Function of the Spill Valve
(Ventilator Pressure Relief Valve, Exhaust Gas Valve)
Because the APL valve in the breathing system is either closed or
isolated during ventilator operation, i.e., mechanical ventilation,
each ventilator must contain a spill valve for venting excess gases
into the scavenging system. During the expiratory phase of the mechanical
ventilation cycle the spill valve vents excess gas; during inspiration
it closes. In an upright (not hanging) bellows, the spill valve is
weighted such that its opening pressure of 2-4 cm H2O is higher than
the pressure required to fill and lift the bellows back to its full
position. Therefore, the bellows will fill first and reach the top
of its casing before the spill valve will open. The opening pressure
of the spill valve causes the "intrinsic PEEP" of 2 -4 cm
H2O observed with a standing bellows even if no PEEP is being applied.
In a system with no significant leaks, the amount of excess gas vented
should approximate the incoming fresh gas flow.
The behavior of the spill valve is modeled in
the simulation and it can be seen to open during the expiratory phase
of mechanical ventilation.
Function of the Proportional Flow Control
Not to be confused with the manually operated flow control valve of
the oxygen flowmeter, the proportional flow control valve in the ventilator
controls the amount of drive gas (O2 in the case of the Ohmeda 7800
ventilator) that is admitted during mechanical inspiration to the
bellows casing to "squeeze" the bellows, and therefore the
tidal volume that is delivered to the patient. The exhalation valve
is closed during mechancial inspiration and during the inspiratory
pause if one is used. The exhalation valve opens during exhalation
allowing the drive gas to vent to atmosphere, thus depressurizing
the bellows casing to atmospheric pressure.
In the simulation, the flow control valves can be
seen off to the right side of the bellows housing in the driving gas
Different Locations of the APL Valve in Modulus
I and II Machines
In the Ohmeda Modulus II machines, the adjustable pressure limiting
(APL, aka as "pop-off", valve) is located at a position
such that it is in pneumatic conection with the breathing circuit
only during manual ventilation (i.e., the selector knob is set to
Bag mode). When the selector knob is set to Ventilator mode, the APL
valve is no longer part of the circuit. Even if the APL valve is left
open, no gas is able to escape out of the APL valve.
In the Modulus I design, the APL valve remains in pneumatic connection
with the breathing circuit even when the selector knob is set to Ventilator
mode. If the APL valve is accidentally left open during mechanical
ventilation, gas will escape to the scavenging system and the patient
might be hypoventilated. The delivered tidal volume might be considerably
less than that set on the ventilator.
The location of the APL valve in this simulation
reflects that of the Modulus II machines.
Function of the APL Valve
The APL (Adjustable Pressure Limiting) valve is only used during manual
(or spontaneous) ventilation in the Modulus II machines. As its name
indicates, the APL valve limits the amount of pressure buildup that
can occur during manual ventilation. When the user adjusts the APL
valve to trap more gas inside the breathing circuit, a spring inside
the APL valve is compressed according to how much the user turns the
APL valve. The degree of spring compression exerts a proportional
force on a sealing diaphragm in the APL valve. The pressure inside
the breathing circuit must generate a force that exceeds the spring
compression force for the APL valve to open. As pressure continues
to build from the combination of fresh gas flow and manual compression
of the breathing bag, the opening pressure of the APL valve will be
exceeded and excess gas will be vented to the scavenging system.
With the selector knob set to manual ventilation
and the APL valve in half open position, the pressure build-up in
the circuit can be seen to overcome the APL opening pressure generated
by the compression of the spring inside the APL.
Positions of the APL Valve
The APL valve can either be completely closed (no gas vented), completely
open (all gas vented - this makes it impossible to ventilate the patient
- no gas buildup is possible and the manual breathing bag stays fully
deflated), or any position in between.
In this simulation only three positions of the
APL valve have been modeled: completely open, completely closed, and
Increase in Compliance of the Manual Breathing
With Increasing Bag Volumes
With the APL valve completely closed and the selector knob in the
Bag mode, no gas can be vented at all and the breathing bag continues
to increase in size. Pressure buildup, however, is quite slow because
the compliance of the bag actually increases with increasing bag size.
The pressure-volume characteristics of rubber bags are such that the
patient is somewhat protected from excessive pressures in the breathing
system. Adding volume to a bag causes a negligible rise in the pressure
until the nominal capacity is reached. When more volume is added,
the pressure rises quickly but then reaches a plateau with the pressures
remaining between 30-50 cm H2O. However, non-elastic bags might exceed
Inflation of the manual bag will initially cause
a rapid increase in the inspiratory pressure which can be observed
on the inspiratory pressure gauge. After reaching a certain bag volume,
the pressures will increase much more slowly.