1) Veterinary patients are often placed in plastic boxes with supplemental oxygen to increase their fractional inspired oxygen concentration (FIO2). However, these boxes do not remove carbon dioxide (CO2) produced by the patient. 2) Without measuring CO2 levels, these enriched oxygen environments can lead to undesirable CO2 concentrations that may compromise patient health and recovery. 3) The author recommends measuring CO2 levels in these boxes, as common oxygen flow rates used can result in CO2 levels of 2000-3000 ppm, which may cause health effects in patients. Higher oxygen flows or changing the air source are needed to prevent elevated CO2.

1. Letter to the Editor Journal of Veterinary Emergency and Critical Care 24(5) 2014, pp 491–492
doi: 10.1111/vec.12232
Undesirable concentrations of carbon dioxide
in enriched oxygen environments in the
clinical setting
Dear Editor,
As emergency clinicians we often need to provide sup-
plemental oxygen to our patients. While most of us have
proper oxygen cages (that circulate air and scrub carbon
dioxide [CO2]), very small patients are commonly placed
in various sized plastic boxes with a fresh gas line from
an anesthesia machine to increase the fractional inspired
oxygen concentration (FIO2). Oxygen concentration me-
ters are readily used to assess the FIO2, but [CO2] is not
commonly measured. Given the lack of CO2 removal in
such settings, I became concerned about possible expo-
sure to increased [CO2] to patients.
A “quick and dirty” calculation of the CO2 produced
by a given patient can be performed in one of two ways.
First, we know that our common sized veterinary pa-
tients utilize 10–20 mL/kg/min of oxygen at rest, and
more with illness or fever. If we assume that they pro-
duce the same amount of CO2 (not strictly a stoichiomet-
ric reaction, but close enough for this purpose), then a 1
kg kitten/bunny/puppy would produce 20 mL/min
of CO2. If the container is 30 L (the size of our smallest
boxes), then every minute they use 0.07% of the oxy-
gen and replace it with the same volume of CO2. If the
system were closed, in 15 minutes the [CO2] would be
1% (.07% × 15 min = 1.05%). If we want the inspired
[CO2] to always be 0.1% (ie, 1,000 ppm), we need to
reduce the [CO2] by changing/diluting the air at least
10-fold with our incoming flow every 15 minutes:
30 L × 4 × 10 = 1200 L/hour = 20 L/min.
where 30 L is the size of the box, 4 is the number of
changes per hour, based on the “15 minute” calculation,
and 10 relates to our desire to reduce the [CO2] 10-fold
from 1% to 0.1%.
This scenario may be somewhat simplistic because
we are not closing the system and only periodically
flushing it, but such a system would have a continuous
flush, so a lower flow rate might be acceptable, perhaps
10–15 L/min. However, the flow through the box is not
uniform and pockets of higher concentration of CO2 may
form, especially around the patient with their head in
the corner of the box or under a blanket. CO2 is heavier
than O2, so will tend to accumulate at the bottom of the
chamber.
Another approach to estimate CO2 production is
to calculate it based on tidal and minute volumes.
For our same 1 kg patient, assume tidal volume of
10 mL/kg/breath × 30 breaths/min = 300 mL/min. Ex-
haled breath is usually 4% CO2, so the patient makes
300 × .04 = 12 mL of CO2/min or 180 mL CO2/15 min.
This is less than the previous calculation based on O2
consumption. If the volume of the box is 30 L, then
180/30 L = 0.6% CO2 ( = 6,000 ppm), which is again 6×
maximum acceptable. Therefore, you would still need to
change/dilute the box at least 6× every 15 minutes:
30 L × 4 × 6 = 720 L/hr = 12 L/min.
If the patient is even mildly tachypneic, the require-
ment might double. We should also add something for
a safety factor because the flow in the box is not uni-
form. Typically, the oxygen flow is adjusted to achieve
the desired FIO2 and is frequently as low at 2–6 L/min.
