VLAS (Vertebral Left Atrial Size) in Dogs: Complete Measurement Guide

1. Introduction

Assessment of cardiac size is a cornerstone of veterinary cardiology, particularly in dogs with suspected or confirmed heart disease. Among the cardiac chambers, the left atrium (LA) plays a uniquely important role as an early indicator of hemodynamic burden and disease progression. Enlargement of the left atrium reflects chronic volume overload, most commonly associated with myxomatous mitral valve disease (MMVD), and closely correlates with the risk of developing congestive heart failure (CHF).

Traditionally, echocardiography has been considered the gold standard for evaluating left atrial size, using measurements such as the left atrial-to-aortic root ratio (LA:Ao). However, access to echocardiography may be limited by availability, cost, or the need for specialist expertise. Thoracic radiography, on the other hand, is widely available in general practice and already routinely performed in many cardiac patients.

The Vertebral Left Atrial Size (VLAS) measurement was developed to bridge this gap. Introduced as a radiographic, objective, and reproducible method for assessing left atrial size, VLAS provides a standardized way to quantify left atrial enlargement using familiar vertebral scaling—similar to the widely accepted Vertebral Heart Size (VHS). Rather than relying on subjective interpretation of left atrial bulging or the classic “backpack sign,” VLAS offers numerical data that can be trended over time.

Importantly, VLAS is not intended to replace echocardiography. Instead, it serves as a complementary diagnostic tool, enhancing the diagnostic and prognostic value of thoracic radiographs. Numerous studies have demonstrated strong correlations between VLAS and echocardiographic indices of left atrial size, validating its clinical utility.

As left atrial enlargement closely parallels disease severity, progression, and prognosis, particularly in MMVD, accurate and repeatable assessment of LA size is critical. This guide provides a comprehensive, step-by-step explanation of VLAS measurement, interpretation, and clinical application, enabling veterinarians to confidently integrate VLAS into everyday practice.

2. Why Left Atrial Size Matters

The left atrium is far more than a passive conduit between the pulmonary veins and the left ventricle. It serves as a reservoir, conduit, and booster pump, modulating left ventricular filling throughout the cardiac cycle. Any pathology that increases left ventricular filling pressures—most notably mitral valve insufficiency—inevitably impacts left atrial size and function.

Left Atrial Enlargement as a Marker of Volume Overload

In dogs with mitral regurgitation, blood leaks backward from the left ventricle into the left atrium during systole. Over time, this chronic volume overload causes progressive left atrial dilation. Unlike transient changes in ventricular size that can occur with hydration status or stress, left atrial enlargement reflects sustained hemodynamic stress, making it a reliable indicator of disease burden.

Connection to Myxomatous Mitral Valve Disease (MMVD)

MMVD is the most common acquired heart disease in dogs, particularly in small and toy breeds. Numerous studies have shown that left atrial size is one of the strongest predictors of clinical outcome in MMVD. Dogs with larger left atria are significantly more likely to progress from asymptomatic disease (ACVIM Stage B) to overt CHF (Stage C).

Prognostic Significance

Left atrial enlargement is not merely a diagnostic finding—it carries profound prognostic implications:

Larger LA size correlates with shorter time to onset of CHF
Increased risk of pulmonary edema
Higher likelihood of arrhythmias, particularly atrial fibrillation in larger breeds
Reduced overall survival time

Why LA Size Predicts CHF Development

CHF develops when compensatory mechanisms can no longer offset elevated filling pressures. Because the left atrium directly reflects chronic diastolic burden, its enlargement precedes many clinical signs of failure. This makes left atrial size one of the earliest and most reliable predictors of impending CHF.

For these reasons, accurate assessment of left atrial size is central to modern canine cardiology—and this is precisely where VLAS has proven invaluable.

3. What Is VLAS? Technical Definition

Vertebral Left Atrial Size (VLAS) is a radiographic measurement of left atrial dimension obtained from a right lateral thoracic radiograph. It quantifies the size of the left atrium by measuring a specific anatomic distance and expressing it in vertebral body units, analogous to the method used for VHS.

Objective Radiographic Quantification

Unlike subjective radiographic signs—such as left atrial bulging, elevation of the mainstem bronchi, or loss of the cardiac waist—VLAS provides a numerical value. This objectivity improves consistency between observers and allows for reliable serial monitoring.

