• Delay time: Time to achieve 10% of a step change in reading at the gas monitor
• Rise time/response time: Time required for a change from 10% to
90% of the total change in a gas value
• Total system response time: DT + RT

• 2 types: sidestream (diverting) or mainstream (nondiverting)
• Sensor located directly in the gas stream (only for oxygen and CO
2
)
• Carbon dioxide: Using infrared technology with the sensor located between the breathing system and the patient
• Also available for the non-intubated patient: sensor attaches to a disposable oral and nasal adaptor
• Oxygen sensor: Uses electrochemical technology
 Usually placed in the breathing system inspiratory limb.
 Can measure both inspired and exhaled oxygen

• Advantages:
 Fast response times, no delay time (waveform has better fidelity)
 No gas is removed from the breathing system, so not necessary to scavenge these devices or to increase the fresh gas flow
 Water and secretions rarely cause a problem (although secretions on the windows of the cuvette can cause erroneous readings: problem with CO
2 sensor)
 Sample contamination by fresh gas is less likely
 Standard gas is not required for calibration (Oxygen sensor: calibrated by using room air)
 Use fewer disposable items than diverting monitors

• Disadvantages:
 Adaptor placed near the patient: cause traction/ weight
 Dead space
 Leaks, disconnections, circuit obstructions
 Condensed water, secretions, blood on the windows of the cuvette interferes with light transmission
 Sensor may become dislodged from the cuvette
 Expensive, vulnerable to damage
 Available only for oxygen and CO
2
 CO
2 sensor must be cleaned and disinfected between uses (potential for cross contamination), disposable become expensive
 Prolonged contact of the CO
2 sensor assembly with the patient could cause pressure injury

• Aspirates gas from the sampling site, sensor (located in the main unit)
• Sampling tube: short- decreases delay time and more satisfactory waveforms
• Usually zeroed using room air and calibrated using a gas of known composition
• Traps, filters and hydrophobic membranes
• Water droplets and secretions may increase resistance/ affect the accuracy
• Clear or purge the sample line, or tube may be changed

• Accuracy at 20-40 bpm and short length, > 40 bpm decreases accuracy
• Sampling flow rate: less than 150 mL/minute should not be used.
Elevated baseline, falsely low peak readings on lower flows
Devices used:
• Face mask: Large dead space relative to tidal volume, difficult to obtain accurate end-tidal values
 can be attached to upper lip or placed in patient's nares or the lumen of an oral or nasopharyngeal airway under the mask
 With a breathing system, most often attached to a component between the mask and the breathing system

• Tracheal Tube: Sampling site must be between the patient and the breathing system (measurement for both exp and insp)
 Sampling site should be away from the fresh gas port (in mapleson circuits can l/t errors)
 Tracheal tubes that incorporate a sampling lumen that extends to the middle or patient end of the tube are available
 Tracheal tube connectors with an attachment or hole for a sampling tube are available or can be created
• Supraglottic devices: sampling tube can be inserted through the connector (preferred site is the distal end of the shaft)
 may be inserted into a nasal airway

• Oxygen Supplementation Devices:
 OxyArm: allow simultaneous administration of oxygen and carbon dioxide monitoring
 In both nose and mouth breathers
 A nasal cannula can be modified to accept a sampling tubing (may l/t choking hazard)
 Sampling tube may be connected to mask outlet, inserted through a vent hole or a slit in the mask
 Accuracy affected by: Mouth breathing, airway obstruction, and oxygen delivery through the ipsilateral nasal cannula

OxyArm

• Jet Ventilation: an injector incorporating a sampling lumen or a sampling tube placed in the airway may be used
 Ventilatory frequency may need to be lowered to measure the endtidal CO
2
• Other ways:
 Sampling line can be placed in front of or inside the patient's nostril
 In mouth breather: in the nasopharynx or hypopharynx
 Catheter placed in the trachea after extubation for CO
2 monitoring
 Bite block can be modified to accommodate a sampling line
 Sampling line can be placed over a tracheostomy stoma

• Advantages of Diverting devices:
 Calibration and zeroing usually automatic (Occasional calibration is necessary, usually easy)
 Added dead space is minimal.
 Potential for cross contamination between patients low.
 Several gases can be measured simultaneously, allows automatic correction for nitrous oxide and oxygen.
 Sampling port can be used to administer bronchodilators
 These devices can be used when the monitor must be remote from the patient (e.g., during MRI)

