The ventiPAC is a time-cycled flow generator. A high-pressure gas supply drives a mechanical oscillator that controls the respiratory cycle. During inspiration, an adjustable flow restrictor limits the rate of gas delivery from the pressurized supply. Pressurized cylinders are used for most critical care transfers, and exploiting their potential energy in this way is relatively efficient.
The operating controls on the front panel are calibrated under nominal loading conditions. If delivering 100% oxygen, the ventilator will continue to provide reasonably accurate tidal volumes despite conditions of increased airway resistance or reduced lung compliance. However, if 'air mix'is selected, the back-pressure imposed on the Venturi in these conditions may result in reduced tidal volumes. Therefore, it is necessary to monitor the adequacy of ventilation, using end-tidal CO2 monitoring and, where possible, arterial blood gas sampling.
The flow-generator model enables effective ventilation of patients with severe adult respiratory distress syndrome including those being transferred to specialist centres. However, airway pressures may increase and must be carefully monitored during transport.
The use of pneumatic components to detect inspiratory effort and control the respiratory cycle conveniently allows MRI-compatibility. However, this type of ventilator may not effectively support the patient's own attempts to breathe. The ventiPAC incorporates a demand valve opened when pressure changes caused by inspiratory effort are transmitted back along the circuit to the ventilator. Fresh gas is provided to the patient but the ventilator provides no additional assistance so the work of breathing is not reduced. Spontaneous breathing does, however, retard the oscillator controlling the respiratory cycle and may improve synchrony between patient and ventilator.