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
End-tidal CO2 monitoring
- validation of proper endotracheal tube placement
- detecting and monitoring of respiratory pathophys
- hyper/hypoventilation
- 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)
- hypothermia
- decreased CO (cardiac arrest)
- pulmonary embolism
- bronchospasm
- increased minute ventilation
normal arterial CO2:
paCO2 values
35-45 mmHg
- 7-6.0 kPA
- 6-5.9%
ETCO2
-capnograph
30-43 mmHg
4.0-5.7 kPa
4-5.6%
Capnography
- measurement and display of both ETCO2 value and capnogram (CO2 waveform)
- mesaured by capnograph
- **picture of waveform
Capnometry
- measurement and display of ETCO2 value (no waveform)
- measured by capnometer
Value of capnogram
- provides validation of ETCO2 value
- visual assessment of patient airway integrity
- verification of proper ETT placement
- assessment of ventilator/breathing circuit integrity
Quantitative vs. Qualitative ETCO2
Quantitative: provides actual numeric vale, found in capnographs and capnometer
Qualitative: only provides range of values, termed “CO2 Detector”
Draw normal CO2 waveform and label parts
-A-B baseline B-C expiratory upstroke C-D expiratory plateau D- ETCO2 value D-E inspiration begins
CO2 with esophageal intubation
-little or no CO2 is present
CO2 waveform with inadequate seal around ETT
cause: leaky or deflated ET or trach cuff
- artificial airwway too small for patient
Hypoventilation causes what in ETCO2?
Increase in ETCO2 -possible cause: decreased RR decreased TV increased in metabolic rate rapid rise in body temp
Rebreathing– possible cause
faulty expiration valvue
inadequate inspiratory flow
insufficient expiratory flow
malfunction of CO2 absorber system
Obstruction-possible causes
kinked or occluded artificial airway
presence of foreign body in the airway
obstruction in expiratory limb of breathing circuit
bronchospasm
Spontaneous breathing Effort with Controlled Ventilation
-curare cleft:
appears when muscle relax being to subside
-depth of clef is inversely proportional to degree of drug activity
Sudden loss of waveform
#1 thought= airway disconnection -apnea, airway obstruction, dislodged airway, ventilator malfunction, cardiac arrest
Colormetric CO2 detector
- NOT A MONITOR
- uses chemically treated paper that exchanges color hen exposed to CO2
- must match color to a range of values
- requires 6 breaths before determination
- gold is golden*
