Scireq flexivent, User Manual, Page: 23 / 86

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volume of air displaced by the piston is slightly higher than the volume delivered to the subject.
Before any perturbations are run with the subject attached, the ventilator compartment is
characterized for each perturbation. When the subject is attached and a perturbation
subsequently run, the contribution of the subject may be isolated by subtracting that of the
ventilator compartment.
1.4.4 SERVO-CONTROLLED PISTON
The module of the flexiVent FX contains the piston/cylinder set. Movement of the piston is
servo-controlled through a closed feedback-control loop as in Figure 1-2. The piston is driven by
the linear actuator in the base unit and its position is measured by an optical sensor. The
differential pressures in the cylinder and ventilator compartment are measured by transducers
within the module. When the piston moves, it causes a change in the pressures measured by the
transducers. These pressures are measured for the purpose of calculating respiratory
mechanics, but also to provide feedback so the piston’s movement may be adjusted as needed
(e.g. it will move less to generate a given pressure following administration of a
bronchoconstrictor).
1.4.5 CONTROLLING EXPERIMENTAL CONDITIONS
Section 1.4.3 discusses a technical challenge in pre-clinical respiratory mechanics. An additional
physiological challenge is addressed by controlling experimental conditions. Mechanics
assessments using other methods may be based on measurements taken breath-by-breath in
spontaneously breathing subjects. As breathing frequency increases, resistance decreases.
Respiratory mechanics also vary with tidal volume. If breath-by-breath measurements are taken
following any variation in the experimental conditions, it is not possible to distinguish
mechanics changes caused by that variation from mechanics changes that may be introduced
due to an increase or decrease in breathing frequency, for example. As discussed in section
1.4.2
the
flexiVent
takes measurements during perturbations. Perturbation properties are set in the
software, so little to no variation exists between subjects.