What type of ventricular septal defect (VSD) is expected in a patient with a late inferior ST-elevation myocardial infarction (STEMI)?

Ventricular Septal Defect in Late Inferior STEMI

In a patient with late inferior STEMI, you should expect an inferior-basal (posterior septal) ventricular septal defect, which carries significantly higher surgical mortality than anterior-apical defects. 1

Location and Anatomic Characteristics

Inferior STEMI produces inferior-basal (posterior septal) VSDs, which occur in the lower portion of the ventricular septum near the base of the heart. 1 This contrasts sharply with anterior STEMI, which typically produces anterior-apical defects in the muscular septum. 1

The inferior-basal location results from occlusion of the right coronary artery (or less commonly, the left circumflex), leading to infarction of the inferior wall and adjacent posterior septum. 2, 3, 4

Critical Prognostic Implications

Mortality risk is substantially higher for inferior-basal defects compared to anterior-apical defects. 1 The surgical mortality for inferior-basal VSDs ranges from 20% to 87%, with the higher end of this range specifically associated with posterior location and cardiogenic shock. 1

The worse prognosis stems from:

• More friable, necrotic myocardium in the basal septum making surgical repair technically challenging 2
• Greater likelihood of right ventricular involvement and dysfunction 5
• Higher rates of hemodynamic instability and cardiogenic shock 4

Timing Considerations

The term "late" in your question is critical. Post-MI VSD typically occurs within the first 24 hours in patients receiving fibrinolytic therapy, but can present up to one week after infarction. 1 In non-reperfused inferior MI (which may be more common given delayed presentation), rupture can occur on days 3-7 when myocardial necrosis is maximal but before significant scar formation. 3

Clinical Presentation

Expect a loud systolic murmur with heart failure or cardiogenic shock, depending on defect size and degree of biventricular dysfunction. 1 The murmur may be accompanied by a palpable thrill. 6 However, in severe cases with equalization of ventricular pressures or profound shock, the murmur may be soft or absent. 6

Right ventricular involvement is common with inferior MI, compounding hemodynamic compromise. 5

Diagnostic Approach

Transthoracic echocardiography establishes the diagnosis and should be performed emergently when VSD is suspected based on new murmur and clinical deterioration. 1, 5 Echocardiography will demonstrate:

• The inferior-basal septal defect location 2, 7
• Left-to-right shunt with color Doppler 6
• Biventricular function and degree of RV involvement 5
• Associated complications (mitral regurgitation, RV infarction) 1

Immediate Management

Emergency surgical repair is necessary even in hemodynamically stable patients because the rupture site can expand abruptly, causing sudden collapse. 1 The ACC/AHA guidelines are unequivocal on this point. 1

Temporizing measures while preparing for surgery:

• Immediate IABP insertion for hemodynamic stabilization 5, 2, 4
• Inotropic support (dobutamine or milrinone) 5
• Vasodilators if blood pressure tolerates 5
• Avoid excessive volume loading which worsens left-to-right shunt 5

Concomitant coronary revascularization should be performed during VSD repair when feasible, as this improves outcomes. 2

Surgical Approach

The trans-aneurismal approach through the infarcted inferior wall is typically used for inferior-basal VSDs. 3 The repair involves:

• Debridement of necrotic tissue 2
• Patch closure using bovine pericardium or Dacron 2
• Reinforcement with felt strips given friable tissue 2

Despite optimal surgical technique, expect 30-day mortality of 11-40% for inferior-basal defects, with higher rates in patients presenting with cardiogenic shock. 2, 4

Common Pitfalls

Do not delay surgery waiting for "stabilization" beyond what is needed for IABP insertion and operating room preparation—the defect can extend catastrophically at any moment. 1

Do not miss concomitant papillary muscle rupture, which also occurs more frequently with inferior MI (posteromedial papillary muscle has singular blood supply). 1 Both complications can present with new systolic murmur. 1

Residual shunts are common post-repair (occurring in approximately 30% of cases), but most are small and do not require reintervention if patients remain asymptomatic. 2