Capnography

Question 1

Capnography

Definition, what reflects

Answer 1

• Aka infrared spectography
• A rapid and accurate method to measure exhaled CO2 throughout the respiratory cycle.
• Typically measured at the junction between the patient and the breathing system
• Directly shows elimination of CO2 by the lungs
• Indirectly reflects elimination of CO2 by tissues and adequate perfusion of lungs
• etCO2 is the CO2 at the end of expiration

Question 2

etCO2 compared to arterial CO2 in healthy adults with normal lungs

Answer 2

etCO2 is 0.3-0.6kPa less than the arterial CO2

Question 3

Capnograph vs capnogram vs capnometer

Answer 3

Capnograph = records and shows the graphical display of waveform of CO2. Displays value of etCO2
Capnogram = graphical plot of CO2 partial pressure (or percentage) vs time (see picture)
Capnometer = instrument which only shows numerical concentration of CO2 (no waveform)

Question 4

Capnography mechanism: absorption of radiation by CO2

Answer 4

• Capnograph works via infrared spectrometry
• Gases with molecules that contain at least 2 dissimilar atoms (eg. carbon and oxygen in CO2) absorb radiation in the **infrared region of the spectrum **
• CO2 absorbs radiation at wavelength of 4.3 micrometers
• Beer-Lambert law: the absorption of light depends on the properties of the material through which the light is travelling -> amount of radiation absorbed is proportional to the number of CO2 molecules (partial pressure of CO2)

Question 5

Components of a capnograph (4)

Answer 5

Light source
* Infrared radiation emitted by a hot wire or light source
* Often microprocessor controlled to produce stable source with constant output

Interference filter
* Particular frequency of radiation required is obtained by passing radiation through an interference filter
* An optical filter in which the wavelengths not transmitted are removed by interference phenomena (i.e. rather than absorption or scattering)

Sample chamber and reference chamber
* Windows made from material transparent to infrared radiation e.g. sodium chloride, silver bromide or sapphire (because glass absorbs infrared radiation)
* Infrared passes simultaneously through sample chamber (containing sample gas) and reference chamber (containing room air). Absorption from each chamber is compared -> CO2 values calculated

Photodetector: thermopile detector or sensor
* After passing through the sample and reference chambers, radiation is focused on a photodetector
* Greater absorption of infrared radiation by gas in sample chamber -> less radiation falls onto detector
* Thermopile detector produces heat which is proportional to partial pressure of CO2 in the mixture in the sample chamber. Heat is measured by a temperature sensor
* -> changes in electrical output and voltage in a circuit

Note: due to large amount of infrared absorption by CO2 in the sample chamber, little infrared finally reaches the detector

Question 6

Other methods to measure CO2 (4)

Answer 6

• Raman spectrometry
• Mass spectrometry
• Photo acoustic spectrometry
• Chemical colorimetric analysis

Question 7

Raman spectrometry

Answer 7

Alternative method to measure CO2
* Gas exposed to monochromatic light from an argon laser
* Energy from the light is absorbed by the intermolecular bonds and partially re-emitted at new wavelengths by the molecules
* Wavelength shift and scattering can be used to measure hte concentration of gases in the system

Advantages:
* allows measurement of all the gases in breathing system (CO2, O2, N2O, volatile anaethetic agents)
* Small and portable
* Gases returned to breathing system unchanged

Disadvantages: very slow response rate

Question 8

Mass spectrometry

Answer 8

Alternative method to measure CO2
* Gas drawn into the spectrometer where it is ionised, then exposed to magnetic field in a vacuum chamber
* Various gases are separated according to their mass to charge ratio
* Concentration of various gases at known mass to charge ratio then calculated

Advantages:
* Highly accurate, reproducible
* Measurement of many gases

Disadvantages:
* Equipment is very bulky and expensive
* Susceptible to damage from water and some drugs
* Ionized gases cannot be returned to the breathing system, must be scavenged

Question 9

Photo acoustic spectrometry

Answer 9

Alternative method to measure CO2
* Sample gas is irradiated with a suitable wavelength pulsatile infrared radiation
* -> periodic expansion and contraction -> pressure fluctuation of aurible frequency that can be detected by a microphone in proportion to the amount of gas present

Advantages:
* Extremely stable, calibration remains constant over much longer periods of time compared to conventional infrared absorption spectrometry
* V fast rise and fall times -> more accurate representation of any change in CO2 concentration

Question 10

Chemical colorimetric analysis

Answer 10

Alternative method to measure CO2
* CO2 hydrated -> carbonic acid -> can be measured by pH sensitive means
* pH sensitive dye undergoes a colour change in the presence of CO2. Usually metacresol purple, changes to yellow in presence of CO2

Advantages:
* Detectors are small and portable
* Sometimes used in the field/pre-hospital

Disadvantages:
* Only gives crude assessment of CO2 level (low, normal, high)

Question 11

Capnograph response time

Transit time, rise time

Answer 11

To be clinically useful, capnograph needs rapid response time, which has two components: transit time and rise time.