• Disadvantages of Diverting devices:
 Leaks, sampling tube obstruction, or failure of the aspirator pump or can kink (use elbow connector)
 Sampling line can be connected to the wrong place
 Leak in sampling line can l/t mixing with air and so dilution of sample
 Aspirated gases must be either routed to the scavenging system (need to inc fresh gas flow) or returned to the breathing system
 Some delay time
 Supply of calibration gas
 Disposable items (adaptors and catheters) used
 Fresh gas dilution
 More variable differences between arterial and end-tidal CO
2 level

• Infrared Analysis
 Black body Radiation technology
 Microstream technology
• Paramagnetic oxygen analysis
• Electrochemical oxygen analysis
 Galvanic cell
 Polarographic electrode
• Peizoelectric Analysis
• Refractometry

• Most common technology in use
• Principle: Gases with two or more dissimilar atoms in the molecule
(nitrous oxide, CO
2
, and the halogenated agents) have specific and unique infrared light absorption spectra.
• Amount of infrared light absorbed is proportional to the concentration of the absorbing molecules, the concentration can be determined
• Nonpolar molecules cannot be measured
• 2 technologies available:
 Black body radiation
 Microstream technology

• Utilizes a heated element called a blackbody emitter as the source of infrared light, produces a broad infrared spectrum (redundant radiation to be removed using filters)
• Optical detectors must be calibrated to recognize only infrared radiation that is modulated at a certain frequency by using a spinning chopper wheel
• Analyzer selects the appropriate infrared wavelength, minimize absorption by other gases that could interfere with measurement of the desired component

• Then an electrical signal is produced and amplified, and the concentration is displayed
• For halogenated agents: separate chamber to measure absorption at several wavelengths (single-channel, four-wavelength infrared filter photometers) have filter for each anesthetic agent and one to provide a baseline for comparison

Diverting type:
• Gas to be measured is pumped continuously through a measuring chamber
• Filtered and pulsed light is passed through the sample chamber and also through a reference chamber (has no absorption characteristics)
• Light is focused on an infrared photosensor (partly absorbed by the sample gas acc to conc)
• Changing light levels on the photosensor produce changes in the electrical current running through it
• Provides hundreds of readings for each respiratory cycle (Continuous waveform produced)

• Monochromatic analyzers use one wavelength to measure potent inhalational agents (unable to distinguish between agents)
• Polychromatic analyzers use multiple wavelengths to both identify and quantify the various agents
• Measuring cell is calibrated to zero (using gas that is free of the gases of interest, usually room air) and to a standard level (using a calibration gas mixture)
Non Diverting Type:
• Gas stream passes through a chamber (cuvette) with two windows, placed b/w the breathing system and the patient
• Sensor (has both the light source and detector) fits over the cuvette
• Sensor is heated slightly above body temperature (to prevent condensation)

• Infrared light passes through window on one side of the adaptor, sensor receives the light on the opposite side
• Then light goes through three ports in a rotating wheel, containing
(a) a sealed cell with a known high CO
2 concentration
(b) a chamber vented to the sensor's internal atmosphere
(c) a sealed cell containing only nitrogen
• Then passes through a filter (to isolate CO
2 information)
• Signal amplified and sent to the display module
• Calibration done using: low calibration cell contains 100% nitrogen, high cell contains a known partial pressure of CO
2
• Corrections for nitrous oxide and oxygen entered manually

Infrared Mainstream Analyser

• Uses laser-based technology to generate infrared rays that match the absorption spectrum of CO
2
• Smaller sample cell, low flow rate
• Emission source: Glass discharge lamp coupled with an infrared transmitting window
• Electrons (generated by a radio frequency voltage) excite nitrogen molecules  Carbon dioxide molecules are excited by collision with the excited nitrogen molecules  These drop back to their ground state and emit the signature wavelength of CO
2
• This emission now passes through main optical detector and reference detector (compensates for changes in infrared output)

• Measurements made every 25 msec
• As low sample flow and small sample cell, useful for measuring:
 CO
2 in very small patients
 high respiratory rates
 low-flow applications
 unintubated patients
• Readings not affected by high concentrations of oxygen or anesthetic gases

• Multigas Capability
• Volatile Agent Detection
• Small, compact, lightweight
• Quick response times (faster for CO
2
)
• Short warm-up time
• Convenience (no complicated calibrations)
• Lack of interference from other gases (argon, low conc NO)
• Detecting anaesthetic agent breakdown (desflurane to CO will show as wrong or mixed agent)

• O
2 and N
2 cannot be measured
• Gas interference :
 O
2 causes broadening of CO
2 spectrum l/t lower readings
 N
2
O absorption spectrum overlaps with CO
2
(l/t higher vlues): so need either automatic or manual correction for N
2
O
 He l/t underestimation of CO
2
• Other substances l/t inaccuracies (ethanol, methanol, diethyl ether, methane): give high volatile agent reading, polychromatic less affected
• Water vapors: Absorb infrared rays (l/t lower values), use water traps, hydrophobic membranes
• Slow response time (with rapid resp rates)
• Difficulty in adding new volatile agents

