25.3 Capnography + Light Absorption

Question 1

a) Outline the principles of capnography using infrared light absorption. (30%)

Answer 1

1&raquo_space; Molecules containing dissimilar atoms
absorb infrared light,
resulting in molecular vibration.

2&raquo_space; A small proportion of the patient’s
expired gas is diverted to the
capnography sample chamber.

3&raquo_space; Infrared (generated by a heated wire)
is filtered to the appropriate wavelength
and passed through the sample chamber
and a reference chamber.

4&raquo_space; The chamber windows are made of crystal,
as glass will absorb infrared,
thus affecting overall absorption.

5&raquo_space; The proportion of infrared absorbed
by the carbon dioxide depends
on the Beer-Lambert law,

i.e. the concentration of the carbon dioxide
present in the chamber

(the variable being measured)
and the path length
(which is the distance within the chamber
and is therefore fixed).

6&raquo_space; The remaining radiation is focused
onto a photodetector and an
electronic monitor displays the
exhaled carbon dioxide concentration and
waveform.

7&raquo_space; Modern capnographs use a disc of
rotating filters in order to measure the
concentration of volatile agents as well

Question 2

b) What diagnostic information can be gained from capnography in anaesthetic practice? (40%)

Answer 2

Information is gained from the absolute value of end-tidal carbon dioxide
(etCO2) and the waveform.

1. Ventilatory sufficiency

High if underventilated, low if overventilated.

Normal waveform,
but height proportional to value of
etCO2.
Rate will be demonstrated by the number of
waveforms per unit time.

2. Respiratory disease,
   ranging from stable
   chronic obstructive pulmonary disease to
   acute bronchospasm under anaesthesia

May be normal or elevated.
Gradually upsloping phase 3,
failure to achieve plateau.

3. Acute reduction or loss of cardiac output
   Rapidly reducing etCO2 value.
   Normal morphology but waveform rapidly
   reducing in size
   with sequential breaths.
4. Soda lime exhaustion or inadequate
   fresh gas flow rate
   Elevated etCO2.
   Baseline not returning to zero between breaths.
5. Disconnection of breathing system/accidental
   extubation/dislodgement of tracheostomy tube

No etCO2.
Sudden loss of trace.

6. Inadequate paralysis

If inadequate paralysis interferes with
effective ventilation, etCO2 will rise.

Clefts in the plateau phase of the trace.
Extra, small waveforms interspersed in the overall trace.

7. Incorrect tube placement

Reducing value of etCO2, if any at all.

It is possible that there may be approximately three
smaller-than-usual waveforms after oesophageal
intubation (as carbon dioxide present in the stomach is
expelled), or there may be no trace at all.

8. Malignant hyperthermia

Rapidly rising etCO2 value.

Morphology of the trace will remain normal,
but the height of the waveform will
rise with successive breaths.

Question 3

c) In which clinical situations and locations should continuous capnography be available for use? (30%

Answer 3

> > All anaesthetised patients,
regardless of the airway device
used or the location of the patient.

> > All intubated patients, regardless of location.

> > All patients undergoing moderate or deep sedation.

> > All patients undergoing advanced life support.

> > Continuous capnography should be
available wherever patients are recovered from anaesthesia and moderate or deep sedation.