Phase 2: Rise
Phase 1: Baseline Phase 1: Baseline
Phase 3: Plateau
THE MVP OF VITAL SIGNS
in the lungs rise up past the sensor.
Phase 3 is when the sensor is receiving the
CO2-rich gas that was in the alveoli. Because
this is a fairly stable amount, the graph levels
off into a plateau. The measurement at the
end of the tide of respiration, the peak measurement at the very end of phase 3, is the
After the end of phase 3, the patient inhales
again, bringing clear air past the sensor, dropping the graph back down to zero to start over
again at phase 1.
Although it can be intimidating to try and
memorize what each phase (and the angles
between them) represents, you can think of
it as follows: The left side shows how quickly
and easily air is moving out of the lungs; the
right side shows how quickly and easily air is
going in; the top shows how easily the alveoli
If all we wanted to read from capnography
was ventilation, this would be enough, but to
indirectly measure a patient’s perfusion and
metabolic status we must understand how CO2
gets to the lungs to be exhaled.
PUTTING ON THE PRESSURE
Many factors affect how oxygen gets into the
body and CO2 gets out; however, the biggest
influence is the partial pressures of these gasses.
Although hemoglobin, myoglobin and
other body chemicals play a part in transporting gasses, it can be helpful to begin by
just picturing the partial pressures pushing the
gasses from one part of the body to the next. 3
The normal partial pressure of oxygen in
ambient air is approximately 104 mmHg. It
gets humidified and absorbed by the body as
it’s inhaled, bringing the partial pressure down
to 100 mmHg by the time the oxygen reaches
the alveoli. The partial pressure of oxygen in
the alveoli is known as PaO2.
Oxygen is then pushed from the partial pressure of 100 mmHg in the alveoli to the lower
partial pressure of 95 mmHg in the capillaries surrounding the alveoli. Oxygen gets carried through the circulatory system, getting
absorbed along the way.
By the time the oxygen gets to the end of its
journey, it has a partial pressure of approximately
40 mmHg, still high enough to allow it to move
into muscles and organs that have a lower partial pressure of approximately 20 mmHg. 4 (See
Figure 2, p. 49.)
If the organs are functioning normally, the
oxygen is metabolized, producing the CO2 that
we’re ultimately going to measure. Although the
journey back involves CO2 moving primarily
through the body’s buffer system as bicarbonate (HCO3-) its movement is still largely governed by partial pressures. 3
The partial pressure of carbon dioxide
(PCO2) as it leaves the organs is approximately
46 mmHg, just high enough to push it into the
capillaries which have a partial pressure of only
45 mmHg. 4 CO2 travels through venous circulation largely untouched.
In the end it moves from 45 mmHg at the
capillaries surrounding the alveoli into the alveoli themselves. From the alveoli to exhalation
the CO2 is approximately 35–45 mmHg. 4 At
this level it will get exhaled and measured by
the EtCO2 sensor, letting us know that the
patient’s metabolism, perfusion and ventilation are all working properly taking up oxygen, converting it to CO2 and releasing it at a
normal rate (or not).
If you were to know one more thing about
oxygen and CO2 transport, it’s that high CO2
reduces the affinity of hemoglobin for oxygen.
Referred to as the Bohr effect, during normal
body function this is a good thing, (the high
CO2 in muscles and organs help hemoglobin
release needed oxygen). However, prolonged
periods of high CO2 and associated acidosis
make it hard for hemoglobin to pickup and
transport oxygen. This can be seen as a shift
of the oxyhemoglobin dissociation curve to the
right. 4, 5 (See Figure 3, p. 50.)
Conversely, if the patient has low CO2,
perhaps because of hyperventilation, it will
cause an increased affinity for oxygen, allowing hemoglobin to pick oxygen up more easily. However, if the low CO2 is prolonged, the
hemoglobin may not release the oxygen into
the organs. This is referred to as the Haldane An end-tidal capnography waveform measures and displays the peak amount of CO2 at the end of exhalation.
Figure 1: Normal end-tidal capnography waveform