By Granger, Wesley
Publication:
FOCUS: Journal for Respiratory Care & Sleep Medicine
Date: Wednesday, June 22 2005
|
A colleague recently asked me what
the latest recommendations were for ABG temperature correction.
I had to admit that I was not sure what the current thinking
was, so I went on a journey through the literature to see if I
could determine the current recommendations. As I discuss
temperature correction there are two terms I will be using that
I should define accurately here at the beginning.
When I use the term "Corrected" I
will be referring to the ABG values as corrected to the
patient's body temperature by the blood gas analyzer and
"non-corrected" will refer to the ABG analyzer readings at 37C.
One of the interesting facts that I
came across was discussed in detail back in the 1960s. When the
temperature of a blood sample changes in a closed system the pH,
PCO2, and PO2 will change, but the SO2, O2 Content, CO2 Content,
and HCO3- will not change. A closed system occurs within the
blood vessels of the patient and within the sampling chamber of
the blood gas analyzer. You must also keep in mind that the
oxyhemoglobin dissociation curve will shift with changes in body
temperature. When the patient's temperature decreases the
oxyhemoglobin dissociation curve shifts to the left resulting in
an increased affinity of hemoglobin for oxygen and this means
that at the tissue level the hemoglobin will not release oxygen
as easily which could result in tissue hypoxia.
THE ALPHA-STAT & pH-STAT MECHANISMS
Hypothermia has been studied in more
detail than hyperthermia because of the clinical interest in
induced hypothermia during surgery, especially during
cardiopulmonary bypass. Also, temperature increases are limited
to about 4[degrees]C while decreases may be induced down to
25[degrees]C. Some the early studies looked at how different
animals respond to hypothermia. In endothermic animals
(including humans) as the body temperature is lowered the
corrected pH will increase and the corrected PCO2 will decrease.
This mechanism is thought to attempt to keep the ratio of
[H+]/[HCO3-] at a constant value which assures continued
functioning of proteins (enzymes) at normal levels. If you look
at the non-corrected pH, PCO2, and PO2 values they will be
normal (i.e. 7.40, 40 and 95). This mechanism has been called
alpha-stat regulation.
The research also found that animals
that have a hibernation cycle will change their ventilation as
their metabolism changes so that as their body temperature drops
the temperature corrected pH and PCO2 will remain close to
normal (e.g. 7.40 and 40) this means that the non-corrected
values will show an increased PCO2 and decreased pH (e.g. 7.24
and 72 measured at 37C when body temperature 25C) indicating a
respiratory acidosis. This mechanism appears to maintain
cerebral blood flow and better cerebral oxygenation during
hypothermia and has been given the name pH-stat regulation. The
alpha-stat and pH-stat mechanisms have both been studied with
induced hypothermia during surgery. Studies have found better
cerebral perfusion and oxygenation with the pH-stat method but
more cardiac arrhythmias. The pH-stat method requires changes in
minute ventilation or during bypass requires the addition of CO2
into the oxygenator of the bypass pump. The debate and
controversy about which method is best to use even continues in
the current literature. The current general consensus seems to
be that with moderate hypothermia (down to 30C-32C) the
alpha-stat method is preferred while with deep hypothermia
(below 30C) the pH-stat method may be preferred to better
maintain cerebral oxygenation. |
PROS and CONS OF TEMPERATURE
CORRECTION
The biggest problem with using
temperature corrected blood gas values is the lack of knowledge
about what is "normal" at temperatures other than 37C. Based on
the fact that the SO2, O2 Content and HCO3- do not change with
changes in temperature, it is argued that acid-base status does
not change when temperature changes under the alpha-stat
regulation mechanism. However, with pH-stat regulation, the
change in ventilation or the addition of CO2 results in a
respiratory acidosis. To use some examples, if a person is
cooled from 37C to 25C, starting with normal acid-base balance,
the pH will change from 7.40 to 7.57, the PCO2 will change from
40 to 23, the PO2 will change from 95 to 23 but remember that
HCO3-, SO2, CO2 content and O2 content will all remain close to
normal (< 2% change). Therefore, it can be argued that at 25C a
pH of 7.57, PCO2 of 23, and PO2 of 23 (assuming room air)
represents the "normal" for that temperature. We cannot use the
traditional "normal" ranges except at 37C. Therefore, several
articles I reviewed recommend that assessment of acid-base and
oxygenation status be carried out on non-corrected (37C) ABG
values regardless of the patient's actual temperature. The 37C
ABG results will show if and what kind of acid-base disorder is
present and the assessment is conducted in the same manner using
the well recognized "normal" values. If a patient is under going
induced hypothermic surgery with the surgical team using the
pH-stat protocol, then they will want to see the temperature
corrected values since the goal is to keep the corrected pH
close to 7.40 and the corrected PCO2 close to 40.
A review of this topic in 1995 by
Shapiro (Respir Care Clinics N Am, 1995; 1(1):69-76) pointed out
that oxygenation is a little harder to assess during
hypothermia. There is no data describing or quantifying the
balance between tissue oxygen demand and oxygen delivery at
temperatures other than 37C. Advocates of the pH-stat method
argue that since hemoglobin's affinity for oxygen increases as
temperature decreases the addition of CO2 or decrease in
ventilation (resulting in a respiratory acidosis) during this
method will counter the left shift and therefore oxygen release
at the tissues will be improved. However, there is no
quantitative data that shows O2 Delivery is significantly
affected. Shapiro recommends that oxygen content and oxygen
indices involving oxygen content (e.g. Shunt fraction) should be
calculated using the non-corrected ABG values. When you want to
calculate oxygen tension based indices such as the A-a Gradient,
PAO2/PaO2 ratio or PaO2/FIO2 ratio the non-corrected results
should be used. The comparison of end-tidal PO2 and PCO2
measurements to arterial values will require that the
temperature corrected values be used since the end-tidal values
are measured at the actual body temperature.
FINAL RECOMMENDATIONS
·
There is no evidence
that we should routinely temperature correct ABG results.
·
When assessing acid-base
status the non-corrected results at 37C should be used.
·
Oxygen contents, Shunt
fraction and oxygenation assessments using the A-a Gradient,
PAO2/PaO2, or the PaO2/FIO2 should use the non-corrected
results.
·
If assessment
comparisons involve end-tidal measurements then use the
temperature corrected ABG results.
·
Temperature correct when
requested to do so by the physician (e.g. during surgery
with pH-stat regulation), but also report the non-corrected
values. The correct interpretation of the results is the
responsibility of the requesting physician.
·
When reporting
temperature corrected results mark them clearly as
temperature corrected and also report the non-corrected
results.
·
If you are comparing
blood gas results during hypothermia or hyperthermia between
intravascular sensors and a temperature controlled ABG
analyzer, then the analyzer results must be corrected for
temperature since the intravascular sensors are reading
values at actual body temperature.
·
Get an accurate
temperature reading if you need to report temperature
corrected results. If the patient's correct temperature is
not given the corrected results are useless.
by Wesley Granger PhD, RRT
