1.10.9 Cardiac Murmur Definition

Cardiac Murmur Definition is a description of the prolonged, often whooshing or rasping sound produced by turbulent blood flow within the heart or great vessels, distinguishing it from the brief, discrete clicks of the normal heart sounds. Murmurs arise whenever blood flow deviates from smooth, laminar movement and becomes turbulent, a condition that can occur across an abnormal valve, through a narrowed vessel, or as a result of markedly increased flow velocity even across normal structures.

Physical Basis of Murmur Generation

Turbulence, rather than the volume of flow itself, is the essential physical requirement for murmur production.

Laminar Versus Turbulent Flow

Under normal conditions, blood moves through the heart and vessels in smooth, laminar layers that generate no audible sound. When flow velocity exceeds a critical threshold, or when it passes through an irregular or narrowed opening, the orderly layers break down into turbulent eddies.

Turbulence-Generating Conditions

Turbulence is promoted by increased flow velocity, decreased blood viscosity, and structural irregularities such as narrowed or incompetent valves, abnormal communications between chambers, or dilated vessels.

Classification by Timing

Murmurs are classified according to when in the cardiac cycle they occur relative to the heart sounds.

Systolic Murmurs

Systolic murmurs occur between S1 and S2, during ventricular systole, and are associated with conditions such as semilunar valve stenosis or atrioventricular valve regurgitation.

Diastolic Murmurs

Diastolic murmurs occur between S2 and the subsequent S1, during ventricular diastole, and are associated with conditions such as semilunar valve regurgitation or atrioventricular valve stenosis.

Continuous Murmurs

Continuous murmurs span both systole and diastole without interruption, typically arising from abnormal communications that maintain a persistent pressure gradient and flow throughout the cardiac cycle.

Classification by Mechanism

Beyond timing, murmurs are also characterized by the underlying valvular or structural mechanism producing them.

Stenotic Murmurs

Stenotic murmurs result from flow being forced through an abnormally narrowed valve opening, increasing flow velocity and generating turbulence.

Regurgitant Murmurs

Regurgitant murmurs result from incompetent valve closure, allowing retrograde flow through a gap that would normally be sealed.

Diagrammatic Summary

Clinical Relevance

Careful characterization of a murmur's timing, location, intensity, pitch, and radiation allows clinicians to infer the specific valve involved and the likely underlying mechanism, making murmur assessment a central component of the cardiovascular physical examination alongside evaluation of the primary heart sounds.