Specifications

Aestiva

7-2 1006-0939-000

7-2

Ventilator theory

General The ventilator pneumatics are at the rear of the breathing system.

A precision valve controls gas flow to the patient.

During inspiration, this gas flow closes the exhalation valve and pushes the

bellows down.

During expiration, a small flow pressurizes the exhalation diaphragm to supply

PEEP pressure.

Volume and pressure measurements come from flow sensors in the flow sensor

module. Two tubes from each sensor connect to a transducer that measures

the pressure change across the sensor, which changes with the flow. A third

transducer measures airway pressures at the inspiratory flow sensor.

With circle circuit modules, volume monitoring uses the right (expiratory) flow

sensor. The ventilator uses the other sensor to adjust its output for changes in

fresh gas flow, small leaks, and gas compression upstream of the breathing

circuit. There is no adjustment for compression in the patient circuit. If

necessary, add the compression loss to the tidal volume setting (volume

control mode). The average volume changes from compression in the breathing

circuit is small (0.5 to 1.25 mL/cmH

O).

For better precision:

•When the fresh gas mixture includes Heliox, use the Heliox mode (Ventilation

setup menu). Heliox, used on some ANSI Models of the Aestiva, changes the

data collected by the flow sensors. When Heliox mode is selected, the

ventilator adjusts the flow sensor data to correct for these changes.

•A small quantity of gas bleeds through a resistor to help keep the pressure on

the exhalation valve constant. At high airway pressures, this can cause a

slight hiss during inspiration.

Do not try to silence the pneumatic resistor. If it is blocked, the

ventilator can malfunction and cause patient injury.

Always connect the expiratory flow sensor. If it is not connected,

the patient disconnect alarm can not operate correctly.