Vaporizers Flashcards
Volatile Anesthetic Agent
- liquid at room temperature
- at room temperature, all the gaseous forms of common anesthetic agents exist as vapors. Somehow we need to get these to the patient*
flow of vaporizer is dependent on what?
- Flow rates or carrier gas flow rate
Vaporizer is in which part of the pressure system?
- Low flow system
Temperature compensated concentration-calibrated variable bypass vaporizer:
- most common type of modern day vaporizer
- fresh gas enters vaporizer where its flow is split between a larger bypass flow and a smaller flow to the vaporizing chamber or sump
- int he sump is the agent at its SVP
- saturated vapor mixes with bypass flow, which dilutes it to the concentration dial setting
Saturated Vapor Pressure
- when placed in a closed container at normal atmospheric pressure and room temp, a potent inhaled anesthetics is in liquid form
- some anesthetic molecules escape from the surface of the liquid to enter the space above as a gas or vapor
- at constant temp, an equilibrium is established between the molecules in the vapor phase and those in liquid phase
- the gas phase above the liquid is said to be saturated when it contains all the anesthetic vapor it can hold at a given temperature, at which time the pressure exerted by the vapor is referred to as its SVP at that temp*
SVP:
Sevo
Iso
Des
Sevo: 160 mmHg
Iso: 238 mmHg
Des: 660 mmHg
partial pressure/total pressure= volume %
What concentrations do we aim for?
MAC: minimum alveolar concentration: the MAC value for an inhalation agent is on that causes a lack of response to painful stimulation in 50% patient. The painful stimulation would be equivalent to a surgical incision
- Typically, in order to cover nearly the entire population, agents will be administered above 1 MAC (1.2-1.3 MAC)
- dont forget synergy of narcotism and other IV drugs
Variable Bypass (Flow over method)
-controls how much gas goes through vaporizer
Splitting ratio
vaporizing chamber flow rate: by pass path way flow rate
Why temperature compensated?
Heat of vaporization:
- energy (heat) is required to promote the vaporization process
- heat is drawn from the liquid anesthetic itself as well as the container in vaporization process
- as vaporization proceeds, the cooling of the liquid anesthetic and the container would tend to slow the vaporization process
- since the vaporization is a temperature dependent process, if no heat is added, vaporizer output would decline
- accordingly, many vaporizers have temperature compensatory systems to ensure that heat loss due to vaporization is compensated and therefore does not reduce output
simple variable bypass: issues
- high flows would result in inadequate vaporization
wiking: increases the surface area (wicking absorbs more gas so there’s more surface area) - accuracy of concentration with mechanical control knobs
Electronic Vaporizer: Aladin Cassette
-similar to the mechanical variable bypass vaporizer, except a computer controls flows through the vaporizing chamber
Why is des different than other gases?
-Very high SVP= a conventional vaporizer would require really high flows to dilute it down to acceptable level
-it has a low boiling point (23.5 C)
at room temp, it will intermittently boil resulting in large fluctuations in agent delivery. When boiling, there is be excessive agent delivery; however, it will then cool due to large loss of latent heat of vaporization, resulting in an exponential decrease in SVP and under-delivery of agent
Ohmeda Tec 6
- overcomes problems with des by using electrical filament that heats the des to 39C, raising its SVP to 194 kPa, that is nearly, 2 ATM
- in addition to providing a stable SVP, this high pressure removes the need for pressurized carrier gas-instead, the fresh or diluent gas is entirely separate from vaporizing chamber
Safety in vaporizers
-color coded and each has its own key code
