Ventilators, historical
Mechanical ventilators began to be used increasingly in anaesthesia
and intensive care during the 1950's. Their development was stimulated
both by the need to treat polio patients and the increasing use
of muscle relaxants during anaesthesia. Relaxant drugs paralyse
the patient and improve operating conditions for the surgeon, but
also paralyse the respiratory muscles and stop breathing. In the
US the Bird ventilator was an early gas driven model, it required
no electrical power source. In the UK the East Radcliffe and Beaver
models were early examples, the later using an automotive wiper
motor to drive the bellows used to inflate the lungs. Electric motors
were however a problem in the operating theatres of that time, their
use caused an explosion hazard in the presence of flammable anaesthetics
such as ether and cyclopropane. In 1952 Roger Manley of the Westminster
hospital, London, developed a ventilator which was entirely gas
driven, and became the most popular model used in Europe. It was
an elegant design, and became a great favourite with European anaesthetists
for four decades, prior to the introduction of models controlled
by electronics. It was independent of electrical power, and caused
no explosion hazard. The original Mark I unit was developed to become
the Manley Mark II in collaboration with the Blease company, who
manufactured many thousands of these units. It's principle of operation
was very simple, an incoming gas flow is used to lift a weighted
bellows unit, which falls intermittently under gravity, forcing
breathing gases into the patient's lungs. The inflation pressure
can be varied by sliding the moveable weight on top of the bellows,
it can be seen in the photograph. The volume of gas delivered is
adjustable using a curved slider, which restricts bellows excursion.
Residual pressure after the completion of expiration is also configurable,
using a small weighted arm visible to the lower right of the front
panel. This was an excellent and robust unit and it's availability
encouraged the introduction of positive pressure ventilation techniques
into mainstream European anaesthetic practice.
antibiotics nursing pharmaceutical product items drug pharmacy medicines Veterinary Vitamins
|