Causes of Left Parasternal Holosystolic Murmur
A left parasternal holosystolic murmur is most commonly caused by ventricular septal defect (VSD) or tricuspid regurgitation (TR), with the key distinguishing feature being respiratory variation—TR increases with inspiration while VSD does not.
Primary Pathophysiologic Mechanism
Holosystolic murmurs occur when blood flows between chambers with widely different pressures throughout systole 1. At the left parasternal border specifically, this represents either:
- Flow from left ventricle to right ventricle (VSD)
- Regurgitant flow from right ventricle to right atrium (TR)
Main Causes
1. Ventricular Septal Defect (VSD)
- Creates flow from high-pressure left ventricle to low-pressure right ventricle throughout systole
- Murmur heard at lower left sternal border 2, 1
- In large VSDs with pulmonary hypertension, shunting may become insignificant at end-systole, limiting the murmur to early-midsystole 1
- Small muscular VSDs may also produce shortened murmurs 1
- Post-MI ventricular septal rupture presents acutely with hemodynamic compromise 3
2. Tricuspid Regurgitation (TR)
- Most common valvular cause at this location
- Critical distinguishing feature: TR murmurs increase with inspiration (right-sided murmurs respond to increased venous return) 1, 4
- Without pulmonary hypertension, TR may produce only an early systolic murmur rather than holosystolic 1, 5
- Causes include:
3. Congenitally Corrected Transposition (CCTGA)
- Systemic atrioventricular valve (morphologic tricuspid valve in systemic position) regurgitation produces holosystolic murmur at apex or lower left sternal border 2
- Consider when dextrocardia is present
- Often missed diagnosis—66% not diagnosed until adulthood 2
4. Mitral Regurgitation (MR) with Eccentric Jets
- While typically apical, eccentric jets directed toward the septum can radiate to left parasternal area
- Acute severe MR from papillary muscle rupture, chordal rupture, or endocarditis may have short or absent murmur due to rapid pressure equalization 7
Diagnostic Algorithm
Step 1: Assess respiratory variation
- Increases with inspiration → TR
- No change → VSD or MR
Step 2: Look for associated findings
- Prominent jugular venous CV waves, hepatomegaly, peripheral edema → TR 6, 8
- Fixed split S2 with midsystolic component → consider ASD with functional TR
- Dextrocardia → consider CCTGA 2
- Recent MI with hemodynamic collapse → VSD or papillary muscle rupture 7, 3
Step 3: Obtain echocardiography
- Transthoracic echo is the definitive diagnostic test 1
- TEE if TTE nondiagnostic, especially for endocarditis or acute MR 7
Critical Pitfalls to Avoid
Assuming all holosystolic murmurs are loud: Severe acute MR or TR may have soft or absent murmurs due to pressure equalization 7
Missing acute pathology: Post-MI VSD rupture or acute severe MR requires urgent intervention—look for hemodynamic instability with hyperdynamic (not depressed) LV function 7, 3
Overlooking endocarditis: Any new holosystolic murmur with fever, especially in injection drug users, warrants blood cultures and echocardiography 1, 4, 8
Ignoring respiratory variation: This simple bedside maneuver distinguishes right-sided (TR) from left-sided lesions (VSD, MR) 1, 4
Relying solely on murmur characteristics: Physical examination has limitations—echocardiography is necessary for definitive diagnosis and severity assessment 9
The distribution pattern on the chest wall relative to the 3rd left parasternal space is diagnostically valuable, with the "left lower sternal" pattern specifically suggesting significant TR 9.