Why does the blood pressure cuff's needle spike during the systolic and diastolic pressures?

Question

Does anyone know?

Thanks.

Thanks Aiden RN!!

Answer 1

Because of fluctuations in pressure in the tubing caused first by arteries opening then by congested veins opening up "beneath" the cuff:

Above the systolic pressure, when you release the pressure (smoothly) the needle comes down smoothly without "spiking" until the pressure of the cuff reaches the systolic pressure and a turbulent (arterial) blood flow passes under the cuff as elastic arterial walls open up and blood rushes (from proximal to distal) through the cuff.

Then, if you look carefully, as you deflate the cuff further, you'll see it "bob" faintly with each pulse until the cuff pressure is (roughly) the same as the diastolic pressure -- even though no Korotkoff sounds are audible at diastole, turbulent venous blood (that has been congested by the cuff) flows towards the cuff, again causing the spiking on the needle that you've just described in your question (by exactly the same mechanism, but from distal to proximal).

The needle only moves visibly due to pressure changes in air pressure *in the tubing* of the sphygmomanometer that is continuous with the cuff -- the cuff is physically a part of the "closed-tube circuit" and transmits fluctuations in air pressure in the tube to the gauge (which is an aneroid barometer) ... see:

Wall-mounted BP cuff:
http://www.hmint.com/Images/11-675D.jpg

Aneroid barometer
http://www.cabinboyinstruments.com/barometers.asp

Hope this helps you!

Answer 2

Pulse transmitted along the artery gives that spike in needle. One can actually measure the pulse rate that way.

Answer 3

sounds like its call a heart beat?