Many flow meters on anesthetic machines do not allow
for flow rates over 4 L/minute, but “bubbler” systems
used for nasal oxygen supplementation frequently go to
15 L/min. These bubbler systems may be a better choice
than using the fresh gas line from an anesthetic machine.
Additionally, they will (minimally) humidify the air and
not take an anesthetic machine away from its intended
uses.
We obtained a CO2 meter, started to measure the
[CO2], and were surprised to discover that with our
commonly used oxygen flow rates, our plastic box cham-
bers rapidly developed [CO2] of 2,000–3,000 ppm. Rais-
ing the flow rate to the 10 L/minute helped considerably,
but also resulted in FIO2 levels near 80%–100%. Even in
our large, recirculating oxygen chamber with a soda lime
(Carbolime) tray, the levels of CO2 increased rapidly, de-
pending on the size and number of patients contained
within. We have found that a medium to large dog (20–30
kg body weight) will need to have the soda lime changed
as often as every 30 minutes to keep [CO2] 1,000 ppm.
Previously, we changed the soda lime on a weekly basis
or less frequently. Multiple patients, very large patients,
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Veterinary Emergency and Critical Care Society 2014 491

2. Letter to the Editor
Measured CO2 (ppm) Percentage Expected effect
250–350 0.035 Background (normal) outdoor air level
350–1,000 0.1 Typical level found in occupied spaces with good air exchange
1,000–2,000 0.2 Drowsiness, poor concentration, loss of attention, increased heart rate, and slight nausea
2,000–5,000 0.5 Headaches, sleepiness, sensation of stagnant, stale, stuffy air
10,000 1.0 Mild increase in respiratory rate
20,000 2.0 Respiratory rate increases by 50%, headache, lethargy
30,000 Respiratory rate increased by 100%, dizziness, elevated heart rate and blood pressure,
increased headache, impaired hearing
40,000–50,000 4–5 Rapid respiratory rate, choking sensation after 30 minutes at this level
50,000 –100,000 5–10 Odor of CO2 can be detected (pungent/stimulating odor like fresh carbonated water),
exhaustion, labored breathing,severe headache, tinnitus, impaired vision, confusion,
unconsciousness
or patients with significant panting will raise the [CO2]
rapidly.
The levels of CO2 in the air and potential health prob-
lems are listed in table above, which was compiled from
multiple sources.a
It is important to note that levels of 2,000–5,000 ppm
can rapidly occur and it is inherently difficult to deter-
mine if sick patients are exhibiting signs of mild or early
toxicity, or if their clinical appearance is due to the dis-
ease for which they presented and are being treated.
We commonly use apparent improvement in respiratory
function (ie, decreased respiratory rate and/or effort) as
a sign that the [FIO2] can be decreased as part of our
weaning protocol to get patient out of an enriched FIO2
environment. Consequently, if CO2 levels are increased
resulting in increased respiratory rate, this protocol will
be compromised.
Portable CO2 metersb
are readily available for under
$200 USD and can safely be placed in high oxygen
environments (per manufacturer). Users of enriched
FIO2 chambers may want to consider monitoring their
[CO2] to ensure sure that patients are not being subjected
to increased [CO2] that may further compromise their
recovery.
An additional benefit of the monitors is to be able
to monitor the [CO2] in the work environment, as we
have noticed that during operating hours when many
workers are present, the [CO2] in the room can of-
ten increase above recommended levels and should be
addressed.
Mitzi M. Howard DVM
VCA Emergency Animal Hospital Referral Center
San Diego, CA 92108
Footnotes
a
http://en.wikipedia.org/wiki/Carbon_dioxide#Toxicity
http://en.wikipedia.org/wiki/Carbon_dioxide_poisoning
http://inspectapedia.com/hazmat/CO2_Health_Effects.htm
b
TIM10 CO2, Temperature and Humidity Monitor, model AZ-004, CO2
Meter, Inc., Ormond Beach, FL, www.CO2meter.com.
492 C
Veterinary Emergency and Critical Care Society 2014, doi: 10.1111/vec.12232