Measurement Principle

VLAS measures the distance between:

The ventral-center of the carina (tracheal bifurcation)
The most caudal aspect of the left atrium

This distance is then transposed onto the vertebral column starting at the cranial edge of T4 and measured caudally, parallel to the vertebral canal. The final value is expressed in vertebral units (v).

Validation Against Echocardiography

Multiple studies have demonstrated a strong correlation between VLAS and echocardiographic LA:Ao ratios, validating VLAS as a reliable surrogate marker for left atrial size. While echocardiography remains more precise, VLAS has shown excellent clinical relevance, particularly for identifying dogs with clinically significant left atrial enlargement.

In summary, VLAS represents a validated, objective, and accessible method for radiographic assessment of the left atrium.

4. How to Measure VLAS Step-by-Step

Accurate VLAS measurement requires attention to positioning, anatomy, and technique. The following step-by-step guide outlines the recommended method.

Prerequisite: Right Lateral Thoracic Radiograph

Patient positioned in true right lateral recumbency
Forelimbs extended cranially
Neck extended to avoid tracheal flexion
Ideally obtained at end-expiration

Step 1: Identify the Carina

The carina is the anatomic landmark where the trachea bifurcates into the left and right mainstem bronchi.

Key features:

Appears as a radiolucent circular or ovoid structure
Located at the junction of the left and right mainstem bronchi
Best visualized just dorsal to the cardiac silhouette

Accurate identification of the carina is critical, as it serves as the cranial starting point for VLAS measurement.

Step 2: Identify the Caudal Aspect of the Left Atrium

The caudal border of the left atrium is identified at:

The point where the left atrium meets the dorsal border of the caudal vena cava
Typically just cranial to the diaphragm

This point represents the maximum caudal extent of the left atrium on the lateral projection.

Step 3: Draw the Measurement Line

Using digital calipers or measurement tools:

Draw a straight line from the ventral-center of the carina
Extend the line caudally to the most caudal aspect of the left atrium

Ensure the line reflects the long-axis dimension of the left atrium.

Step 4: Transfer the Measurement to the Vertebral Column

Copy the measured line
Place it starting at the cranial edge of the fourth thoracic vertebra (T4)
Align it parallel to the vertebral canal
Extend caudally along the vertebral bodies

Step 5: Count Vertebral Bodies

Count the number of vertebral bodies spanned by the line
Round to the nearest tenth
Express the final result in vertebral units (v)

Example:
A measurement spanning 2.6 vertebrae = VLAS = 2.6 v

Tips for Accurate Identification

Use high-quality, properly exposed radiographs
Avoid rotated or obliqued images
Confirm bronchial anatomy when identifying the carina
Compare with prior studies when monitoring progression

Common Measurement Errors

Misidentifying the carina
Including the pulmonary veins instead of the left atrium
Using the cranial instead of caudal LA border
Measuring on left lateral radiographs (less validated)

Consistent technique is essential for reliable results.

5. Normal VLAS Values & Clinical Cutoffs

Reference Interval in Healthy Dogs

Studies in clinically normal dogs have established the following reference interval:

Normal range: 1.4–2.2 v
Median value: 1.9 v

These values apply to the general canine population and provide the foundation for clinical interpretation.

Published Values by Study

Malcolm et al.:
- Median VLAS: 2.1 v
- Interquartile range (IQR): 1.8–2.3 v
Radiographic reference ranges:
- 1.8–2.3 v

Minor variations between studies likely reflect population differences and measurement techniques.

Clinical Cutoffs and Their Meaning

>2.3 v
- Suggestive of left atrial enlargement
- Warrants further evaluation and monitoring
≥2.5 v
- High specificity (~84%) for LA:Ao ≥1.6
- Strongly indicative of clinically relevant LA enlargement
≥3.0 v
- Accepted ACVIM Stage B2 criterion
- Can be used as an alternative to echocardiography when echo is unavailable

Sensitivity and Specificity

VLAS demonstrates:

Good sensitivity for detecting moderate-to-severe LA enlargement
Increasing specificity at higher cutoff values

This makes VLAS particularly useful for identifying dogs at risk of CHF.

Correlation with VHS

Moderate positive correlation with VHS (r = 0.53)
Reflects that overall cardiomegaly and LA enlargement often—but not always—progress together

Importantly, VLAS adds independent diagnostic value beyond VHS alone.

6. Breed-Specific VLAS Values

Breed-related differences in thoracic conformation and cardiac size necessitate careful interpretation of VLAS values.

