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Technology
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Technology |
Gases
(air) are measured by virtue of the pressures that they exert. When gases are mixed they each contribute
to a total pressure. Each gas contributes a partial pressure (P).
Total atmospheric air pressure at sea level is 760 mmHg (millimeters
of mercury). At sea level partial pressure carbon dioxide,
written PCO2, is 0.3
mmHg. Capnographs (capnometers) provide for continuous
measurement of PCO2, while breathing, during both inhale and the exhale, a
waveform known as the capnogram. During the inhale, readings are effectively
“zero” (0.3 mmHg), while during the exhale readings rise sharply to 35 – 45
mmHg (in healthy people). During the
transition between the exhale and the inhale, the PCO2 continues to slowly
rise (the alveolar plateau), eventually reaching a peak value immediately
prior to the next inhale. This peak
value is equivalent to the average alveolar PCO2 retained in the lungs. It
represents PCO2 at the “End”
of the “Tide” of exhaled air, and
is known as End
Tidal PCO2, written ETCO2. In healthy people, pulmonary capillary PCO2
equilibrates to alveolar PCO2. Thus, ETCO2
and arterial PCO2, known as PaCO2, are highly correlated, nearly
equivalent. Note that capnograph
instrumentation does NOT indicate how much CO2 is being exhaled; it indicates
the level retained. Actual quantities
of CO2 excreted by the body vary considerably based on metabolism, e.g.,
exercise, although the PaCO2 values required for maintaining acid-base
balance (35-45 mmHg) remain the same (see physiology). The capnogram
permits observation of gasping, sighing, breath holding, aborted exhaling,
rhythmicity, and breathing rate. The CapnoTrainer® presents this
information, along with ETCO2 values, in real time, for helping people learn
about their breathing habits, how these habits are affecting the, and how to
learn new habits consistent with good respiratory fitness. The CapnoTrainer is an educational
capnograph, a learning tool, not a medical monitoring instrument. . |
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