A mechanical ventilator keeps a patient with respiratory failure alive by pumping precisely controlled amounts of air (or an air/O2 mixture) at controlled pressure into the patients lungs. During intake (inspiration), the ventilator meters the flow of air and the duration of the flow to deliver a controlled tidal volume of air that ranges from 0.2 to 0.8L. During the exhaust (expiration) phase, the flow is turned off and a path is opened to allow the patient to exhale to the atmosphere – possibly with a positive pressure maintained (via a PEEP valve). The timing of the breaths can be entirely managed by the ventilator or a new breath can be initiated by the patient.
A ventilator is fundamentally an airflow controller with pressure constraints. The respiratory rate, breaths per minute (bpm), is adjustable from 10 to 40 and the inspiratory to expiration time (i:e) ratio is adjustable from 1 to 4. This gives an intake time that ranges from 300ms to 3s. The required flow rate is the tidal volume divided by the intake time. The usable range of settings can be achieved with an intake flow rate of 50L/min. During the intake, pressure is monitored and the flow rate is reduced as required to keep the pressure below a maximum pressure (in the range of 20-60cm H20).