The left anterior descending artery (LAD), also known as the anterior interventricular artery, is the largest and most important of the coronary arteries. It supplies approximately 40-50% of the left ventricular myocardium, including the interventricular septum and the apex.
Origin and Course
The LAD arises from the bifurcation of the left main coronary artery, continuing in the same direction as the LMCA. It descends in the anterior interventricular groove toward the cardiac apex.
Course segments:
- Proximal LAD: From origin to the first major septal perforator
- Mid LAD: From first septal to the second diagonal branch
- Distal LAD: From second diagonal to the apex
The LAD typically wraps around the apex to supply a variable portion of the inferior apical wall. In approximately 70-80% of hearts, the LAD extends to the diaphragmatic surface (wrap-around LAD).
Segmental Anatomy
Proximal LAD
The proximal segment extends from the LMCA bifurcation to the origin of the first major septal perforator. This segment:
- Gives off the first septal perforator (most important branch to the conduction system)
- May give off the first diagonal branch
- Lies deep in the epicardial fat of the anterior interventricular groove
- Sometimes courses intramyocardially (myocardial bridge)
Mid LAD
The mid segment extends from the first septal perforator to the second diagonal branch. This segment:
- Gives off the second and third septal perforators
- Gives off diagonal branches to the left ventricular free wall
- May have a superficial or deep course
- Is the most common site for LAD stent placement
Distal LAD
The distal segment extends from the second diagonal branch to the apex. This segment:
- Tapers significantly in diameter
- Supplies the apical myocardium
- May anastomose with posterior descending artery branches
- Often wraps around the apex
Branches
Septal Perforators
The septal perforators arise from the LAD at approximately 90-degree angles to penetrate the interventricular septum.
Characteristics:
- 3-10 perforators in total
- First septal perforator is the largest (diameter 1-2 mm)
- Supply the anterior two-thirds of the interventricular septum
- Supply the atrioventricular bundle (Bundle of His) and the proximal bundle branches
- The first septal perforator is a critical target in alcohol septal ablation for hypertrophic cardiomyopathy
Diagonal Branches
Diagonal branches arise from the LAD at acute angles and course diagonally across the anterior left ventricular free wall.
Characteristics:
- 2-6 diagonal branches
- Numbered sequentially (D1, D2, D3, etc.)
- D1 (first diagonal) is usually the largest
- Supply the anterolateral left ventricular wall
- May arise from the LMCA bifurcation as a ramus intermedius
Other Branches
Right ventricular branches: Small branches to the right ventricular free wall near the septum
LAD to pulmonary artery: Tiny branches to the pulmonary trunk (vasa vasorum)
Dimensions
| Segment | Typical Diameter |
|---|---|
| Proximal LAD | 3.0-4.5 mm |
| Mid LAD | 2.0-3.5 mm |
| Distal LAD | 1.0-2.0 mm |
| First septal perforator | 1.0-2.0 mm |
| Diagonal branches | 1.5-3.0 mm |
Myocardial Territory Supplied
| Territory | Supply |
|---|---|
| Anterior left ventricular wall | Diagonal branches |
| Anterior two-thirds of septum | Septal perforators |
| Apex | Distal LAD |
| Anterior papillary muscle (mitral) | Diagonal branches |
| Bundle of His and proximal bundle branches | First septal perforator |
| Moderator band | Proximal LAD (via septal) |
| Anterior right ventricle (minor) | Small RV branches |
Anatomic Variants
Myocardial Bridging
The LAD may course intramyocardially (within the myocardium) for a variable length, most commonly in the mid segment.
Incidence: 15-30% of hearts (by autopsy), 1-5% by angiography
Characteristics:
- The bridge is a band of myocardium overlying the artery
- Systolic compression of the bridged segment
- Diastolic relaxation allows normal flow
- Usually benign, but can cause ischemia in hypertrophic states
Wrap-Around LAD
In 70-80% of hearts, the LAD extends around the apex to supply the inferior apical wall.
Types:
- Type I: LAD wraps around apex by < 1 cm
- Type II: LAD wraps around apex by 1-3 cm
- Type III: LAD wraps around apex by > 3 cm (significant inferior wall supply)
Dual LAD
Rare variant where two LADs exist:
- One short LAD ending in the proximal anterior interventricular groove
- One longer LAD arising separately and running parallel
Clinical Significance
LAD Myocardial Infarction (Anterior STEMI)
The LAD is the most common culprit vessel in ST-elevation myocardial infarction (STEMI).
Presentation:
- ST elevation in precordial leads (V1-V4)
- Anteroseptal: V1-V2 (septal involvement)
- Anterior: V3-V4
- Extensive anterior: V1-V6, I, aVL
- High-risk features: New left bundle branch block, Q waves
Complications by Segment:
| Segment Occluded | Complications |
|---|---|
| Proximal LAD (before first septal) | Massive anterior MI, heart failure, cardiogenic shock, complete heart block, ventricular septal rupture |
| Mid LAD (after first septal) | Anterior MI, preserved septal function, lower risk of heart block |
| Distal LAD | Apical MI, smaller infarct size |
| Diagonal branch | Limited anterolateral MI |
The Widowmaker
Proximal LAD occlusion is often called the widowmaker due to:
- Large myocardial territory at risk (40-50% of LV mass)
- High risk of cardiogenic shock
- High mortality without prompt revascularization
- Door-to-balloon time is critical (< 90 minutes)
Stenting Considerations
Bifurcation lesions: LAD-diagonal bifurcations are common and technically challenging Ostial LAD: May require precise stent placement Myocardial bridge: Stenting is contraindicated (risk of perforation) CTO (chronic total occlusion): LAD CTO recanalization has high success rates
Collateral Circulation
The LAD can receive collateral flow from:
- Posterior descending artery (via septal collaterals)
- Obtuse marginal branches (via epicardial collaterals)
- Right coronary artery (via conus branch to septal perforators)
- Left circumflex artery
First Septal Perforator in Hypertrophic Cardiomyopathy
Alcohol septal ablation targets the first septal perforator to create a controlled septal infarction, reducing the left ventricular outflow tract gradient.