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.

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