|MadSci Network: Medicine|
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. M 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. NIGE
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