The capnogram

The capnogram is the curve obtained by continuous recording of PCO2 in a sample of expiratory air (PeCO2).

The normal capnogram has a rectangular wave pattern exhibiting 5 main phases (after a brief inapparent stage "E1", correponding to expiration of the anatomic dead space) :

  • E2 : ascending slope, almost vertical (mixed air),
  • Angle Q : almost 90°, usually very distinct,
  • E3 : alveolar plateau (alveolar gas),
  • R : end-tidal peak (PetCO2) close to PACO2 (5%),
  • I 1 : vertical descending slope (start of next inspiration)

Capnographic phases

The capnographic wave can exhibit two types of  deformation depending on respiratory conditions : modifications in PetCO2 related to PACO2, deformations of the rectangular shape in presence of obstructive disease.

The capnogram in COPD

In obstructive diseases, the capnogram looses its rectangular crenellated shape for a shark-fin appearance. This deformation 1) is proportional to the bronchial obstruction, 2) disappears with it.


Examples (opposite) :

  • above : normal waveform
  • below : significant bronchospasm

Main characteristics of the deformation are :

  • loss of vertical E2,
  • opening and blunting of angle Q,
  • tilting of E3.


Capnograms : normal (upper) and during an attack of asthma (lower)


This deformation is due to parallel heterogeneity of VA/Q associated with bronchial obstruction.

Computerized measurement of relevant indices of this deformation allows reliable assessment of obstructive syndromes (bibliography). So, various shape indices were evaluated and compared ; the most significant were incorporated into the capnographic software.

The capnogram PaCO2 relationship

Regardeless of physiological shunt-effect, PaCO2 may be assimilated with PACO2, itself measurable at the peak of a capnogram recorded during deep expiration ( "PedCO2"). So PaCO2=PedCO2.

Capnogram during a deep expiration

In absence of obstructive disease, the horizontal plane of the alveolar plateau also allows the following approximation to be made : PetCO2 =PedCO2. So giving : PetCO2 = PaCO2.
Obstructive disease results in alveolar plateau steepness and in PetCO2 << PedCO2. In this case the difference between PetCO2 and PaCO2 increases with the plateau's slope.


Capnographic analysis provides two simultaneous informative data :