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Related Experiment Videos

Siggaard-Andersen algorithm-derived p50 parameters: perturbation by abnormal hemoglobin-oxygen affinity and acid-base

T J Morgan1, Z H Endre, D M Kanowski

  • 1Department of Anesthesiology and Intensive Care, University of Queensland, Royal Brisbane Hospital, Australia.

The Journal of Laboratory and Clinical Medicine
|October 1, 1995
PubMed
Summary

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The Siggaard-Andersen algorithm for p50 calculation may inaccurately assess tissue oxygen availability in critical illness. Its accuracy is compromised by altered hemoglobin-oxygen affinity and acid-base balance, potentially overestimating oxygen delivery.

Area of Science:

  • Critical care medicine
  • Clinical chemistry
  • Physiology

Background:

  • The p50 and derived indexes are crucial for assessing tissue oxygen availability in critical illness.
  • These indexes are calculated using the Siggaard-Andersen algorithm from arterial blood gas tensions and hemoglobin-oxygen saturation.
  • The algorithm's accuracy across various pathophysiologic conditions requires thorough investigation.

Purpose of the Study:

  • To evaluate the accuracy of the Siggaard-Andersen p50 algorithm.
  • To assess its performance under diverse pathophysiologic conditions, including altered hemoglobin-oxygen affinity and acid-base balance.
  • To determine if the algorithm provides reliable tissue oxygen availability assessments in critical illness.

Main Methods:

  • Blood gases, cooximetry, and p50 calculations were performed on blood samples with normal, increased, and reduced hemoglobin-oxygen affinity.

Related Experiment Videos

  • Samples included those from patients with diabetic ketoacidosis.
  • Log p50 in vivo/pH plots were used to analyze the Bohr effect, with and without base deficit correction.

    • Main Results:

    • Standard p50 values exhibited high variability (CV > 5.9%) and saturation-linked bias, except in normal affinity specimens (CV < 1.7%).
    • The algorithm overestimated p50 by up to 11 mm Hg as saturation approached 97%.
    • Base deficit correction improved accuracy but did not eliminate saturation-linked bias, distorting the Bohr effect.

    Conclusions:

    • Siggaard-Andersen p50 calculations can be misleading in critical illness due to disturbances in hemoglobin-oxygen affinity and acid-base balance.
    • The algorithm may significantly overestimate oxygen availability, particularly in cases of metabolic acidosis with high hemoglobin-oxygen affinity.
    • Clinical interpretation of these indexes requires caution when significant pathophysiologic alterations are present.