Introduction to Acoustics

portance. The Strouhal number has been used for the
prediction of the residual lumen diameters of carotid
stenoses. If the frequency of a bruit is divided into the
number 500 [mm Hz], the result is the minimum lumen
diameter [21.18–25] in mm. In clinical testing, this
method accurately predicted the stenosis in hundreds of
cases. However, because cumbersome technology is involved,
and because the method fails in tight stenoses,
which are most clinically important, this clever method
has been abandoned.
Heart murmurs are auscultated at a series of locations
on the precordium (the front middle of the chest
around the sternum). They are graded from grade 1
(barely audible) to grade 6 (no stethoscope needed).
Murmurs may result from leaky (incompetent, regurgitant)
valves, stenotic (narrow) valves, or other holes
between heart chambers. The differential diagnosis of
heart murmurs is based on the timing of the murmur
and the locations on the chest wall [21.26, 27] where
the murmur is best heard. A murmur can be early systolic,
mid-systolic, late systolic, or early diastolic, or
mid-diastolic. If a murmur is best heard in the left second
intercostal space (the space between the second rib
and the third rib along the left boarder of the sternum),
it is most probably an aortic stenosis [21.28–30]. Each
of the other valve pathologies has a corresponding location
where the murmur is likely to be highest. Of course,
there is a great deal of variability between patients, and
the examiner must apply a full set of detective skills to
sort out a proper diagnosis.
There are some common systolic heart murmurs.
Athletes, with temporary increased blood flow due to
exercise programs have physiologic systolic heart murmurs,
as do pregnant women, who have a 30% increase
in cardiac output to supply the placenta. Hemodialysis
patients also often have systolic heart murmurs because
of the increased cardiac output required to supply the
dialysis access shunt [21.31] in addition to the supply to
the body. In these cases, remodeling of the blood vessels
to increase the flow diameter has not had time to occur.
Bruits are almost always associated with stenoses in
arteries, which often require treatment. They are classified
by their pitch and duration. The occurrence of bruits
in the head, in the abdomen after eating and in the legs
are all indications of arterial stenoses, or high-velocity
blood entering vascular dilations. The presence of a bruit
heard in the neck is usually indicative of a carotid stenosis.
The carotid arteries are the major vessels located on
the left and right side of the neck that supply blood to
the brain. The development of an atherosclerotic plaque
causes luminal narrowing and the corresponding onset
Medical Acoustics 21.2 Medical Diagnosis; Physical Examination 845
of disturbed flow. This lesion is associated with a 20%
risk of stroke in two years from material that can be released
by the atherosclerotic plaque and travels to the
brain to occlude a branch artery.
Carotid bruits are usually present when the residual
lumen (stenotic) diameter is greater than 50% of the
original lumen diameter (called a 50% diameter reduction),
but less than 90% of the original lumen diameter
(90% diameter reduction) In this range, the blood flow
rate to the brain through the carotid artery is approximately
5 cm 3 /s. If the pressure [21.32] drop across the
stenotic region is about 20 mmHg, then the power dissipation
at the stenosis is 13 mW, which can appear as
sound energy in a bruit. Unfortunately, as the stenosis
becomes more severe, the flow rate along the artery decreases,
reducing the acoustic power for the bruit, so
the bruit is no longer detectable, even though this more
severe stenosis carries a higher risk for stroke. The decreased
blood flow will not cause symptoms in most
people because 95% of the population have collateral
connections (the circle of Willis) that can provide the
required blood flow that is not delivered by the stenotic
artery. In the remaining 5% of the population, a severe
carotid stenosis causes impaired mental function.
One of the barriers to new methods in the classification
of carotid stenosis is that the first description
of the relationship between carotid artery stenosis and
apoplexy (stroke) was provided by Egaz Moniz in 1938.
Moniz used percent diameter reduction and everyone has
followed suit, through a confusing and contentious evolution,
even when more-rational and useful alternatives
have been developed. Thus, a new method that predicts
residual lumen diameter will not be easily adopted by
the medical community because of the tradition of using
percent diameter reduction.
Bruits may be generated in the abdomen by stenoses
in the renal arteries, which supply the kidneys. A renal
artery stenosis will cause renovascular hypertension because
the kidney is a transducer for detecting low blood
pressure. The pressure drop across a renal artery stenosis
causes the kidney to measure a low pressure, and
deliver the signals to raise blood pressure in the arterial
system, causing hypertension in all arteries except those
in the affected kidney. The transduction system is called
the renin-angiotensin-aldosterone system.
Some cardiovascular sounds are so loud that they
cannot be missed. A patient in renal failure, refusing
dialysis, had a cardiac friction rub (friction in the
pericardial sack from inflammation). The 1 kHz squeak
occurring with each systole could be heard from the
doorway of the patient’s room.
Part F 21.2