Acid-Base Changes (also see Chapter
41
)
The pH of neutral water ([OH-
] = [H+
]) increases
0.017 U for each 1°C reduction in temperature; the pH of blood in a closed system
(e.g., test tube or artery) changes similarly. Cold-blooded animals allow the pH
to vary with body temperature as it would in vitro (i.e., blood
becomes more alkalotic as the temperature decreases), whereas homeotherms, which
decrease body temperature during hibernation, maintain an arterial pH near 7.4.
Interpretation of arterial pH in hypothermic humans is difficult because it is unclear
which strategy is optimal.[192]
To mimic the compensatory
mechanisms used by hibernating homeotherms, blood pH (which is measured by electrodes
at 37°C) has traditionally been "corrected" to the patient's actual body temperature.
Without correction, tissue oxygen availability decreases because hemoglobin's affinity
for oxygen increases approximately 1.7%/°C. This effect is small when compared
with the 5.7%/°C increase in oxyhemoglobin affinity caused by hypothermia itself.
Fortunately, the combined increases in affinity are offset by the 8%/°C reduction
in metabolic rate caused by hypothermia. Tissue hypoxia is thus unlikely, with or
without correction, and has not been demonstrated experimentally.
The ectothermic strategy is also known as "alphastat" because
the dissociation constant of the α-imidazole group in histidine changes in
parallel with that of water. Maintaining constant imidazole ionization results in
optimal enzyme function as temperature changes. In contrast, homeothermic dynamics
significantly decreases metabolic function, and animals are essentially anesthetized
by cold. Constant relative alkalinity also maintains a stable intracellular-to-extracellular
gradient that promotes the removal of acidic products of intracellular metabolism.
No studies have convincingly indicated that an ectothermic strategy
is better than that adopted by hibernating homeotherms. However, most anesthesiologists
now use "uncorrected" values. This technique facilitates comparison between serial
blood gas values because "normal" arterial pH remains approximately 7.4 and "normal"
PaCO2
remains about 40 mm Hg at any temperature.
However, both techniques work well, and physiologic differences between them appear
to be subtle and have minimal if any effect on patient outcome.
