Articles Page Number Selection:| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | |
||||
Suite 323 2600 Philmont Avenue Huntingdon Valley, Pennsylvania 19006 Page 3 |
||||
Measurement of Blood
Pressure Mercury
is about 13.6 times denser, or heavier, than blood or water.
Even modern blood pressure measuring devices,
sphygmomanometers, which use an air pressure system (aneroid
manometers) still record the pressure as "millimeters of
mercury". This refers to the height of a mercury column that
could be balanced by the pressure. "Millimeters of mercury"
is often abbreviated mm. Hg., since "Hg" is the chemical
symbol for mercury. The nine-foot column of blood observed
by Stephen Hales is equivalent to 2,743.2 millimeters
(304.8, the number of millimeters in a foot, times 9). Since
mercury is approximately 13.6 times heavier than blood or
water, the blood pressure of the horse, in modern
terminology, was about 202 mm. Hg. (2,743.2 divided by
13.6).Stephen
Hales did not pursue the measurement of blood pressure. He
became interested the movement of gases in the lungs. It was
he who first demonstrated "artificial ventilation" or
"artificial respiration" in animals. The modern importance
of this discovery is well known, as it applies to mechanical
ventilation during general anesthesia and in resuscitation
of persons near
death. In
spite of Rev. Hales' experiment with the horse, it
was to be 143 years before an inventor, named
Ritter von Basch, produced a measuring device, a
sphygmomanometer, which was practical and accurate,
and did not require insertion into an artery! This
apparatus was introduced in 1876. A "blood pressure
cuff" similar to modern instruments, was introduced
in 1896 by Scipione Riva-Rocci. The
early sphygmomanometers used an inflatable cuff
that was placed on the upper arm. It was inflated
to a pressure sufficient to compress the artery and
obliterate the pulse. The actual pressure was
measured by balancing the pressure in the cuff with
a reservoir of mercury. The mercury was displaced
upward into a calibrated glass tube from which air
had been evacuated. The systolic pressure was
measured by observing the pulse as pressure was
released from the upper arm. The systolic pressure
is produced in the circulatory system during
contraction, or systole, of the main pumping
chambers of the heart, the ventricles. This is the
higher pressure recorded. A lower pressure, called
the diastolic pressure, is the pressure in the
system when the heart is relaxed, or in
diastole. The
systolic pressure could then fairly well be
determined by feeling for the pulse, and noting the
pressure when the pulse first returned during
deflation of the cuff. To measure more accurately
required the use of a listening instrument, the
stethoscope. The first stethoscope was simply a
tube made of stiff rolled paper, later devised as a
wooden cylinder with an ear-piece. It was invented
in 1816 by René
Théophile Hyacinthe
Laënnec,
a French physician. Before the invention of the
stethoscope, the physician placed his ear against
the bare chest to examine the sounds of the lungs
and heart. While the stethoscope improved the
quality of the sounds heard, the real reason for
its invention was to allow the physician to examine
the chests of his female patients more discreetly.
It also made it easier to examine those patients
who were very obese, and to put some distance
between the physician and patients with
particularly loathsome or malodorous diseases. The
Laënnec stethoscope evolved into an instrument
with flexible tubing, using both ears, by 1826. A
stethoscope very similar to those now used was
available by 1855. Twentieth Century
Medicine |
||||
Articles Page Number Selection:| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | |