Why monitor CO2?

 

Global well-being.

The excretion of CO2 is the final common pathway of metabolism. As such it provides a useful global indication that all is well.

  • ventilation must be sufficient to carry oxygen into the lungs
  • oxygen is being transported to the mitochondria (cardiovascular function)
  • aerobic metabolism is consuming this oxygen and producing carbon dioxide
  • CO2 is being transported to the lungs (cardiovascular function)
  • CO2 in the expired air gives an indication of adequate ventilation

Note: CO2 monitoring gives no information on regional blood flow. Cerebral or coronary ischaemia is possible in the presence of normal capnography.

 

Risk management

The Dutch looked at preventable anaesthetic deaths in healthy people having minor surgery. They demonstrated on the way that there is no such thing as minor anaesthesia! They found the commonest problems were with the airway and that the earliest detection would be with CO2 analysis. Their response was to make CO2 analysis compulsory in 1980. Their work has been demonstrated again by the AIMS data.

The college of anaesthesia in Australia took an interesting approach in recommending that a CO2 analyser be available for every intubated AND ventilated patient by 1997.

 

Specific diagnostic patterns

Carbon dioxide analysis can assist diagnosis of a number of patient problems. These will be described below.

 

How do CO2 monitors work?

A light is shone through the expired air and the degree of absorption of a certain frequency of infra-red light is proportional to the concentration of CO2. The light may be split with half passing through a reference cell. The light may also be 'chopped' so that it is not continuously heating the gas in the reference cell.

The analyser may be placed in one of 2 places: in-line, and out of circuit at the end of a sampling tube.

 

In-line analyser

Advantages Disadvantages
No sampling tube to block bulk
  needs to be heated
  windows fogging up
  can't be used on non-intubated patients

 

Sampling
The sampling variety has the big disadvantage of a sampling tube which tends to get blocked with condensation. Some companies offer a special tube that allows water to escape. They are an expensive gimmick.


There are two main graphs that we look at which are a function of the sweep speed.
These are waveforms and CO2 trends.

At high sweep speed we get a wave form of the CO2 from each breath which is known as the capnogram. There is only one normal shape:

cap1

 

At first there is a rapid rise as the dead space gas comes out of the major airways. Then there is a plateau which is allowed to have a slow rise.

Finally there is a rapid decline as the next breath enters the patient.

 

Arterial pCO2

The plateau is essential for accurate analysis.
The normal end tidal value is about 40 mm Hg or 5%. In the absence of significant cardiac shunts there is no significant alveolar to arterial CO2 gradient so what you are seeing is also the arterial CO2 concentration.

In a patient with chronic airways disease the slope may be increased and the end tidal value somewhat higher. This is useful to record at the beginning of an anaesthetic to prevent over ventilation. It is surprising how easy it is to hyperventilate an elderly patient. The standard 15 ml/kg 12 breaths /Min will drop their CO2 to 25 mm Hg. This will make them slow to breath at the end of the case. The other real danger is the cerebral vasoconstriction caused by the low CO2. Perhaps that has been the reason that granny is sometimes 'not quite herself' after the operation.

 

Intubation

First of all if you get a capnogram you can be sure you have intubated the trachea. It is said you may get three or four breaths of CO2 from the stomach. We attach the CO2 analyser to the ET tube before intubation. This is very comforting when I am supervising someone else intubating a patient. There are case reports of oesophageal intubation where a supervising consultant performed all the known tests to verify tube placement.

 

Poor plateau

  • Kinked tube
  • Herniated cuff
  • Bronchospasm
  • ie Any obstruction that limits expiration
cap8

'Curare cleft'

  • Usually seen with high CO2
  • It is a diaphragmatic twitch pulling some fresh gas past the sampling tube
  • Not to be confused with cardiogenic oscillations
cap9

Cardiogenic oscillations

  • Caused by the beating of the heart against the lungs (c/f. helicopters and HFV)
  • Said to be more readily seen as relaxant wears off and tone returns to chest and abdominal walls and diaphragm.
  • It is more common in paediatrics because the heart takes up relatively more space in the chest.
cap10

Camel capnogram

  • Unequal emptying of lungs
  • Lateral position
  • Tube touching carina
cap10


There are two main graphs that we look at which are a function of the sweep speed.
These are CO2 waveforms and trends.
At slower speed we get a trend image of the peeks and troughs of expired CO2.
The trend of end tidal CO2 is the most useful graph to watch to follow ventilation. and there are a few patterns that are diagnostic.

 

Slow decrease in CO2

  • Hyperventilation
  • Fall in body temperature
  • Falling lung or body perfusion
cap7

 

A sudden drop in end tidal CO2 to zero.

Spontaneous breathing and ventilated patients

  • Kinked ET tube
  • Kinked or disconnected sampling tube
  • Patient extubated
  • Total anaesthetic circuit disconnect

 

In a ventilated patient

  • The ventilator has failed
cap2

 

A sudden drop in end tidal CO2 but not to zero

  • Leak in circuit eg deflated cuff
  • Obstruction eg acute broncho-spasm.
  • Leak in sampling tube drawing in room air
cap3

 

A sudden rise in baseline

  • Stuck valve in circle absorber system
  • Exhausted CO2 absorber
  • Calibration error in monitor
cap4

 

An exponential decrease in CO2

  • Circulatory arrest: cardiac or hypovolaemic
  • Embolism: air or clot
  • Sudden severe hyperventilation
    On the other hand, when all other monitors fail, it is comforting to see CO2 coming out. It can be seen to rise in external cardiac massage.
cap5

 

Gradual increase in CO2

  • Hypoventilation
  • Absorption of CO2 from peritoneal cavity
  • Rapidly rising body temperature
cap6

 

Sudden increase in CO2

  • Injection of sodium bicarbonate
  • Release of tourniquet
  • Sudden increase in blood pressure

 

cap12

 

Article by Dr. David Sainsbury

Snr. Staff Anaesthetist, Dept.of Paediatric Anaesthesia
Women & Childrens Hospital, N.Adelaide, Australia.
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