Biomedical Engineering Reference
In-Depth Information
Although tidal volume is probably easier to record with a spirometer, the dynamics
of respiration are better displayed using a pneumotachograph, which offers less resistance
to the air stream and a much shorter response time. The response is so fast that cardiac
impulses are often clearly identifiable on the flow rate versus time record. The volume is
obtained by integrating the flow rate.
A typical pneumotachograph offers a resistance of between 5 and 10 mmH
2
O and can
measure flow rates of up to 200 L/min. Response times of between 15 and 40 ms are usual.
Other ways of measuring flow using turbines, a heat conduction, or Doppler methods
are discussed in Chapter 2.
9.6
MECHANICAL VENTILATION
It is reasonable to classify modern ventilators into two main groups. The first, and largest
group, are those used in intensive care to support patients after surgical procedures or
to assist patients with acute respiratory disorders. The second includes less complicated
machines used at home to help treat patients with chronic respiratory disorders.
9.6.1 Early History
But so that life may in some measure be restored to the animal, you must attempt an
opening in the trunk of the trachea and pass into it a tube of rush or reed, and you
must blow into this so that the lung may expand and the animal draw breath after a
fashion; for at a light breath the lung in this living animal will swell to the size of the
cavity of the thorax, and the heart take strength afresh, and exhibit a great variety of
motions. Vesalius (1543)
This is an English translation of a section of the last chapter of Andreas Vesalius famous
anatomical treatise,
De humani coporis fabrica.
It is probably the first account of mechan-
ical ventilation and also the first description of the physiological effects resulting from
ventilation on a collapsed lung.
Robert Hooke's rather brutal experiments, described in the proceedings of the Royal
Society of October 24, 1667, showed that if the thorax of a dog was opened it was unable
to breathe. However, the dog could be kept alive for hours using bellows to inflate the
lungs periodically.
John Mayow was probably the first scientist to really understand the mechanics of
respiration. In 1670, he showed that air is drawn into the lungs by enlarging the thoracic
cavity. He demonstrated the principle by building a model using bellows inside which was
inserted a bladder. Expanding the bellows caused air to fill the bladder, and compressing
the bellows expelled air from the bladder.
Throughout most of human history, people have sought the ability to restore breath
into the bodies of those who have stopped breathing. The devices and theories used in
these experiments were built on ideas that came about mostly during the late eighteenth
century and were focused on the recovery of the apparently drowned or dead.
Initially, doctors established some basic methods of resuscitation, including warmth,
inflation (very similar to modern rescue breathing), fumigation, friction, stimulants, bleed-
ing a vein, and encouraging vomiting. However, most were not effective and thus fell from
favor after a few years.
In 1782, the Royal Humane Society of England supported the use of bellows as
the best means of inflation for artificial respiration. This method was widely supported
