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

 

List:

·        Effects of hypothermia

·        alpha-stat

·        pH-stat

 

Effects of hypothermia:

Solubility

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

·     Solubility coefficient ∝ 1/temperature (T)

Ex vivo (assuming stable gas content):

·     ↓1°C -> ↓PaO2 5mmHg

·     ↓1°C -> ↓PaCO2 2mmHg

In vivo (assuming stable gas partial pressures):

·     ↓T -> ↑CaO2:PaO2 (minor factor, since most is bound to Hb)

·     ↓T -> ↑CaCO2:PaCO2 (minor factor, since most is bound to carbamino or as HCO3-)

Metabolism

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

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

pH

·     ↑pH 0.015 per ↓1°C (Rosenthal equation)

o  ↓Temp -> ↓H2O dissociation

o  ↓Metabolic rate -> ↓VCO2 ->↓PaCO2

o  ↑CO2 solubility -> ↓PaCO2 : CaCO2

 

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

Advantages

·     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

 

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

·     Luxury cerebral perfusion (↑PaCO2 -> vasodilation)

·     Systemic vasodilation -> fast and homogenous cooling (↑PaCO2)

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

 

 

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