Physical Causes

1. What is the cause of murmurs?

ANS: Sufficient flow in the cardiovascular system to generate enough turbulent energy in the walls of the heart or blood vessels to produce sounds. High-energy turbulence can be produced by either obstruction to blood flow or flow into a distal chamber of larger diameter than the proximal one.

Causes of turbulence. In A, turbulence occurs in fluids flowing at high velocity through tubes of uniform caliber, according to the formula for Reynolds numbers. In B and C, either a local obstruction or fluid flowing into a channel of larger diameter can produce turbulence at much less flow velocity. In D, a combined effect of obstruction and abrupt increase in caliber, as in poststenotic dilatation, produces turbulence at relatively low flow velocity.

Causes of turbulence. In A, turbulence occurs in fluids flowing at high velocity through tubes of uniform caliber, according to the formula for Reynolds numbers. In B and C, either a local obstruction or fluid flowing into a channel of larger diameter can produce turbulence at much less flow velocity. In D, a combined effect of obstruction and abrupt increase in caliber, as in poststenotic dilatation, produces turbulence at relatively low flow velocity.

2. What happens to the velocity of flow at an orifice?

ANS: It increases, just as it does when you narrow the nozzle of a hose.

3. How does a decrease in orifice area affect the volume of flow across it, the rate of flow across it, and the loudness of the murmur?

ANS: With a decrease in orifice area, the volume of flow will tend to be maintained by a compensatory increase in pressure upstream from the stenotic area. The smaller the orifice, the greater is the velocity of flow at the orifice and the greater the turbulence and the loudness of the murmur.

Note: Turbulence is also affected by the following factors:

a. Viscosity. The greater the viscosity, the less is the turbulence. It is to be expected, then, that the high hematocrit in patients with cyanotic congenital heart disease can increase blood viscosity enough to attenuate murmurs.

b. The irregularity and sharpness of the edge of the orifice. The greater the irregularity or sharpness at the orifice edges, the louder the murmur. This may account for the surprisingly loud ejection murmurs in some elderly patients with aortic valve sclerosis.

4. How does the frequency or pitch of a murmur relate to the (a) gradient and (b) flow?

ANS: a. The greater the gradient, the higher are the frequency and pitch produced. High gradients with little flow volume produce "blowing murmurs."

b. The greater the flow, the more low and medium frequencies are produced ("the greater the flow, the more the low"). When low gradients produce murmurs that are highly dependent on flow, the result is a "rumbling murmur," e.g., mitral stenosis diastolic murmurs.

c. A combination of high gradient and high flow produces mixed frequencies which, if loud, can result in harsh murmurs.

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