Murmur at the 4th Left Intercostal Space: Hypertrophic Cardiomyopathy Until Proven Otherwise
A murmur with maximal intensity at the 4th left intercostal space (LIS) is the classic location for hypertrophic cardiomyopathy (HCM) and requires echocardiography to rule out this potentially life-threatening condition. 1
Differential Diagnosis by Location
The 4th left intercostal space is a critical landmark that distinguishes HCM from other causes of left ventricular outflow tract obstruction:
- Hypertrophic cardiomyopathy: Maximal thrill and murmur at 4th LIS 1
- Valvular aortic stenosis: Maximal at 2nd right intercostal space 1
- Supravalvular stenosis: Maximal at 1st right intercostal space 1
- Discrete subvalvular stenosis: Maximal at 2nd right intercostal space 1
Key Distinguishing Features of HCM
When examining a patient with a murmur at the 4th LIS, look for these specific findings that strongly suggest HCM:
Physical Examination Findings
- Carotid pulse: Brisk, jerky, with systolic rebound (not the slow-rising parvus et tardus of valvular AS) 1
- Fourth heart sound (S4): Commonly present 1
- Paradoxical splitting of S2: Rather common in severe cases 1
- Ejection click: Uncommon or absent (unlike valvular AS where it's usually present) 1
Dynamic Auscultation (Critical for Diagnosis)
- Valsalva maneuver: Murmur increases in intensity (opposite of all other causes of LV outflow obstruction) 1
- Standing: Murmur becomes louder 1
- Squatting: Murmur softens or disappears 1
- Post-VPB or long R-R interval in atrial fibrillation: Pulse pressure decreases (opposite of valvular stenosis where it increases) 1
These dynamic maneuvers are essential—a murmur that increases with Valsalva or standing requires immediate workup for HCM. 2
Mandatory Echocardiography Indications
Echocardiography is required for any murmur at the 4th LIS if:
- Grade 3 or louder 2
- Any symptoms present: syncope, angina, heart failure, or family history of sudden cardiac death 1, 2
- Abnormal dynamic auscultation: increases with Valsalva or standing 2
- Abnormal ECG or chest X-ray: ventricular hypertrophy or other abnormalities 2
- Abnormal physical findings: S4, paradoxical splitting, or abnormal carotid pulse 2
Even grade 1-2 murmurs at this location warrant echocardiography if any of the above features are present. 2
Treatment Approach
Treatment depends entirely on the underlying diagnosis confirmed by echocardiography:
If HCM is Confirmed
- Avoid dehydration and vasodilators (these worsen outflow obstruction)
- Beta-blockers or calcium channel blockers for symptomatic relief
- Implantable cardioverter-defibrillator (ICD) consideration for high-risk patients with family history of sudden death
- Septal reduction therapy (surgical myectomy or alcohol septal ablation) for severe symptomatic obstruction refractory to medical therapy
- Activity restriction from competitive athletics
If Other Pathology is Found
Treatment is directed at the specific valvular or structural abnormality identified. 1
Critical Pitfalls to Avoid
- Do not assume it's innocent: Unlike murmurs at the left sternal border or base, a murmur maximal at the 4th LIS is pathologic until proven otherwise 1
- Do not rely on intensity alone: Even soft murmurs at this location can represent significant HCM 2, 3
- Do not skip dynamic auscultation: The response to Valsalva and positional changes is the most important bedside diagnostic tool 1, 2
- Do not miss the family history: Sudden cardiac death in young relatives is a red flag requiring aggressive evaluation 4
- In elderly patients: The classic parvus et tardus pulse may be absent even with severe AS due to vascular aging, making location and dynamic maneuvers even more critical 1