Anaesthesia Machines Flashcards
Goals of Anaesthesia
1) Optimal conditions for patient
- loss of consciousness
- pain free
- preservation of vital functions
2) Optimal conditions for surgeon
- muscle relaxation
Types of Anaesthetic Circuit
Rebreathing Systems:
1) semi-closed anaesthetic circuit: FGF - V’patient > 0
2) closed circuit (total rebreathing): FGF = V’patient
FGF = Fresh Gas Flow
Gas Dosing - Vapor
Humidifier: control bypass flow around the anaesthetic agent –> remaining flow is 100% saturated with anaesthetic vapor
Ventilator Concepts for Rebreathing Systems
especially for long-term support, type of rebreathing system important to keep function of lung up
1) bag in bottle: works periodically
2) Piston: works periodically
3) Blower: works continuously, very high dynamics
Machine Oriented Control Loop - Control Concept
1) Pressure Control: based on pressure sensor
2) Volume Control: based on Piston position
- PID control
- breathwise adaption with compliance correction
3) Closed-loop control of respiration
- ventilation of patient is considered a disturbance
Electrical vs. pneumatic equivalences
current source = flow source
voltage source = pressure source
Patient-oriented control - Minimal Flow Anaesthesia
what is controlled?
1) Low flow: FGF < 1 l/min
2) minimal flow: FGF < 0,5 l/min
- control of inspiratory and expiratory concentrations
- patient is not part of the loop but interferes as disturbance
Minimal Flow - Advantages
- reduced environmental impact
- reduced gas consumption
- lower costs
- less body heat loss
Minimal Flow - Disadvantages
passive vs. active rebreathing systems
- slower system dynamics –> PT1Tt
- depends on FGF
- possible safety risk
–> in passive rebreathing systems the ratio between transport and mixing times might be adverse for the control
However: active rebreathiing systems (with blowers) dynamically behave like PT1 system –> easy to control
Control of the target organ effect - Patient in the loop
- Patient is the target variable
- control:
- depth of anaesthesia (not yet in practice)
- -> EEG
- Bispectral Index BIS
Which parameters does a Vaporizer control?
Pressure and Temperature control
Which volume can be set as a reference in the user settings?
Tidal Volume
Anaesthetic Gas (AGAS)
What is the advantage of minimal flow with the AGAS concentration?
- dynamics of respiratory system are not linear
- gas sensors to control AGAS concentration
In Minimal Flow anaesthesia, the change of the Fresh Gas Flow together with the AGAS concentration enables a faster adaptation of the expired
anaesthetic concentration to the target value
What is the general behavior of the respiratory system?
P T1 Tt behavior