• Paramagnetic substances: Substances which locate themselves in the strongest portion of the field when introduced into a magnetic field
• Oxygen is paramagnetic
• Principle: When a gas that contains oxygen is passed through a switched magnetic field, the gas will expand and contract, causing a pressure wave that is proportional to the oxygen partial pressure
• Pressure difference is detected by the transducer and converted into an electrical signal that is displayed as oxygen partial pressure or volumes percent.

• Short rise time so both inspired and end-tidal oxygen levels can be measured
• Desflurane disturbs the paramagnetic oxygen sensor and it reads higher than expected

• Consists of a sensor, analyzer box, display, and alarms
• Sensor: A cathode and an anode surrounded by electrolyte
• Sensor is placed in the inspiratory limb
• Gel held in place by a membrane (nonpermeable to ions, proteins, but is permeable to oxygen)
• Older ones respond slowly to changes in oxygen pressure, so cannot be used to measure end-tidal concentrations (not so with new analyzers)
• Technology:
 Galvanic cell/ fuel cell
 Polarographic electrode

• Principle: Oxygen diffuses through the sensor membrane and electrolyte to the cathode, where it is reduced, causing a current to flow
• Current generated is proportional to the partial pressure of oxygen in the gas
Cathode: O
2
+ 2H
2
O + 4e → 4OH -
Anode: 4OH + 2Pb → 2PbO + 2H
2
O + 4e -
• Cathode is the sensing electrode, anode is usually consumed
• The current is strong enough to operate the meter so a separate power source is not required to operate the analyzer
• The chemical reaction is temperature dependent (a thermistor may be connected in parallel with the sensor.)

Galvanic cell
Fuel cell Oxygen Analyzer

• Sensor comes packaged in a sealed container that does not contain oxygen
• Its useful life is cited in percent hours: the product of hours of exposure and oxygen percentage
• Sensor life can be prolonged by removing it from the breathing system and exposing it to air when not in use
• Whole sensor cartridge must be replaced when it becomes exhausted

• Components: anode, a cathode, an electrolyte, and a gas-permeable membrane
• Needs power source for inducing a potential between the anode and the cathode
• Same principle as galvanic cell
• May be either preassembled disposable cartridges or units that can be disassembled and reused by changing the membrane and/or electrolyte
Advantages:
 Easy to use, low cost, compact
 No effect of argon

Disadvantages:
 Maintenance (more in polarographic)
 Need to be calibrated every day (every 8 hrs)
 Slower response time

• Uses vibrating crystals that are coated with a layer of lipid to measure volatile anesthetic agents
• Principle: When exposed to a volatile anesthetic agent, the vapor is adsorbed into the lipid  resulting change in the mass of the lipid alters the vibration frequency
• By using an electronic system consisting of two oscillating circuits, one has an uncoated (reference) crystal and the other a coated (detector) crystal, an electric signal that is proportional to the vapour concentration is generated
• Diverting devices

 Accuracy
 Fast response time
 No need for scavenging
 Short warm up time
 Compact
 Only one gas measured
 No agent discrimination
 Inaccuracy with water vapours

• Consists of a pH-sensitive indicator
• Principle: When the indicator is exposed to carbonic acid that is formed as a product of the reaction between CO
2 and water it becomes more acidic and changes color
Technology:
 Hygroscopic
 Hydrophobic
Uses:
• For confirming tracheal intubation when a capnometer is not available
• Disposable so it may be useful to confirm tracheal intubation in patients with respiratory diseases (e.g.SARS)

Advantages:
• Easy to use, small size, low cost
• Not affected by N
2
O, volatile anaesthetics
• Offers minimal resistance to flow
• CO doesn’t interfere
Disadvantages:
• Recommended to wait six breaths before making a determination
• False-negative results may be seen with very low tidal volumes
• Drugs instilled in the trachea or gastric contents can cause irreversible damage to the device
• False-positive results can occur if CO
2 in the stomach
• Semiquantitative, cannot give accurate measurement of CO
2
(So use limited to check endotracheal intubation)

• Optical interference refractometer (interferometer): Light beam passes through a chamber into which the sample gas has been aspirated, also passes through an identical chamber containing air.
• Vapour slows the velocity of light, the portion passing through the vapor chamber is delayed
• Beams form dark and light bands, position of these bands yields the vapor concentration
• Used primarily for vaporizer calibration
• Sensitive to nitrous oxide (so cannot be used to measure halogenated agents in a O
2
, N
2
O, agent mixture)

