Anesthesia Gas Monitoring Flashcards
Patient Safety: Guiding Concern in Development of Monitors
- Oxygen concentration
- disconnect alarms
- end tidal CO2
- pulse ox
- peak pressure monitoring
- anesthesia gas monitoring
- -N2O
- -desflurane
- -sevo
- -iso
- -oxygen
- -CO2
- -nitrogen
Prevention and Detection
-most adverse outcomes come from misuse by practitioner and/or failure to detect equipment failure when it happens
Non diverting gas monitor
- Mainstream, in-line
- sensor is located directly in the gas stream
- only CO2 and oxygen are monitored with this mode (cannot monitor volatile gases)
- oxygen: fuel cell (electrochemical)
- CO2 infrared
Diverting gas monitor
-sidestream
-gas is aspirated from sampling site and through a tube to sensor located inside or on top of machine
-ALL gases can be monitored this way
oxygen: paramagnetic
volatiles, nitrous oxide, and CO2 infared
Infrared Analysis (Diverting or NonDiverting)
- most often used analysis for CO2, nitrous oxide and volatile agents
- molecules containing dissimiliar atoms will absorb infrared radiation
- this technology does NOT work for oxygen and nitrogen
- tend to underestimate inspired levels and overestimate expired levels at high respiratory rates
Infrared Analysis
-most molecules will absorb infrared at specific wavelengths and hence the molecule can be identified and its concentration measured
Beer-Lambert Law
- absorption is according to this
- there is a logarithmic dependence between the transmission of light through a substance and concentration of that substance
IR Side Stream Sampling Diverting
- continuously aspirates a sample of the gas from patient circuit, usually near where breathing circuit is connected to the airway device
- 50-250 ml/min aspirated (may be returned to patient or to scavenging)
- sample direct to place between infrared emitter, optical filter, and infrared detector, which outputs a signal proportional to remaining infrared energy not absorbed by the gases
- to quantify and identify multiple gases simultaneously multiple optical filters are required
- detected signal then amplified and interpreted via microprocessors
The Good on side stream sampling
- automatical calibration and zeroing
- quick response time and short warm up
- minimal added dead-space
- low potential for cross-contamination between patients
What could be better on side stream sampling
- multiple places that leaks may occur
- more variability in CO2 readings than with in line sampling- accurate with RR 20-40, decreased with increased rate
- slower response to changes than with in line sampling
- water contamination (water traps)
Gas Monitoring
-to monitor CO2 the sensor must be positioned between the patient and the circuit, ideally closest to the patient end as possible
why- dead space
Dead space: wasted ventilation
-ventilated areas which do not participate in gas exchange
Total deadspace= anatomic + alevolar + mechanical
Anatomic deadspace
-airways leading to alveoli
Alveolar deadspace
-ventilated areas in lungs without blood flow
Mechanical deadspace
-artificial airways including ventilator circuits
Inspired Oxygen Analysis
- FiO2 monitor is extremely important in patient safety
- first line of defense against detecting hypoxic mixtures
- but.. ventilation and oxygenation must be considered as two separate entities
- pulse oximetry is a late indicator of hypoxemia
Low V/Q
-shunt perfusion: alveoli perfused but not ventilated
ET tube in mainstream bronchus
V/Q= .8
normal
alveoli perfused and ventilated
High V/Q
deadspace ventilation
-alveoli ventilated but not perfused
(cardiac arrest)
Two types of oxygen analyzers
- Paramagnetic (Diverting)
- more expensive, no need to calibrate, fast enough to differentiate oxygen concentrations - Electrochemical (Non Diverting)
- galvanic (fuel cell)
- calibration needed
Paramagnetic Oxygen Analysis
- unpaired electron in the oxygen molecule is attached to magnetic field
- when oxygen passed through magnetic field it goes to the strongest portion of that field
- expansion, contraction of the gas creates a pressure wave that is a proportional to the oxygen’s partial pressure
Paramagnetic Oxygen Analyzers
- both inspired and end tidal oxygen levels to be measured even at rapid respiratory rates
- auto calibrates with reference gas (air or known concentration oxygen)
- many monitors combine diverting IR analysis of CO2, volatiles and nitrous oxide with a paramagnetic oxygen analysis using the same side stream sample
Electrochemical Oxygen Analysis (Fuel cell or Galvanic)
- oxygen diffuses through sensor membrane and electrolyte to cathode ray tube
- reduced there (gains electrons), allowing a current to flow
- rate at which oxygen enters cell and generates current is proportional to the partial pressure of the gas outside of the membrane
Electrochemical Oxygen Analyzer
-usually placed on or near the carbon dioxide canister on the inspiratory side
the good: cheaper
the bad: calibration every 8 hours, need frequent changing
