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Flashcards in Anesthesia Gas Monitoring Deck (43)
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Patient Safety: Guiding Concern in Development of Monitors

-Oxygen concentration
-disconnect alarms
-end tidal CO2
-pulse ox
-peak pressure monitoring
-anesthesia gas monitoring


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

-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

alveoli perfused and ventilated


High V/Q

deadspace ventilation
-alveoli ventilated but not perfused
(cardiac arrest)


Two types of oxygen analyzers

1. Paramagnetic (Diverting)
-more expensive, no need to calibrate, fast enough to differentiate oxygen concentrations
2. 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


End-tidal CO2 monitoring

-validation of proper endotracheal tube placement
-detecting and monitoring of respiratory pathophys
-cardiac function, circuit disconnection or leaks
-adjustment of parameter settings in mechanically ventilated patients
-Estimate PaCO2


ETCO2 and cardiac resuscitation

-Non survivors- average ETCO2: 4-10 mmHg
Survivors (to discharge): average ETCO2: >30 mmHg
-if patient is intubated and pulmonary ventilation is consistent with bagging, ETCO2, will directly reflect CO
-flat waveform can establish PEA
-configuration of waveform will change the obstruction


Increase in ETCO2

-increased muscular activity (shivering), MH
-increased CO (during resuscitation)
-bicarbonate infusion
-tourniquet release
-effective drug therapy for bronchospasm
-decreased minute ventilation


Decrease in ETCO2

-decreased muscular activity (muscle relaxants)
-decreased CO (cardiac arrest)
-pulmonary embolism
-increased minute ventilation


normal arterial CO2:
paCO2 values

35-45 mmHg
4.7-6.0 kPA



30-43 mmHg
4.0-5.7 kPa