Smiths Medical Pneupac babyPAC 100, User Manual

Page: 39 / 64

504-2056A 39
SECTION 8 TECHNICAL INFORMATION
(a) Principles Of Operation
The principle of operation of the babyPAC ventilator is illustrated by the block diagram in Fig 4,
page 42. Compressed oxygen and/or air enter the ventilator from the input hoses at the inlet
connectors
1 and 2 . The priority valve 3 supplies either oxygen or, if available, air to the internal
pressure regulator 4 where it is regulated to a constant pressure before being fed through to the
CMV valve 5 and the CPAP valve 23 .
When CMV is selected valve 5 is opened and gas passes through to the oscillator 6 .
The oscillator is a miniature pneumatic device which switches on and off at time intervals
determined by the settings of the precision needle valves T
INS
7 and T
EXP
8 .
When the oscillator is ‘on’, gas passes to the entrainment gas mixing device 9 where the gas
mixture is made up to the selected oxygen concentration at a fixed output flow of 10 L/min. From
here the gas passes to the ventilator gas output port and hence to the inspiratory limb of the patient
circuit.
The pressure in the patient circuit is set by means of the exhalation valve 17 . This is controlled by
means of the precision needle valve 19 working in conjunction with bleed restrictor 21 . Valve 19 is
fed, during the inspiratory phase, from the pneumatic switch valve 18 which is in turn actuated by
the oscillator output.
At the end of the inspiratory phase the oscillator output switches ‘off’ and hence the output flow
ceases and the valve
18 switches to supply the precision needle valve 20 instead of valve 19 . Valve
20 , in conjunction with bleed 21 , sets the PEEP pressure level on valve 17 and so once valve 18 has
switched the patient pressure will drop down to the PEEP level and the patient will exhale.
During the inspiratory phase the flow amplifier valve
11 supplies oxygen to the mixer control valve
12 to ensure that the required oxygen flow is supplied to the mixer 9 . The mixer draws in the
required volume of fresh air through inlet 10 .
The pressure supplied to the patient is limited by the adjustable upper pressure limiting valve 13 and
is monitored by means of the manometer 14 . Gas discharged from the pressure limiting valve is
used to sound the audible alarm.
The function switching valves
5, 23 and 24 are all actuated by the rotation of the function selector
switch shown as 4 in Fig 1(a).
When CMV+ACTIVE PEEP is selected then both valve
5 and valve 23 are opened. When the
oscillator is in its ‘on’ state its output pressure switches valve 27 ‘off’. When the oscillator switches
‘off’ valve 18 switches as above but valve 27 subsequently opens and provides a gas supply to the
gas mixing device 9 and the flow amplifier valve 11 , thereby maintaining patient circuit flow
throughout expiration.
When CPAP is selected, the CMV valve
5 is closed and CPAP valve 23 is opened. In this way the
oscillator is bypassed and gas pressure is supplied continuously to the gas mixer 9 . The non-return
valve prevents this pressure actuating valve 18 so the PEEP/CPAP valve remains energised whilst
CPAP is selected and the setting of this valve determines the CPAP level.
When IMV+CPAP is selected all three valves
5, 23 and 24 are open. Valve 5 allows the oscillator
to function but valve 24 connects it to an auxiliary reservoir 25 which increases the expiratory time