Transit time: time for sample to be transferred to analyser.
* Depends on length (i.e. short as possible e.g. 0.2m) and diameter of the sampling tubing, and sampling rate
* Delay of <3.8s is acceptable

Rise time: time for analyzer to respond to the signal. Depends on size of sample chamber and the gas flow

Question 12

Side stream capnograph: set-up

Answer 12

• Sampling chamber connected to distal end of breathing system via sampling tube
• Sampling tube is 1.2mm internal diameter tube that samples both inhaled and exhaled gases at constant rate of ~150-200ml/min. Made of teflon (so impermeable to carbon dioxide, does not react with anaesthetic agents)
• Light weight adaptor connects sampling tube to breathing system with small increase in the dead space.
• Moisture trap with an exhaust port allows gas to be vented to the atmosphere or returned to the breathing system

Key points for use
* Sampling tube should be placed close to patients trachea for accuracy
* Other gases and vapours CAN also be analyzed from the same sample

Question 13

Main stream capnograph: set-up

Answer 13

• Sampling chamber positioned within patient’s gas stream -> increasing the dead space
• Heated to about 41 degrees C to prevent water vapour condensation on its windows

Key points
* No sampling tube: no transport time delay in gas delivery to sample chamber
* CANNOT measure other gases and vapours simultaneously

Question 14

Side stream vs main stream capnography: advantages

Answer 14

Side stream capnograph:
* Easy to connect
* Can be used in awake or non-intubated patients
* Easy to use when patient is in unusual positions e.g prone positioning
* No problems with sterilisation
* Multiple gas analysis (but calibration gas needed)

Main stream capnograph:
* Suitable for neonates and children
* No sampling tube -> no transport time delay
* No effect due to pressure drop
* No effect due to changes in water vapour pressure

Note, both produce a capnogram

Question 15

Label the phases of this normal waveform

Answer 15

• I, baseline: Inhalation ends, exhalation begins, dead space air is eliminated first and no CO2 is present
• II, ascending phase, early exhalation: alveolar gas begins to mix with dead space air, sharp upstroke is produced
• III, alveolar plateau: alveolar air predominates, CO2 level plateaus. etCO2 is noted at end of exhalation (D)
• IV, descending phase, inhalation: inhalation occurs, CO2 level returns to baseline

Question 16

What is normal etCO2?
Why is etCO2 less than alveolar CO2?
Why is the alveolar CO2 less than arterial CO2?

Answer 16

End-tidal carbon dioxide< alveolar carbon dioxide< arterial carbon dioxide

• Normal ETCO2 is 4.5-5.5 kPa (35-45mmHg)
• End-tidal CO2 is less than alveolar CO2 because the end-tidal CO2 is always diluted with alveolar dead space gas from unperfused alveoli. These alveoli do not take part in gas exchange and so contain no CO2.
• Alveolar CO2 is less than arterial CO2 as the blood from unventilated alveoli and lung parenchyma (both have higher CO2 contents) mixes with the blood from ventilated alveoli. In healthy adults with normal lungs, end-tidal is 0.3-0.6 kPa less than arterial CO2. This difference is reduced if the lungs are ventilated with large tidal volumes.

Question 17

Uses of waveform analysis of capnography (7)

Answer 17

• Measure respiratory rate -> alarms for hypo- or hyperventilation
• Confirm tracheal intubation. NB in oesophageal intubation, some CO2 might be present in stomach, may have 5-6 waveforms with abnormal shape decreasing in amplitude
• Discconection alarm for ventilator or breathing system
• Diagnose lung embolism: sudden decrease in end-tidal CO2, assuming BP remains stable
• Diagnosed malignant hyperthermia (dramatic increase in etCO2 in ventilated patient/ dramatic increase in RR in spontaneously breathing patient)
• Indicate rebreathing (baseline of capnogram >0) due to exhausted soda lime, inadequate fresh gas flows, or faulty valve
• Monitor recovery in patients recovering from neuromuscular blockade during PPV (clefts in plateau phase of capnogram) or cardiogenic oscillations in patient starting to breathing spontaneously

etc

Question 18

Causes of inaccuracy in capnography

Answer 18

• Obstructive airway disease -> waveform shows sloping trace, does not accurately reflect the end-tidal CO2
• Collison broadening (where absorption spectrum of CO2 is effectively broadened by presence of nitrous oxide in anaesthetic gas mixture)
• Can be difficult to produce and interpret during paediatric anaesthesia
• Equipment problems -> variation in normal capnographs

Question 19

What equipment problem does this show

Answer 19

Sticky inspiratory valve

Question 20

What equipment problem does this show

Answer 20

Leak in sampling tube

Question 21

What does this capnogram show and why

Answer 21

Obstructive airway disease.
Ascending plateau on capnograph usually indicates impairment of the ventilation:perfusion ratio because of uneven emptying of alveoli

In patients with chronic obstructive airways disease, the alveoli empty at different rates because of the differing time constants in different regions of the lung with various degrees of altered compliance and airway resistance.

Question 22

How can the presence of nitrous oxide affect the measurement of end-tidal partial pressure of CO2?

Answer 22

• Capnography: Nitrous oxide absorbs infrared radiation and can artificially increase the reading on a capnograph (collision broadening).
• Mass spectrometry: Nitrous oxide has the same molecular weight as carbon dioxide and can lead to an overestimate of the measurement of carbon dioxide by mass spectrometry.

Question 23

Which condition is shown in each capnograph?

Answer 23

COPD/asthma i.e. obstructive airways disease
Lung embolism (sudden decrease in etCO2)
Rebreathing (baseline > 0)
Patient recovering from neuromuscular blockade
Oesophageal intubation