Hamilton G5 - CO Elimination (VCO

624074/07 E-3
E.2 CO
2
elimination (V’CO
2
)
To convert a time-based capnogram into a volumetric capno-
gram, CO
2
must be combined with flow. Figure E-3 shows the
volume of CO
2
exhaled in one breath, combining a typical
FetCO
2
versus time curve (Figure E-1) with the flow curve
(Figure E-2) for a mechanically ventilated patient. The area
under the expiratory curve (B) minus the area under the inspira-
tory curve (A) is the net transfer of CO
2
out of the lungs per
breath, or VCO
2
.
CO
2
elimination (V’CO
2
) is obtained by adding VCO
2
over sev-
eral breaths and dividing the sum by the total time in minutes
(Noe 1963). Steady-state conditions are essential to interpret
the V’CO
2
values (Brandi 1999). V’CO
2
thus represents CO
2
elimination but not necessarily CO
2
production. Normal values
for V’CO
2
can be found in the reference literature or in
Table E-1.
Figure E-1. Typical capnogram of patient on pressure-
controlled ventilation, showing fractional concentration
of CO
2
plotted against time
1
1. Inspiration starts at time 0; exhalation, at approximately 2.75 sec. Notice
that inspiratory gas initially contains CO
2
(rebreathing) that is washed out of
the Y-piece.