2013C13 How does a fall in temperature influence blood gas solubility and acid-base values?

 

List:

·         Effects of hypothermia

·         alpha-stat

·         pH-stat

·         Advantages of each

 

Effects of hypothermia:

Solubility

·  Henry’s law: amount dissolved = partial pressure x solubility coefficient

·  Solubility coefficient 1/temperature (T)

·  ↓1°C -> ↓PaO2 5mmHg

·  ↓1°C -> ↓PaCO2 2mmHg (-> ↑pH)

Metabolism

·  ↓T -> ↓ rate of oxidative phosphorylation -> ↓VO2 -> ↓MR (7% per -1°C)

·  ↓T-> ↓ rate of Krebs cycle -> ↓VCO2 (-> ↓PaCO2 -> ↑pH)

pH

·  Direct effect:

o ↓T -> ↓H2O dissociation -> ↑pH

o Rosenthal’s equation: ↓T 1°C -> ↑pH 0.015

·  Indirect effect:

o ↑CO2 solubility -> ↓PaCO2 -> ↑pH

o ↓VCO2 -> ↓PaCO2 -> ↑pH

 

Alpha stat:

Hypothesis

·  pH ICF = pN ≈ 6.8

·  pKa imidazole groups of ICF proteins ≈ 6.8

·  Alpha = ratio unpronated:protonated residues = 0.55 normally

·  Hypothesis: that ‘α’ is unchanged across the temperature range; i.e. as temperature decreases, pH and pKa of imidazoles should increase at the same rate

Management

·  Aim: ensure ECF pH will be 7.4, PaCO2 will be 40mmHg once rewarmed

·  Hence: as ↓T, no change minute ventilation, allow ↑pH, ↓PaCO2

·  ABG machine: enter T = 37 °C

 

pH stat:

Hypothesis

·  That ECF pH 7.4 and ICF pH 6.8 are optimal at any temperature

Management

·  Aim: ensure plasma pH 7.4 across temperature changes

·  Hence: as ↓T, hypoventilate to ensure the above

·  ABG machine: enter T = actual °C

 

Advantages of each:

Alpha stat

pH stat

·   Ensures trapping of metabolic intermediates in cytosol
(Davis hypothesis)

·   Preserves enzyme and ion channel function

·   Ensures optimal ICF buffering

·   Preserves CBF autoregulation, minimises ICP

·   Minimise microembolisation e.g. stroke

·   Prevent ischaemic damage

·   Luxury cerebral perfusion (↑PaCO2 -> vasodilate)

·   Systemic vasodilation -> fast, even cooling (↑PaCO2)

·   Right shift OHDC (↑PaCO2) offsets left shift (↓T, ↑pH)

 

 

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