Reading & interpreting end-tidal
By Rommie L. Duckworth, LP
Capnography is a great way to confirm air way device placement and moni- tor ventilation, but it can do so much
more. Carbon dioxide (CO2) is a product of
metabolism transported via perfusion and
expelled through ventilation. End-tidal carbon dioxide (EtCO2) waveform monitoring
allows you to measure all three simultaneously,
making it the most important vital sign you use. 1
To evaluate the metabolism, ventilation and
perfusion of a patient through EtCO2 waveform monitoring you need to read the PQRST:
proper, quantity, rate, shape and trend.
Proper means that you should know the normal readings for quantity, rate, shape and trending of EtCO2. In this case, normal means what
we find in a healthy person with no metabolism,
ventilation or perfusion problems. One of the
great things about EtCO2 is that although ventilation rates vary based on age, normal readings for quantity, shape and trends are the same
for men and women of all age groups, making
them easy to remember.
Quantity; target EtCO2 value should be
Rate of ventilation should be 12–20 breaths
per minute (bpm) for adults if the patient is
breathing on their own and 10–12 bpm if you’re
ventilating them. Children should be ventilated
at a rate of 15–30 bpm; 25–50 bpm for infants.
Ventilating too quickly won’t let enough CO2
build up in the alveoli, resulting in lower EtCO2
readings. Ventilating too slowly will allow extra
CO2 to build up, resulting in higher readings.
Shape of the waveform should normally be a rectangle with rounded corners.
Different waveform shapes can indicate different conditions.
Trending of the quantity, rate and shape of
EtCO2 should be stable or improving.
Although reading EtCO2 waveforms can
be easy, interpreting what you see requires
understanding how the waveforms and numbers are produced.
READING THE WAVES
When it comes to capnography, everyone
knows the normal adult respiratory rate of
12–20 breaths per minute and most people
know, or quickly learn, that the normal quantity of exhaled CO2 is 35–45 mmHg. What
can be intimidating is the idea of reading the
shape of the waveform, but in practice it’s not
difficult at all.
An end-tidal capnography waveform is a
simple graphic measurement of how much CO2
a person is exhaling. The normal end-tidal cap-
nography wave form is basically a rounded rect-
angle. 2 (See Figure 1, p. 48.) When a person is
breathing out CO2, the graph goes up. When
a person is breathing in, it goes back down.
Phase 1 is inhalation. This is the baseline.
Since no CO2 is going out when a patient is
breathing in, the baseline is usually zero.
Phase 2 is the beginning of exhalation. CO2
begins to travel from the alveoli through the
anatomical dead space of the airway causing a
rapid rise in the graph as the CO2.
Phase 2 measures the exhaled CO2 from
the alveoli mixed with the gas that was in the
dead space. This part of the graph goes up as
the more concentrated CO2 gases from lower