What is the Austin Flint murmur?

Austin Flint Murmur

The Austin Flint murmur is a low-pitched, rumbling diastolic murmur heard at the left ventricular apex in patients with severe aortic regurgitation, occurring without mitral stenosis and not accompanied by an opening snap. 1

Characteristics of Austin Flint Murmur

Timing and Quality

• Occurs in mid-diastole or presystole (late diastole)
• Low-pitched, rumbling sound
• May be either middiastolic or presystolic in timing 1
• Best heard at the left ventricular apex
• Unlike mitral stenosis, there is no opening snap preceding the murmur 1

Pathophysiology

Several theories explain the mechanism:

1. Aortic Regurgitant Jet Theory: The murmur is generated when the aortic regurgitant jet collides with the anterior mitral valve leaflet 2
2. Increased Flow Theory: The murmur results from increased flow across a functionally narrowed mitral valve orifice due to the regurgitant volume 3
3. Combined Mechanism: In some cases, the murmur may be generated by aortic regurgitant flow alone, not requiring rapid mitral inflow 3

Clinical Context

• Only occurs in the setting of significant aortic regurgitation 4
• Severity of aortic regurgitation is greater in patients with the Austin Flint murmur than those without it 3
• The direction of aortic regurgitant flow does not correlate with the presence of the murmur 3

Differential Diagnosis

Key Distinctions from Mitral Stenosis

• Opening Snap: Present in mitral stenosis, absent in Austin Flint murmur 1
• Etiology: Austin Flint occurs with normal mitral valve structure but in the presence of aortic regurgitation
• Hemodynamics: Intracardiac sound studies show the Austin Flint murmur is maximal in the left ventricular inflow tract and not recordable in the left atrium 5

Other Diastolic Murmurs to Consider

• True mitral stenosis
• Increased flow across normal mitral valve in conditions like:
  • Ventricular septal defect
  • Patent ductus arteriosus
  • Severe mitral regurgitation 1, 6
• Atrial myxoma 1
• Mid-ventricular obstructive hypertrophic cardiomyopathy (less common) 7

Clinical Significance

Diagnostic Value

• Presence of an Austin Flint murmur suggests severe aortic regurgitation 4
• The murmur may be detected on intracardiac phonocardiography even when not audible externally 5

Response to Maneuvers

• Being a left-sided murmur, it is typically more audible during expiration
• Decreases with Valsalva maneuver
• Increases with exercise (both isotonic and isometric)
• Decreases with standing, increases with squatting 1

Pitfalls in Diagnosis

• The murmur may be confused with true mitral stenosis
• Careful auscultation for the absence of an opening snap is critical for differentiation
• The intensity of the murmur does not always correlate perfectly with the severity of aortic regurgitation
• In some patients, the murmur may be subtle or difficult to detect on external examination 5

Understanding the Austin Flint murmur is important for accurate diagnosis of valvular heart disease and avoiding unnecessary interventions for presumed mitral stenosis when the pathology is actually severe aortic regurgitation.