• Standard requirements:
 Oxygen readings shall be within ±2.5% of the actual level (min for 6hrs together)
 The high and low oxygen level alarms must be at least medium priority, oxygen concentration below 18% (should be high priority alarm)
 It shall not be possible to set the low oxygen alarm limit below 18%
• Technology used:
 Electrochemical Technology
 Paramagnetic Technology: Rapid response time, even for nonintubated

•
 Oxygen monitor provides earlier warning of inadequate oxygen than pulse oximetry
 Problems resulting from hyperoxygenation: patient movement during surgery, awareness, damage to the lungs and eyes, fires
•
 However not dependable
•
 Normal: Difference b/w inspired and expired oxygen is 4% to 5%
•
 Assess pt’s oxygen consumption (Malignant hyperthermia)
 To detect air embolism (inc ET O
2
)

• Means for assessing metabolism, circulation, and ventilation
• ASA guidelines: Correct positioning of ET tube must be verified by identifying CO
2 in the expired gas
• Capnometry: Measurement of CO
2 in gas mixture
• Capnography: Recording of CO
2
Conc versus time
• CO
2 reading shall be within ±12% of the actual value or ±4 mm Hg
• Must have a high CO
2 alarm for both inspired and exhaled CO
2
• Infrared Analysis
• Chemical colorimetric analysis

• Metabolism
• Respiration
• Circulation
• Equipment Function
• Confirming endotracheal and enteric tube placement

• Dec ET CO
2
:
 Impaired peripheral circulation
 Impaired lung circulation (Pulmn embolus)
 Increased patient dead space
 Hyperventilation
 Hypothermia
 Increased depth of anaesthesia
 Use of muscle relaxants
 Leak in sampling line
 Leak around ET

• Increased ET CO
2
:
 Absorption of CO2 from peritoneal cavity
 Injection of NaHCO
3
 Convulsions
 Hyperthermia
 Pain, anxiety, shivering
 Increased muscle tone (reversal of muscle relaxation)
 Hyperventilation
 Upper airway obstruction
 Rebreathing
 Increased circulation from tissues to lung (release of tourniquet)

• Absent waveform:
 Esophageal intubation
 Disconnection
 Apnea
 Blockade of sampling line

• Normal: PaCO
2
– ET CO
2
= 2-5 torr
• Altered with:
 Reduced FRC (Pregnancy, Obese pt)
 Rebreathing
 Neurosurgical procedures
 During one lung ventilation
(In these cases transcutaneous CO
2 monitoring more accurate)

• Examined for:
 Height (Depends on ETCO
2
)
 Frequency (R.R.)
 Rhythm
 Baseline (normally zero)
 Shape (Top hat or Sine wave is normal)

• Phase 1: E (Inspiratory baseline)
• Phase 2: B to C (Expiratory upstroke), S shaped- represents transition from dead space to alveolar space
• Phase 3: C to D (all from alveoli)
• End of Phase 3 (Point D): End tidal point (Max CO
2
)
• Alpha : Angle b/w Phase 2 & 3 (normal 100-110 degree)
• Beta: B/w end of phase 3 & Descending limb (90 degree)

• The slope of phase 3 (C to D) increases:
 With PEEP
 Airway obstruction
 V/Q mismatch
And so angle Alpha also increases
And angle Beta decreases
• Angle Beta increases with:
 Rebreathing
 Prolonged response time

Unusual waveforms:
• Leak in sample line: Brief peak at the end of plateau
• Partially paralysed (making intermittent resp effort) :Curare cleft
• Cardiogenic occilations:
Seen in pediatric pts
(d/t heart beating against
Lungs)

Volatile Anaesthetic Agents:
• Standard Requirements:
 Difference in value shall be within ) 0.2% vol%
 High conc alarm is mandatory, low conc alarm is optional
• Measurement technique:
 Infrared Analysis
 Refractometry
 Peizoelectric Analysis
• Significance:
 Assess vaporizer function and contents
 Information of patient uptake of the agent (insp & exp conc)
 Information on Anaesthetic depth
 Detecting contaminants/ disconnection

Nitrous Oxide:
• Measurement technique:
 Infrared Analysis
• Significance:
 Assess flowmeter function
Nitrogen:
• Previously measured using Raman spectroscopy or mass spectrometry
• Now no longer available
• Significance:
 Verifying adequate denitrogenation before induction (imp for pediatric pt, in lung ds, dec FRC)
 Detecting air emboli (Inc ET N2)