Medical ventilator
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- Ventilator redirects here. For the article on the non-medical gas moving device, see Fan (implement), and for other uses see the disambiguation page at Ventilation.
A medical ventilator is a device designed to provide mechanical ventilation to a patient. Ventilators are chiefly used in intensive care medicine, home care, and emergency medicine (as standalone units) and in anesthesia (as a component of an anesthesia machine).
In its simplest form, a ventilator consists of a compressible air reservoir, air and oxygen supplies, a set of valves and tubes, and a disposable or reusable "patient set". The air reservoir is pneumatically compressed several times a minute to deliver an air/oxygen mixture to the patient; when overpressure is released, the patient will exhale passively due to the lungs' elasticity. The oxygen content of the inspired gas can be set from 21 percent (ambient air) to 100 percent (pure oxygen). Pressure and flow characteristics can be set mechanically or electronically.
Ventilators may also be equipped with monitoring and alarm systems for patient-related parameters (e.g. pressure and flow) and ventilator function (e.g. air leakage, power failure), backup batteries, air and oxygen tanks, and remote control and alarms. The pneumatic system is nowadays often replaced by a computer-controlled turbopump.
Modern ventilators are electronically controlled by a small embedded system to allow exact adaptation of pressure and flow characteristics to an individual patient's needs. Fine-tuned ventilator settings also serve to make ventilation more tolerable for the patient. In Germany, Canada, and the United States, respiratory therapists are responsible for tuning these settings.
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[edit] Life-critical system
Because the failure of a mechanical ventilation system may result in death, it is classed as a life-critical system, and precautions must be taken to ensure that mechanical ventilation systems are highly reliable. This includes their power-supply provision.
Mechanical ventilators are therefore carefully designed so that no single point of failure can endanger the patient. They usually have manual backup mechanisms to enable hand-driven respiration in the absence of power. Some systems are also equipped with compressed-gas tanks and backup batteries to provide ventilation in case of power failure or defective gas supplies, and methods to operate or call for help if their mechanisms or software fails.
[edit] Ventilators, historical
In 1949 John Haven Emerson developed a mechanical assistor for anesthesia with the cooperation of the anesthesia department at Harvard University. Mechanical ventilators began to be used increasingly in anesthesia 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 anesthesia. Relaxant drugs paralyze the patient and improve operating conditions for the surgeon, but also paralyze 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 anesthetics 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 anesthetists 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 movable weight on top of the bellows. 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 anesthetic practice.
