Volumetric Capnography - terminology

1. Evolution of volumetric CO2 measurements
Michael B. Jaffe PhD Philips-Respironics, Wallingford, CT
 As the technology for CO2 and volume/flow measurements in
respired gases evolved from the manual time-consuming
methods of the mid-19th century to the real-time “electronic”
methods of the mid-20th century, the analysis of the CO2 and
flow waveforms have evolved from paper based research
methods to computer-based optimization techniques for robust
determinations of volumetric CO2 measurements such as CO2
elimination (VCO2) and dead space.
 Physiologists of the early 20th century did not have fast
responding infrared gas analyzers available and as such
employed clever approaches to analyze the composition of
alveolar air throughout the respiratory cycle. While methods
were known to estimate VCO2 as early as the late 19th century,
little was published about the shape of the expiratory
volumetric CO2 curve. Aitken & Clark-Kennedy (1928) (1)
collected expired air from exercising subjects into "separate
successive portions (e.g. rubber bags) suitable for
measurement and analysis“ (upper figure below). The resulting
data points were represented by a smooth curve from which
they determined the “size and nature of respiratory dead space”
using an area based method under the expired CO2-volume
curve (similar to the later method of Fowler (2) for N2) (lower
figure below).
 Volumetric capnographic measurements and analysis method
continue to evolve with increased robustness, and reliability
with improvements in technology and algorithms.
Conclusions
DiscussionDiscussionIntroductionIntroduction
 It was Fletcher who, in his doctoral thesis (1980) (5) and in
later publications (6) first presented the concepts of dead
space, effective and ineffective ventilation and VCO2 in a
unified framework using phase terminology of Fowler (2).
His presentation became widely known as the single breath test
(SBT) or single breath CO2 (SBCO2) curve (i.e. the expiratory
portion of the volumetric capnogram).
ReferencesReferences
 1.Aiken, R.S. & Clark-Kennedy, A.E. J. Physiol. (1928). 65,
389-411.
 2.Fowler, W.S. Am. J. Physiol. (1948). 154, 405.
 3.Elam, J.A., Brown, E.L., Ten Pas R.H. Anesthesiology
(1955). 16, 876-885
 4.Berengo, A. & Cutillo, A. , J. Appl. Physiol. (1961) 16, 522-
530.
 5.Fletcher, R. (1980). The single breath test for carbon dioxide
(Thesis). Lund, Sweden.
 6.Fletcher, R. & Jonson, B. Br. J. Anaesth. (1984), 56, 109-
119.
 7.Ream RS, et al. Anesthesiology. (1995);82(1):64-73.
 8.Breen PH, Jacobsen BP. Anesth Analg. (1997);85(6):1372-6.
 While this framework remains in wide use, the terms SBT and
SBCO2 are being replaced with a more descriptive term,
volumetric capnogram. The term “Capnograph” now
synonymous with a carbon dioxide analyzer was originally a
trademark (1st use in commerce in 1959 and registered in
1965) before becoming genericized. The term “volumetric
capnography” started to become widely used in the mid 1990’s
(7) and promulgated as a more appropriate term. Other
terminology such as “carbon dioxide spirogram”(8) have been
suggested as well.
 Analysis methods in use include minimization techniques such
as least-squares for fitting the central portions of phase II and
III as well as the entire expiratory curve to allow for robust and
reliable estimates of clinically useful diagnostic measures of
the slopes of phase II and III.
Elam (3) (1955), using a fast responding breathe-through (i.e.
mainstream) CO2 gas analyzer (Liston-Becker Model 16) and
screen pneumotachograph for flow, published time-based CO2
and flow profiles of human respiration. The flow profile was
used in order to determine an alveolar value of CO2 by
comparison of the time records (see figure below). Their work
on CO2 homeostasis included both normal and abnormal
characteristics of the CO2 profiles and measurements of dead
space and alveolar ventilation.
DiscussionDiscussion
 Single breath volumetric curves for CO2 using fast responding
analyzers appeared as early as 1961 in the literature. Berengo
and Cutillo (1961) noted that other workers focused on the last
portion of the “capnogram” but had not reported any detailed
study of the “capnogram” as a whole including analysis of its
different phases. With data from a CO2 gas analyzer and a
spirometer, the experimental CO2 and expired air volume data
was fitted to a 4th degree polynomial to find the "flexion"
points.