Bellows Ventilator
- pneumatically (gas) driven, electronically controlled
- pneumatic force compresses a bellows, which empties its contents (gas from flometers and vaporizer) into circuit
Driving Gas
oxygen, air, or venturi mix of both (injector)
Venturi Mechanism (injector)
gas flows through constricted area at high velocity, pressure around it drops below atmospheric pressure and air is entrained.
net result = increase in total gas flow leaving outlet
Bellows Ventilator 2
- pneumatically (gas) driven
- inner and outer compartment
- driving gas enters outer compartment and depresses bellows
- inner compartment delivers gas to patient breathing circuit
Movement of bellows is controlled by what?
- Movement is controlled by drive gas which enters outer chamber and pushes bellows containing circuit gas into breathing circuit.
- During exhalation the bellows fills w gas from breathing circuit and fresh gas from flow meters
- Excess gas and pressure is vented out to scavenging system via spill valve
Why can drive gas go to atmosphere and not scavenging system?
Drive gas is not contaminated (oxygen and air to compress bellows) so it can go to atmosphere.
Hanging/Descending Bellows
- weighted
- causes PEEP
- safety: if there is a leak the bellows will descend anyways (weighted) and entrain air
Ascending Bellows
- filing is dependent on exhaled gases from tight circuit
- failure to rise if leak is > than FGF
Piston Ventilator
- do NOT require driving gas!
- driven by electric motor (no electricity = no ventilation)
- computer determines how far piston needs to move to deliver set TV or pressure (more accurate than bellows)
- at end inspiration, piston retracts and cylinder is filled w fresh (from flometers) and exhaled gases that have passed through CO2 absorber
2 Factors that May Effect Delivered TV
- FGF
- Compliance of circuit
FGF
- changes in flow rate, I:E ratios, or RR can alter delivered volume
- modern machines adjust for this (FGF compensation, FG decoupling)
Fresh Gas Decoupling
prevents FGF from entering the breathing system during inspiration so that FGF does not contribute to TV
Compliance of Circuit
modern machines calculate for this and compensate for it
Ventilator Modes: Mandatory Ventilation and Support Ventilation
Mandatory: VC and SIMV
Support: PSV
Vent Settings
- PEEP
- RR
- I:E ratio
- Pressure
- Volume
Normal TV
500-700 mL
Normal Minute Ventilation
5-6 L/min
Normal Setting for TV
7-10 mL/kg
MV
TV x RR
ie: 500 TV at 6 breaths/min = MV of 3L /min
Volume Control Ventilation - VCV
- most common (good for new CRNAs)
- you set TV and RR, peak pressure varies
- minute ventilation remains constant
Pressure Control Ventilation - PCV
- you set peak pressure and RR, TV varies based on pt resistance and compliance
- MV will vary (because TV varies)
- you can underventilate the pt w this setting, but barotrauma is avoided because you’ll never overpressure them (supraglottic airway)
PC vs VC
- square wave pattern in VCV results in higher peak pressure for same TV
- pressure control gets you better volume for your pressure but there is a greater risk of underventilating
Pressure Control - Volume Guarantee - PCV-VG
- based on the fact that PC gives you better volume
- smart mode: you set desired TV and max pressure you will tolerate to get to that TV
- delivers preset TV w lowest pressure using decelerating flow pattern (like PC)
- pt’s compliance determined from 1st breath which is VC and inspiratory pressure is established for all subsequent breaths
- VG ensures that for all mandatory breaths the set TV is applied w minimum pressure required*
- if resistance/compliance changes, pressure gradually adapts over several breaths to restore set TV
Pressure Support Ventilation - PSV
- used for pts spontaneously breathing
- you set PS for machine to deliver during spont breathing
- once ventilator senses inspiratory effort from pt, it provides constant pressure to the airway to relieve WOB
- 2 different ways vent can sense spont breaths: pressure changes (negative pressure) or flow past fow sensor during exhaltion
