MadSci Network: Medicine

Re: What is the ticking sound heard when taking blood pressure?

Date: Mon Jan 17 11:55:22 2000
Posted By: BOTFIELD Nigel, Staff, Haematology, Scunthorpe General Hospital
Area of science: Medicine
ID: 947099235.Me

Hi John,
Some good news and some bad I'm afraid.
First the bad news - I'm unable to find a name for the ticking noise. The
books on nursing techniques I've read don't give a name and none of the
nurses I've spoken to can recall what the name is or even if one was given
during their training - sorry!

[Admin note: In fact the sounds are called "Korotkoff Sounds."]

Now the good news - what causes the sound?
Measuring blood pressure uses a similar process to finding the pulse. You 
trap an artery between the sphgmomanometer cuff and the underlying bone, 
as the pressure in the cuff is released blood can squeeze past the 
obstruction caused by the cuff and the sound you hear is the pulse. As the 
heart beats it forces blood into the main artery ( the aorta ) causing the 
aorta to swell to accomodate the ball ( or bolus ) of blood, Tom and Jerry 
do it all the time when Tom turns on the hosepipe but Jerry squeezes the 
pipe to form a big balloon of water in the pipe which he releases when Tom 
looks down the hose nozzle to see what the problem is. The arterial system 
has many such swellings travelling through it - one for each heartbeat as 
shown below.

HEART aaaaaBaaaaaBaaaaaBaaaaaBaaaaaBaaaaaBaaaaa REST OF BODY

The aaaaa represents the artery, the B shows each bolus of blood caused by
a heartbeat travelling through the artery and M is the point of 
measurement. When taking blood pressure the cuff exerts enough external 
pressure to stop blood flowing the artery. As the cuff pressure is released
it reaches a point where it is less than the pressure the heart generates 
as it beats - this is the systolic pressure. At this point you can hear 
the pulse and the start of this ticking sound is your cue to take a 
reading from the sphygmomanometer for the systolic pressure. As the cuff 
pressure is released further it reaches a point where it is no longer 
squeezing the artery and the ticking sound disappears, taking a reading 
from the sphgmomanometer now will give you the diastolic pressure - that 
is the pressure caused by blood inside the artery between each bolus from 
the heartbeat. You can hear the pulse because of the energy transferred 
from the artery as it pushed against the cuff by the blood, this energy is 
transmitted through the body tissues and air space in the stethoscope until
hits your eardrum and causes your brain to register a sound.
You can think of it this way - you are in a car travelling down the 
highway when you have a bit of a Mulder and Scully moment and start 
floating inside the car. The car passes over a series of speed retarding
bumps in the road. Each impact of tyre with bump causes energy to be 
transmitted through the suspension and into the body of the car, this in 
turn causes the air inside the car to vibrate and when these vibrations
reach your ear you know the car is hitting the bumps because you can hear
a rhythmic THUD THUD sound - this is the equivalent of the systolic 
pressure of the pulse. However, in between the bumps you can still hear 
some white noise caused the tyres running on the road and wind passing 
over and around the car - this background noise is the equivalent of the
diastolic pressure. When your M and S moment is over you settle down into
your seat but you are now aware of the bumps physically as some of the 
impact energy is now being transmitted directly into your body - this is 
the equivalent of feeling your pulse.
Sound does travel through blood and indeed the rest of the body but the 
sound you are asking about has two problems:
1 It is a quiet sound and its energy is absorbed by surrouding tissues so 
the stethoscope has to be placed near to the cuff.
2 The sound is caused by the obstruction created by the cuff, therefore 
the sound does not exist in the body without the obstruction.
Hope this helps.

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