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ED use of blood gases

ED use of blood gases. AWH Teaching Program 2013. ABG or VBG. Treatment is based on clinical parameters i.e. real time observations and response to treatment Almost never a need to do ABG in ED. VBG provides all the information you might need.

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ED use of blood gases

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  1. ED use of blood gases • AWH Teaching Program 2013

  2. ABG or VBG • Treatment is based on clinical parameters • i.e. real time observations and response to treatment • Almost never a need to do ABG in ED. • VBG provides all the information you might need. • SaO2 provides the rest - won’t rule out hyperoxia • see the next slide.......

  3. oxyhaemoglobin dissociation curve • A known saturation will reasonably provide you with the PaO2 • 90% being roughly equivalent to 60mmHg - the point at which the curve flattens

  4. Oxygen Measurement • 1774 - Joseph Priestly first extracted oxygen from blood (see diagram) • 1908 - Krogh used aerotonomoter to measure rabbit arterial blood oxygen tension • 1958 - Clark (and others) created an oxygen electrode

  5. Pulse Oximetry • Method invented in 1972 by Takuo Aoyagi • byproduct of research into non-invasive measurement of cardiac output • finger probes developed in 1979 • SaO2 accurate to within 2.75% of PaO2 in sepsis Wilson et al. The accuracy of pulse oximetry in emergency department patients with severe sepsis and septic shock: a retrospective cohort study BMC Emergency Medicine 2010, 10:9

  6. Values commonly measured from a VBG • pH • PaO2 • PaCO2 • HCO3- • Base excess • COHb • Na+ • K+ • Cl- • Ca++ - ionised • lactate • Hb/Creatinine

  7. STEADY STATE VALUES

  8. Venous pH in the ED Kelly AM, McAlpine R, Kyle E. Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department. EMERG MED J. 2001 SEP;18(5):340-2 • Good correlation between values in range of disease states • multiple small studies • first large study was performed in Australia 2001 • approx. 250 patients had simultaneous ABG and VBG • 200 with respiratory disease • 50 suspected of metabolic derangement • pH values differed by 0.4

  9. Value of ABG in the ED to diagnose dyspnoea • Retrospective study of prospectively collected data • approx 1150 patients presenting to Basel ED with dyspnoea • diagnoses include APO, COPD, Asthma, Pneumonia and Hyperventilation • No ability to differentiate between major diagnoses • ICU admissions were greater with pH <7.33 • mortality was greater with lower pH Burri E, Potocki M, Drexler B, et alValue of arterial blood gas analysis in patients with acute dyspnea: an observational study.Crit Care. 2011;15(3):R145. doi: 10.1186/cc10268. Epub 2011 Jun 9

  10. Predicting ABG values in COPD Ak, A., Ogun, C., Bayir, S. et al Prediction of Arterial Blood Gas Values in Patients with Acute Exacerbation of Chronic Obstructive Pulmonary Disease Tohoku J. Exp. Med., 2006, 210(4), 285-290 • study of 144 comparing ABG and VBG values • good correlation between pH, pCO2, HCO3- • 100% negative predictive value of venousPaCO2 <46 for arterialPaCO2 <46 • poor correlation between PaO2 and SaO2

  11. using VBG instead of ABG in DKA • Review article • attempted to correlate ABG with VBG values in DKA • found good correlation between • pH - 0.02 unit difference • HCO3- - -0.18 difference • data based on small studies • uncertain if true in haemodynamic instability or respiratory failure (not often the case in DKA) KELLY AM. The case for venous rather than arterial blood gases in diabetic ketoacidosis. Emerg Med Aust (2006) 18, 64-67

  12. calculations or corrections • Compensation is really the physiological response to the primary acid/base disorder • It is possible to determine the presence of a mixed or combined acid/base disorder • Following are some formulae to help with that • Follow the links to some more detailed explanations

  13. Calculations • Expected values: • Rule of thumb • calculations • Anion gap • Delta ratio

  14. RULE OF THUMB • In acute respiratory disease • for every ↑ in CO2 of 10mmHg the pH ↓ is 0.1 units • for every ↓ in CO2 of 10mmHg the pH ↑ is 0.1units • true for the range of pH 7.2 - 7.6

