Measurement Variability

Independent of the approach used for interpretations, certain factors result in reliable changes to reported or normal values. These include:

Hypothermia

Hypothermia:

  • Reliably results in a ↓ gas partial pressure for any given gas content
    For every 1° below 37°C:
    • PaO2 ↓ by ~5mmHg
    • PaCO2 ↓ by ~2mmhg
  • Can be managed with two approaches:
    • Alpha-stat
    • pH-stat

Alpha-Stat

Sample is warmed to 37°C:

Alpha stat is generally standard in adult cardiac surgery, and most critical care analysers.

  • Normal reference ranges are used
  • Measured gas concentrations will be higher than “true” concentrations
  • Cellular physiology is unchanged as intracellular buffering varies with temperature

pH-Stat

Sample is corrected to patient temperature:

pH-stat remains used in paediatric cardiac surgery, where ↑ CBF ↓ the risk of ischaemic encephalopathy. It remains less popular in adult cardiac surgery as this ↑ the CVA risk from atheromatous vessels by ↑ the proportion of CO delivered to the brain.

  • No reference range exists
  • CO2 may be added to correct gases to normal values at 37°C
    This results in:
    • Cerebral vascular dilation and ↑ CBF
    • Improving O2 offloading at cells by right-shifting the oxyhaemoglobin-dissociation curve

Pregnancy

Chronic respiratory alkalosis:

  • ↑ pH: 7.4-7.47
  • ↓ PaCO2: 30mmHg
  • ↑ PaO2: 105mmHg
  • ↓ HCO3-: 20mmol/L

Hyperventilation due to the effect of progesterone, which acts on the medullary ventilation centres. PO2 ↑ due to the AGE, and HCO3- ↓ due to renal compensation.

Sources of Error

General:

  • Sampling/technical errors
    • Contaminated
      • Venous blood
        Altered dissolved gas concentrations.
      • Air
    • Transport
      • Clotted
      • Haemolysis
      • Prolonged
        Metabolism in sampled blood changes gas concentration:
        • ↓ BSL
        • ↑ LDH
        • ↑ Total protein
        • ↑ K+
        • ↑ PO4
      • Cooled with ice
        ↑ Solubility of dissolved gases into the polymer syringe.
  • Patient
    • Haemolysis
      Cells are lysed during analysis, and so free Hb is not distinguished from intracellular Hb.
    • Leucocytosis
      Much more metabolically active than erythrocytes, so in large numbers may consume dissolved oxygen and artificially ↓ PaO2 and ↑ PaCO2.
  • Equipment
    • Poor calibration
    • Wrong temperature

Metabolic effects are ↓ by cooling, but this results in other metabolic changes.

Effect of Circulating Drugs:

  • Heparin
    Dilutes sample and artificially produces a metabolic acidosis. Modern gas syringes are pre-filled with very small doses of heparin to ↓ the magnitude of the error.
  • Salicylate overdose
    Compete with chloride for the chloride electrode, creating a negative anion gap.
  • Ethylene glycol
    Causes spurious ↑ in blood gas lactate but formal lactate.

References

  1. Brandis, K. Acid-base pHysilogy. 2015.
  2. Bersten, A. D., & Handy, J. M. (2018). Oh’s Intensive Care Manual. Elsevier Gezondheidszorg.
  3. Marts LT, Hsu DJ, Clardy PF. Mind the Gap. Annals ATS. 2014 May;11(4):671–4.