ABG & Hypothermia

alpha stat

· A pH of 7.4 and a PaCO2 is only normal for 37^o blood

· If cool a sample of blood:

i) CO₂ becomes more soluble

ii) PaCO2 decreases to maintain a constant CO₂ content

iii) pH rises

· note that neutrality of water [ie equal ratio of H⁺ to OH^-] results in a rise of pH with decreasing temperature

—> the same relationship holds for blood

· arterial blood from heart; temp 37^oC; pH 7.4

· skin: temp 25^oC; pH 7.6

· exercising muscle: temp 40^oC; pH 7.35

—> CO2 content remains constant and keep an equal ratio of H⁺ to OH^-

ie acid-base equilibrium is maintained

· intracellular pH parallels extracellular pH

· maintenance of intracellular neutrality despite cooling is by the buffering of the imidazole group of histidine (alpha groups)

· as temperature changes:

i) fraction of unprotonated histidine imidazole groups (alpha) remain constant

ii) total CO₂ remains constant

iii) pH changes

· as observed in poikilotherms (eg reptiles) whose tissues must function over a range of temperatures

pH-Stat

· Alternative method of acid-base management

· Here, pH is maintained constant over varying temperatures

· Therefore, as blood is cooled:

i) CO₂ must be added to maintain a PaCO₂ of 40 and a pH of 7.4

ii) extracellular & intracellular ratio of H⁺ to OH is changed

iii) total CO₂ stores are elevated

· Seen in hibernating mammals in which the ensuring acidosis depresses metabolism in non functioning tissues

[note that during hypothermia, a pH of 7.4 is acidotic]

Which is the better technique?

· maintenance of a constant intracellular electrochemical neutrality appears to be essential for normal cellular function; most enzymes depend on an optimal pH for their function

· the response of cerebral blood flow to changes in PCO₂ remains intact, therefore alpha stat with a decreasing PaCO₂ results in a reduced cerebral blood flow. However, hypothermia reduces cerebral metabolic rate

· cerebral blood flow autoregulation is lost with pH-state and is therefore related to perfusion pressure: it may result in excessive blood flow which may unnecessarily expose the brain to high ICP and microemboli.

Strategy	Aim	Total CO2 content	pH & PaCO2 maintenace	Intracellular state	a-imidazole buffering	enzyme function	effect on ischemic tissue
Alpha-stat	constant OH-/H+	constant	normal uncorrected values	neutral OH-=H+	constant	normal	full protection
pH-stat	constant pH	increases	normal corrected values	acidotic excess H+	excess (+)	decreased	decreased protection

K. C. Potger
Copyright © 2001