1. Introduction (≈150 words)
Vertebral Left Atrial Size (VLAS) is a radiographic measurement used to objectively assess left atrial (LA) enlargement in dogs. It has become an important adjunct to echocardiography, particularly in the screening and staging of myxomatous mitral valve disease (MMVD). By relating left atrial size to vertebral body length, VLAS provides a standardized, repeatable metric that can be applied across patients and time points.
The purpose of VLAS calculation is to improve early detection of clinically relevant LA enlargement and to support evidence-based clinical decision-making, especially when echocardiography is unavailable or as part of a multimodal cardiac assessment. VLAS is now incorporated into ACVIM consensus guidelines, where a value ≥3.0 vertebrae is one of the criteria supporting Stage B2 MMVD.
This guide explains how to measure VLAS step by step on a right lateral thoracic radiograph, interpret the results, avoid common errors, and apply the measurement in clinical practice using both manual and digital tools.
2. Step-by-Step Measurement (≈500 words)
Radiographic Requirements
VLAS is measured on a right lateral thoracic radiograph obtained at peak inspiration. Proper positioning is essential: the sternum should be elevated to avoid cardiac compression, the thoracic limbs pulled cranially, and the spine and sternum aligned to minimize rotation. Poor positioning can significantly alter cardiac silhouette shape and invalidate measurements.
Identifying the Left Atrium
On a right lateral view, the left atrium is located in the caudodorsal aspect of the cardiac silhouette. Although the atrium itself is not directly visualized, its enlargement produces characteristic contour changes. The key landmark for VLAS measurement is the center of the most ventral aspect of the carina, where the mainstem bronchi diverge. This point approximates the dorsal border of the left atrium.
Step-by-Step Measurement Process
-
Locate the Carina
Identify the point where the trachea bifurcates into the left and right mainstem bronchi. Use the midpoint of this bifurcation as the dorsal starting point. -
Identify the Caudal Vena Cava–Aorta Intersection Locate the most caudal extent of the caudal vena cava where it crosses the ventral border of the aorta.
-
Draw the LA Measurement Line Draw a straight line from the center of the carina to the most caudal extent of the caudal vena cava where it crosses the ventral border of the aorta. This line spans the dorsal-to-caudal dimension of the left atrium and represents the measurement used for VLAS.
-
Transpose to the Vertebral Column
Without changing the line length, move it dorsally and align it parallel to the vertebral column, starting at the cranial edge of the fourth thoracic vertebra (T4). -
Count Vertebral Lengths
Measure how many vertebral body lengths the line spans, including partial vertebrae, to the nearest 0.1 vertebra. This value is the VLAS.
Measurement Tips
- Always start vertebral measurements at the cranial edge of T4, consistent with VHS methodology.
- Use the vertebral body length, not intervertebral disc spaces.
- Measure to one decimal place to improve repeatability.
- Confirm that the radiograph is not rotated by checking rib and vertebral superimposition.
Common Pitfalls and How to Avoid Them
- Incorrect radiographic view: VLAS should not be measured on left lateral or DV/VD projections.
- Misidentifying the carina: Avoid using the tracheal wall or bronchial margins instead of the bifurcation midpoint.
- Patient rotation: Rotation can artificially increase or decrease apparent LA size.
- Poor inspiration: Expiratory films may exaggerate cardiac size.
Careful attention to technique significantly improves the reliability of VLAS measurements.
3. Interpretation (≈400 words)
Normal Values
In most dogs, a VLAS <2.5 vertebrae is considered normal, indicating no radiographic evidence of left atrial enlargement. Values between 2.5 and 3.0 are often considered equivocal or borderline and should be interpreted in the context of clinical findings and other diagnostic results.
VLAS ≥3.0 and Stage B2 MMVD
A VLAS ≥3.0 vertebrae is one of the imaging criteria used to support a diagnosis of ACVIM Stage B2 MMVD, when combined with evidence of cardiomegaly (e.g., VHS ≥10.5, breed-adjusted) and echocardiographic changes. Dogs meeting Stage B2 criteria may benefit from early medical therapy, such as pimobendan, making accurate measurement clinically impactful.
Breed Considerations
Breed conformation influences cardiac appearance. Cavalier King Charles Spaniels (CKCS), Dachshunds, and other small breeds predisposed to MMVD are the populations in which VLAS has been most extensively validated. While VLAS is broadly applicable, clinicians should be cautious when interpreting values in:
- Deep-chested breeds, where cardiac orientation differs
- Brachycephalic breeds, where airway anatomy may obscure landmarks
- Very small or very large dogs, where vertebral proportions may vary
Breed-specific trends should be considered alongside absolute cutoffs.
When to Be Concerned
VLAS should raise concern when:
- Values are ≥3.0, especially if increasing on serial studies
- Borderline values are accompanied by a new or louder heart murmur
- Clinical signs or biomarkers (e.g., NT-proBNP) support cardiac remodeling
VLAS is not a replacement for echocardiography but serves as a valuable screening and monitoring tool when interpreted holistically.
4. Practice Examples (≈300 words)
Example 1: Normal Dog
A 7-year-old mixed-breed dog presents for pre-anesthetic screening. A right lateral thoracic radiograph shows a normal cardiac silhouette. The VLAS measurement from the carina to the caudal vena cava–aorta intersection transposes to 2.3 vertebrae starting at T4. This value falls within the normal range, and no radiographic evidence of left atrial enlargement is present. No further cardiac workup is pursued.
Example 2: CKCS with Enlarged Left Atrium
A 9-year-old Cavalier King Charles Spaniel with a grade 3/6 left apical systolic murmur undergoes thoracic radiography. VLAS measures 3.4 vertebrae, and VHS is also increased. Given the breed, murmur intensity, and VLAS ≥3.0, the dog meets imaging criteria consistent with Stage B2 MMVD. Echocardiography confirms LA and LV enlargement, and pimobendan therapy is initiated.
Example 3: Borderline Case
A 10-year-old small-breed dog with a stable grade 2/6 murmur has a VLAS of 2.8 vertebrae. The dog is asymptomatic, and VHS is within normal limits. This borderline VLAS prompts closer monitoring rather than immediate treatment. Repeat radiographs in six months show a stable measurement, supporting continued conservative management.
5. Digital vs Manual Measurement (≈150 words)
Manual VLAS measurement using printed radiographs or basic DICOM viewers is feasible but can be time-consuming and subject to interobserver variability. Digital measurement tools offer improved precision, consistency, and efficiency by allowing accurate line placement, automated vertebral scaling, and decimal-level measurements.
Integrated platforms such as RadAnalyzer streamline the process by combining VLAS calculation with other cardiac metrics, including VHS and clinical decision support. Digital tools also facilitate serial comparisons, making it easier to detect subtle progression over time. For busy practices or clinicians monitoring MMVD patients longitudinally, digital VLAS measurement enhances reliability and supports evidence-based cardiac staging.
Related Resources
- VLAS: Vertebral Left Atrial Size Guide — Comprehensive pillar page on VLAS measurement and interpretation.
- VHS and VLAS vs Echocardiography — Compare radiographic and echocardiographic cardiac assessment.
- MMVD in Dogs: Complete Guide — Understand how VLAS is used in MMVD staging.