitral annulus. It is perhaps superior to the LA:AO for three reasons: a) it uses the PLAX4, which is the first and most useful view of the FCU exam, b) it requires correct acquisition of only one structure (the LA), c) it requires only one measurement. Inter- and intra-observer agreement was excellent for cardiologist-obtained images,(5) though it needs to be studied further in the hands of non-cardiologists. For dogs, the interpretation can be a little tricky, as it needs to be “normalized” (similar to being indexed) to the body weight:

nLAD = LAD (cm) / weight 0.309
Normal nLAD < 1.6 for dogs(5)

Figure 4: PLAX4 showing the LAD measurement. An inner-edge-to-inner-edge technique is used, that is you start at the blood-endothelial interface of the interatrial septum and measure to the blood-endothelial interface of the opposite atrial wall. nLAD, normalized left atrial diameter. Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC.

Left atrial volume (LAV)
LAV is also measured using PLAX4(5) just before the opening of the mitral valve in early diastole (See Figure 5). The internal surface of the LA, at the blood-endothelial interface, is traced and the echo machine uses a formula to calculate LAV(5) – check to see if your machine has this function. LAV < 1.5 ml/Kg is expected in normal dogs.(5) In dogs with mitral valve disease, values < 2.25 ml/Kg mean left congestive heart failure less likely.(3)

Figure 5: a) PLAX4 showing the LAV measurement in a dog with degenerative mitral valve disease ACVIM stage B1 (27 Kg, 1.4 ml/Kg). b) LAV measurement in a dog with advanced degenerative mitral valve disease and massive volume mitral regurgitation. It can be difficult to measure the LAV accurately in advanced LA disease, as the atrial myocardium can hypertrophy leading to an unclear blood-endothelial interface. However, the reported volume here was 8.5 ml/Kg, which is extremely high. Photo courtesy of Dr. Christopher Kennedy, DACVECC, DECVECC.

The “absolutely massive” scale
With FCU, we are looking to support/refute our clinical suspicions, gauge fluid (in)tolerance and contemplate elevated LA pressure as the cause of pulmonary edema. Given our patients are often critical plus the difficulties of acquiring and measuring accurate data, errors happen. Avoid getting bogged down in the little numbers – an LA:Ao or an nLAD of 1.7 might not be relevant; an LA:Ao or nLAD of > 2 is always relevant.(6) Think big, think obvious: use the “absolutely massive” scale (videos 2 & 3).

Video 2: PLAX4 view showing an “absolutely massive” left atrium.

Video 3: PSAX-base view showing an “absolutely massive” left atrium. The image quality is not ideal, which could lead to measurement error. However, in FCU, we do not need to measure this LA:Ao – we should record a video clip and label it with the patient name and date.

References and further reading
1. Bowcock EM and Mclean A. Bedside assessment of left atrial pressure in critical care: a multifaceted gem. Crit Care 2022;26(1):247.
2. 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
3. Höllmer M, Willesen JL, Tolver A, et al. Left atrial volume and function in dogs with naturally occurring myxomatous mitral valve disease. J Vet Cardiol 2017;19(1):24-34.
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. Visser LC, Ciccozzi MM, Sintov DJ, et al. et al. Echocardiographic quantitation of left heart size and function in 122 healthy dogs: A prospective study proposing reference intervals and assessing repeatability. J Vet Intern Med 2019;33(5):1909-1920.
6. 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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