VLAS — Vertebral Left Atrial Size

Reference diagram for marking the VLAS line on a lateral thoracic radiograph

Lateral Thoracic Radiograph — Right Lateral View
SPINE (vertebrae) CARINA (tracheal bifurcation) Left Atrium LA bottom (caudal LA border) VLAS DIAPHRAGM STERNUM DORSAL ↑ VENTRAL ↓ ← CRANIAL CAUDAL →

How to mark VLAS

Click 1
Carina — the point where the trachea splits into the left and right bronchi. On a lateral view it looks like a fork or Y-shape dorsal (above) the heart. Click the tip of the fork.
Click 2
Bottom of Left Atrium — the most caudal (rear) and ventral (lower) border of the left atrium. The left atrium is the dorsal (upper-rear) bulge of the cardiac silhouette. Click the lowest point of that bulge.
Tip
Zoom in first using Zoom In before clicking — the carina is a small structure and precision matters. Use Fullscreen for the clearest view.
Formula
VLAS = VLAS line (mm) ÷ T4 (mm)
Result is in vertebral units (v). The T4 vertebra length is your unit of measurement — same as VHS.
Normal Ranges (dogs)
Normal ≤ 2.3 v
Borderline 2.3 – 2.8 v
Enlarged LA > 2.8 v
Cats: normal ≤ 1.9 v