Breed	Mean VLAS	95% Range
General Population	1.9	1.4–2.2
Chihuahua	1.8	1.3–2.1
Cavalier King Charles Spaniel	1.79	1.68–1.9
Maltese	2.0	1.8–2.1
Border Terrier	2.15	1.6–2.7

Why Breed-Specific Values Matter

Some breeds naturally have:

Slightly larger left atria
Different thoracic shapes that affect radiographic appearance

Without breed context, normal dogs may be misclassified as abnormal—or vice versa.

Breeds with Higher Baseline VLAS

Terrier breeds and some brachycephalic dogs may have higher baseline VLAS values, underscoring the importance of:

Serial measurements
Clinical correlation
Integration with other findings

7. VLAS in MMVD Staging

ACVIM Consensus Guidelines Overview

The ACVIM guidelines for MMVD staging emphasize early identification of dogs at risk for CHF.

Stage B2 Criteria

Dogs are classified as Stage B2 if they have:

No clinical signs of CHF
Evidence of cardiac enlargement, defined as:
- VHS ≥11.5 OR
- VLAS ≥3.0 v

Using VLAS When Echocardiography Is Unavailable

VLAS provides a validated alternative when:

Echocardiography is not accessible
Owners decline advanced diagnostics
Screening in general practice settings

Monitoring Disease Progression

Serial VLAS measurements allow clinicians to:

Track left atrial enlargement over time
Identify progression from B1 to B2
Optimize timing of therapeutic intervention

Predictor of CHF Onset

Increasing VLAS values are strongly associated with:

Shorter time to CHF
Higher likelihood of pulmonary edema

EPIC Study Implications

The EPIC study demonstrated that early treatment of Stage B2 dogs improves outcomes. VLAS helps identify these dogs accurately, supporting evidence-based therapy.

8. VLAS vs Echocardiography

Echocardiography: The Gold Standard

Direct visualization of cardiac chambers
LA:Ao ratio provides precise LA assessment
Evaluates valve morphology and function

When Radiographic VLAS Is Preferred

Limited access to echocardiography
Screening or monitoring in general practice
Serial assessments where cost is a concern

Correlation Between VLAS and LA:Ao

Studies consistently show:

Moderate to strong correlation
Increasing VLAS parallels increasing LA:Ao

Advantages of VLAS

Widely accessible
Low cost
Easily repeatable
High inter-observer reliability

Limitations

Less precise than echocardiography
Two-dimensional projection of a three-dimensional structure
Influenced by positioning and radiographic quality

VLAS should be viewed as complementary, not competitive, with echocardiography.

9. Technical Considerations

Reliability

Intra- and inter-observer ICC ≥0.91
Demonstrates excellent repeatability when standardized technique is used

Importance of Positioning

True right lateral positioning is essential
Rotation can falsely alter measurements

Respiratory Phase

End-expiratory films are preferred
Inspiratory films may slightly increase apparent cardiac dimensions

Radiographic Quality

Adequate exposure
Clear visualization of bronchi and vertebrae
Minimal motion artifact

10. VLAS in Cats

Currently, VLAS has not been validated for feline patients.

Why Not?

Different thoracic anatomy
Variable cardiac orientation
Feline cardiomyopathies affect the LA differently

Ongoing Research

Studies are exploring radiographic LA assessment in cats, but no standardized VLAS equivalent exists yet.

Current Alternatives

Echocardiographic LA diameter
LA:Ao ratio
Subjective radiographic assessment with caution

11. FAQs

When should I measure VLAS vs VHS?

VHS: Overall cardiac enlargement
VLAS: Specifically left atrial size
Ideally, both should be measured together.

Can VLAS replace echocardiography?

No. VLAS is a screening and monitoring tool, not a replacement for echocardiography.

How often should I repeat VLAS measurements?

Every 6–12 months in stable patients
More frequently if clinical status changes or progression is suspected

12. Conclusion

Vertebral Left Atrial Size (VLAS) has emerged as an essential radiographic tool in modern veterinary cardiology. By providing an objective, validated, and accessible method to assess left atrial size, VLAS enhances the diagnostic and prognostic value of thoracic radiographs—particularly in dogs with MMVD.

When measured correctly and interpreted thoughtfully, VLAS supports early disease staging, informed treatment decisions, and effective monitoring of progression. Integrated with tools like RadAnalyzer, VLAS becomes even more powerful, streamlining measurement and improving consistency across practices.

Incorporating VLAS into routine cardiac evaluation empowers veterinarians to deliver earlier, more precise, and more proactive cardiac care.