Fluke Biomedical Cufflink - Manual (Page 138)

Cufflink
Operators Manual
A-2
oscillometric techniques are well known non-invasive methods. These methods are
indirect because they do not couple directly to the artery.
Auscultatory Technique (not available in Analyzer)
The auscultatory technique is based on the sounds caused by the blood flow through the
artery that is surrounded by the cuff. These sounds are known as Korotkoff (K) sounds.
In manual blood pressure measurement these sounds are detected by a human observer
using a stethoscope.
Automated blood pressure monitors use an audio transducer (e.g. microphone) to convert
the K sounds into electric signals. The cuff is inflated to a point that occludes the artery.
The pressure in the cuff is lowered. The cuff pressure at which the K sounds are first
detected is the systolic pressure. The monitor continues to decrease the cuff pressure until
the K sounds disappear. The cuff pressure at this point is called diastolic pressure
Oscillometric Technique
The oscillometric technique does not use K sounds to determine blood pressure. The
oscillometric technique monitors the changes in cuff pressure caused by the flow of blood
thru the artery. The monitor inflates the cuff to a pressure that occludes the artery. Even
when the artery is occluded, the pumping of the heart against the artery can cause small
pressure pulses in the cuff baseline pressure.
The monitor lowers cuff pressure at a controlled rate. As the cuff pressure goes down,
blood starts to flow thru the artery. The increasing blood flow causes the amplitude of the
pressure pulses in the cuff to increase. These pressure pulses continue to increase in
amplitude with decreasing cuff pressure until they reach a maximum amplitude at which
point they begin to decrease with decreasing cuff pressure. The cuff pressure at which the
pulse amplitude is the greatest is known as Mean Arterial Pressure (MAP).
The manner in which the pulse amplitudes vary is often referred to as a pulse envelope.
The envelope is an imaginary line that connects the peak of each pressure pulse and
forms an outline. The shape of the envelope is observed by the monitor using a variety of
techniques to determine the diastolic and systolic blood pressures.
Auscultatory vs. Oscillometric Techniques
The auscultatory response is based on the sounds heard with a microphone. The cuff is
inflated to a pressure much higher than systole. As the cuff is deflated the observer (or
automated monitor) listens for the onset of K sounds. The cuff pressure at which the K
sounds are first detected is treated as systolic pressure.
In Figure A-1, some smaller K sounds are shown to appear before the systolic point. In
many cases there is a threshold point. The point at which the sounds jump from some
very low level to a much larger level is more typical with live subjects. The K sound
amplitude does not change much as the cuff continues to deflate until the point of diastole
is reached. At this point, there is an abrupt drop in K sound amplitude.