Dr. Jeffrey Elliot Field HBSc, D.D.S., Diplomat of the National dental Board of Anesthesia, Fellow of The American Dental Society of Anesthesia

 Carbon dioxide absorbs infrared light with a wavelength of 4.3m m.  Light at this wavelength is shone through a gas sample and the absorption is proportional to the carbon dioxide concentration.  A sample of expired gas is withdrawn from the anaesthetic circuit by a pump and analyzed inside the machine.

 Valuable information can be obtained from the continuous measurement of carbon dioxide.  At the most basic level the regular rise in carbon dioxide at the end of respiration can be used to determine respiratory rate, and regularity of respiration.

 2)The shape of the capnograph (the plot of carbon dioxide against time) is used to assess pulmonary function.  The carbon dioxide level should rise rapidly during the first part of exhalation and then flatten off - the "alveolar plateau". If there is pulmonary disease or poor lung perfusion (secondary to poor cardiac output) the alveolar plateau disappears.  The level of carbon dioxide at the end of expiration (end tidal carbon dioxide, etCO 2 ) is normally within a few mmHg of the arterial carbon dioxide level and therefore a predictable measure of arterial CO2.  EtCO 2 is very useful for assessing adequacy of ventilation both during spontaneous respiration and when using a ventilator.

 But the most important thing ETCO2 tells us in a sedated patient is :  ARE THEY BREATHING  In an intubated patient ETCO2 will initially confirm proper placement of the endotracheal tube.  As the case progresses ETCO2 will continue to confirm that the tube is placed properly and has not moved.  Finally the adequacy of ventilation can be continuously assessed.

 In a sedated patient it is not so much a normal value but a baseline value for that particular patient that we look at. Changes in this baseline value will let us know whether our patient is breathing normally, hyperventilating or hypoventilation.  In an intubated patient a normal end tidalCO2 value is 40 mm of mercury.  ETCO2 Less Than 35 mmHg = "Hyperventilation" ETC02 Greater Than 45 mmHg = "Hypoventilation"

 There are two main graphs that we look at which are a function of the sweep speed.  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 for a capnogram :  At first there is a rapid rise as the dead space gas comes out of the major airways.  Then there is a plateau which has a slow rise.  Finally there is a rapid decline as the next breath enters the patient.

A to B is post inspiration/dead space exhalation, B is the start of alveolar exhalation, B-C is the exhalation upstroke where dead space gas mixes with lung gas, C-D is the continuation of exhalation, or the plateau(all the gas is alveolar now, rich in C02). D is the end-tidal value – the peak concentration, D-E is the inspiration washout.

 Kinked tube (intubated patients only)  Herniated cuff (intubated patients only)  Bronchospasm (intubated and non-intubated patients)  Any obstruction that limits expiration( eg mucous on the tube, COPD, asthma, foreign body obstruction.

Causes- 1)Unequal emptying of lungs( pnemothorax). Seen in intubated and non-intubated patients 2)Lateral position 3)Tube touching carina

Note what happens when the patient stops breathing. The pulse oximeter lags way behind the capnograh in picking up a cessation in breathing.