What does a 1/6 short systolic ejection murmur (SEM) at the base indicate?

A 1/6 Short Systolic Ejection Murmur at the Base

A 1/6 short systolic ejection murmur (SEM) at the base most likely represents a benign or "innocent" murmur that does not indicate significant cardiac pathology. This type of murmur is typically caused by normal blood flow across the aortic or pulmonic outflow tracts and generally does not require further evaluation in the absence of other concerning findings. 1

Characteristics and Significance

Systolic ejection murmurs (SEMs) at the base have specific features:

• Timing: Start shortly after S1 and end before S2
• Configuration: Crescendo-decrescendo (diamond-shaped)
• Intensity: Grade 1/6 is very soft, barely audible
• Location: "Base" refers to the aortic or pulmonic areas

Common Causes of Grade 1/6 Short SEM at the Base:

1. Physiologic/Innocent Murmurs:

   • Increased flow rate due to:
     • Elevated cardiac output (pregnancy, thyrotoxicosis, anemia)
     • Arteriovenous fistula
   • Increased sound transmission through a thin chest wall
   • Normal flow across semilunar valves in children and young adults 1

2. Possible Pathologic Causes (though less likely with grade 1/6):

   • Very mild aortic or pulmonic stenosis
   • Early manifestation of a bicuspid aortic valve
   • Mild functional mitral or tricuspid regurgitation 1

Clinical Approach

When evaluating a patient with a 1/6 short SEM at the base:

Features Suggesting Benign Nature:

• Grade 1/6 intensity (very soft)
• Short duration
• No radiation to neck or axilla
• Normal S1 and S2
• Absence of other cardiac symptoms or signs
• Normal ECG

Features Warranting Further Evaluation:

• Associated symptoms (dyspnea, syncope, chest pain)
• Abnormal S2 (soft A2, fixed splitting)
• Ejection click
• Diastolic murmur
• Other signs of cardiac disease
• Family history of sudden cardiac death or valvular disease 1

Dynamic Auscultation

Dynamic maneuvers can help differentiate the origin:

• Respiration: Right-sided murmurs (pulmonic) increase with inspiration; left-sided murmurs (aortic) are louder during expiration
• Valsalva: Most innocent murmurs decrease in length and intensity
• Exercise: Innocent murmurs may become louder but maintain their benign characteristics 1

Clinical Pitfalls

1. Underestimating Significance: While most grade 1/6 SEMs are benign, they can occasionally represent early manifestations of valvular disease, especially in patients with risk factors.

2. Misinterpreting Associated Findings: Fixed splitting of S2 with a soft SEM may indicate an atrial septal defect; a soft or absent A2 may suggest severe aortic stenosis despite a soft murmur 1.

3. Missing Multiple Lesions: Studies show that clinical examination has limited ability to detect multiple cardiac lesions when present simultaneously 2.

4. Relying Solely on Murmur Intensity: A soft murmur can sometimes be present with significant pathology, particularly in patients with reduced cardiac output 2, 3.

While echocardiography is not routinely indicated for isolated grade 1/6 SEMs without other concerning features, it should be considered if there are any atypical features, associated cardiac findings, or if the patient is at high risk for valvular heart disease 3, 4.