Biomedical Engineering Reference
In-Depth Information
from within the barrel. Unfortunately, it didn't work very well, and the inventor died from
lack of oxygen after being in his own device for 20 minutes.
Just over 50 years later, in 1825, William James designed another self-contained
breather comprising a cylindrical iron hoop attached to a copper helmet. It provided
enough air for a 7-minute dive. This was followed in 1876 by a closed-circuit oxygen
rebreather invented by Henry Fleuss. This was first used in the repair of an iron door
of a flooded ship's chamber and later in a 10 m deep dive. Unfortunately, oxygen under
pressure is toxic, and Fleuss died.
Christian Lambson designed a more successful system in 1939 as part of the military
Self Contained Underwater Breathing Apparatus (SCUBA) program. However, divers
were still injured or killed frequently from oxygen toxicity. It wasn't until 3 years later
when Emile Gagnan and Jacques Cousteau invented a demand regulator that a safe and
effective method of providing fresh air when a diver breathed became available. A year
later they began marketing the Aqua-Lung.
Research undertaken during WWII to deliver oxygen to fighter pilots operating at
high altitude also played a part in the design of the modern positive-pressure ventilator.
Intensive use of positive-pressure ventilation gained momentum during polio epi-
demics in Scandinavia and the United States in the early 1950s. In Copenhagen, patients
with respiratory paralysis were supported by manually forcing a rich oxygen mixture
through a tracheostomy and had reduced mortality rates. However, this activity required
the effort of 1400 medical students recruited from universities. The reduction in mortality
rates from 80 to 25% lead to the development and adoption of positive-pressure machines
(Byrd, Kosseifi et al., 2010).
9.8.2 The Need for Positive-Pressure Ventilation
Positive-pressure ventilators generate inspiratory airflow by applying a positive pressure
(greater than atmospheric pressure) to the airways. During inspiration, the inspiratory flow
delivery system creates a positive pressure in the tubes connected to the patient's airway,
while the expiration system closes the outlet valve to ensure that air flows into the patient's
lungs. When the ventilator switches to expiration, the inspiratory flow is stopped and the
expiration system opens the outlet valve, allowing the patient's exhaled breath to flow to
the atmosphere, as shown in Figure 9-40.
The use of a positive-pressure gradient in creating the flow allows treatment of pa-
tients with high lung resistance and low compliance. As a result, positive-pressure venti-
lators have been very successful in treating a number of different breathing disorders and
have all but displaced the use of negative-pressure ventilation from almost all respiratory
FIGURE 9-40
Simplified schematic
showing operation of
a positive-pressure
ventilator.