  15. Concepts to help explain • Henderson-Hasselbach equation • law of mass action: • CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

  16. How to pick the major disorder CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

  17. CO2 in metabolic acidosis • expected CO2 = 1.5[HCO3-]+ 8 • 12-24 hrs to stabilise • limit of ‘compensation’ - 10mmHg http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php

  18. CO2 in metabolic alkalosis • expected CO2 = 0.7[HCO3-]+20 http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php

  19. HCO3- in respiratory acidosis • Chronic • expected HCO3- • = 24 + 4([CO2]- 40) • 10 • Acute • expected HCO3- • = 24 + ([CO2]- 40) • 10 4:1 RULE - the rise in bicarbonate in stable chronic respiratory acidosis (2-3 days) is 4 times higher than in acute respiratory acidosis (immediate) http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php

  20. HCO3- in respiratory alkalosis 5:2 RULE - the fall in bicarbonate in stable chronic respiratory alkalosis (2-3 days) is higher than in acute respiratory alkalosis • Chronic - not <15mmHg • expected HCO3- • = 24 - 5([CO2]- 40) • 10 • Acute - not <18mmHg • expected HCO3- • = 24 - 2([CO2]- 40) • 10 http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php

  21. anion gap explained • excess of measured positively charged ions - cations • calculated by the formula: • (Na++K+)-(HCO3-+Cl-) • normal range 16-20 (12-16 if K+not included) http://www.anaesthesiamcq.com/AcidBaseBook/ab3_2.php

  22. when is it real? • at 20-29 metabolic acidosis present - in 2/3 patients • >29 considered a wide anion gap acidosis • you can use delta ratio to discover further acid/base disorders

  23. Delta ratio • compares the relative difference between the change in the anion gap with the change in HCO3- • as acidity rises (↑anion gap) bicarbonate should fall • calculate ∆ Anion gap = [18 - measured Anion Gap] • calculate ∆ HCO3- • divide ∆ Anion gap by ∆ HCO3- - the ‘delta ratio’

  24. now what? • < 0.4  - Hyperchloraemic normal anion gap acidosis • 0.4 to 0.8  - Combined high AG and normal AG acidosis • 1  - Common in DKA due to urinary ketone loss • 1 to 2  - Typical pattern in high anion gap metabolic acidosis • > 2 Check for either a: • co-existing Metabolic Alkalosis (which would elevate [HCO3])   • or a co-existing Chronic Respiratory Acidosis (which results in compensatory elevation of [HCO3]) http://www.anaesthesiamcq.com/AcidBaseBook/ab3_3.php

  25. Recap • The clinical scenario, pH and bicarb/CO2 are all needed to determine the primary acid/base disorder • Use of calculations will determine if a secondary acid/base disorder exits • The delta ratio can be used to ‘discover’ additional acid/base disorders - beware of over-interpretation

  26. Examples • The following examples have no workings and are presented for you to have a go.....

  27. example #1 • 26 year old male • type 1 diabetes • moderately unwell with vomiting

  28. example #2 • 56 year old female • under investigation for endocrine disorder • shocked on arrival

  29. example #3 • 56 year old female • Known COPD • Drowsy

  30. Resources • The accuracy of pulse oximetry in emergency department patients with severe sepsis and septic shock: a retrospective cohort study • Wilson et al. • BMC Emergency Medicine 2010, 10:9 • Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department. • Kelly AM, McAlpine R, Kyle E. • Emerg Med J. 2001 Sep;18(5):340-2 • Value of arterial blood gas analysis in patients with acute dyspnea: an observational study. • Burri E, Potocki M, Drexler B, Schuetz P, et al • Crit Care. 2011;15(3):R145. doi: 10.1186/cc10268. Epub 2011 Jun 9 • Prediction of Arterial Blood Gas Values in Patients with Acute Exacerbation of Chronic Obstructive Pulmonary Disease • Ak, A., Ogun, C., Bayir, S. et al • Tohoku J. Exp. Med., 2006, 210(4), 285-290 • The case for venous rather than arterial blood gases in diabetic ketoacidosis. • Kelly AM. • Emerg Med Australas. 2006 Feb;18(1):64-7. Review • http://www.anaesthesiamcq.com/AcidBaseBook/ab9_3.php

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