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Adult intensive care ventilators that deliver gas to the lungs by periodically applying and releasing an external negative pressure to the chest cavity. Some negative-pressure ventilators consist of a tight chamber (e.g., tank or iron lung) where the patient is enclosed from the neck down; then a vacuum pump creates a negative pressure around the body, causing air to be sucked into the lungs. Others consist of a plastic or metallic shell (breastplate or cuirass) externally attached to the patient chest and a vacuum pump that creates a negative pressure within the breastplate. Cuirass negative-pressure ventilators that can control both inspiration and expiration either by providing a negative pressure during the inspiratory phase and a positive pressure during expiration or by intermittent swings of negative pressure (negative pressure oscillatory ventilators) are also available. Adult negative-pressure ventilators typically operate at normal breathing rates, but the oscillatory type can reach respiratory rates up to several hundred cycles/minute.
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