Mechanical Ventilation
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 Valve
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 circuit.
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.
Manual Ventilation
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 half-open.
Increase in Compliance of the Manual Breathing Bag
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 this limit.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.