Effect of temperature on acid balance. 1

Alpha-stat 1

pH stat 1

Significance of blood gas measurement during total body hypothermia. 2

alpha stat 2

pH-Stat 2

Which is the better technique?. 2

Effect of temperature on acid balance

Alpha-stat

1. Neutrality of water

a) Neutral water: [H⁺] equals [OH^-]

b) As temperature decreases, the pH at which water is ‘neutral’ increases 0.017 units for each °C decrease in temperature

i) At 37°C, pH of neutral water equals 6.8

ii) At 25°C, pH of neutral water equals 7.4

2. Constancy of [OH^-] : [H⁺] ratio

a) Human blood has [OH^-] : [H⁺] ratio of 16 : 1 resulting in a pH of 7.4 at 37°C

b) This ratio is maintained despite changes in temperature

c) Therefore, the pH of blood also increases 0.017 units for each °C decrease in temperature?

d)

3. pCO₂ & hypothermia

a) Increased solubility of CO₂ in plasma with hypothermia

b) Therefore at similar pCO₂ see increased dissolved CO₂ and increased total CO₂ in plasma with decreasing temperature.

c) Note that although more CO₂ becomes dissolved in solution with decreasing temperatures, the molecular interaction between water and CO2 is decreased with hypothermia, and therefore formulation of H+ is minimal. p488 Hensley

pH stat

i) Shifts Hb-O₂ curve to right (reverses effect of hypothermia on Hb-O₂ curve)

ii) Results in vasodilation (reverses effect of hypothermia reduced tissue perfusion 2° increased viscosity)

Significance of blood gas measurement during total body 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 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 animals 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 unecessarily 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

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