Using Blood Gases

Using Blood Gases Dr. Jonathan R. Goodall M62 Coloproctology Meeting 2nd April 2004

JRG Hope Hospital April 2004

Teaching Grandma to suck eggs… JRG Hope Hospital April 2004

Introduction • The necessary basics • Clinical examples of ABG use in ICU • Newer considerations in interpretation of acid-base disturbances JRG Hope Hospital April 2004

The Basics - Normal Values • pH 7.35 - 7.45 H+ 35- 45 nmol.l-1 • pCO2 4.5 - 6 kPa • pO2 11 -14 kPa • Actual HCO3 22-26 mmol.l-1 • Standard HCO3 22-26 mmol.l-1 • Base Excess +/- 2.0 • SaO2 ≥ 95 % JRG Hope Hospital April 2004

Oxygenation PaO2 ? SaO2 + Hb more important Interpret only with knowledge of FiO2 The Basics - Interpretation JRG Hope Hospital April 2004

Acid-Base Status pH pCO2 HCO3 (actual or standard) Base Excess Oxygenation PaO2 SaO2 + Hb more important? Interpret only with knowledge of FiO2 The Basics - Interpretation JRG Hope Hospital April 2004

H2O + CO2 H2CO3 H+ + HCO3- JRG Hope Hospital April 2004

The Basics - Standard Bicarbonate • The amount of bicarbonate that would be present if: • PCO2 was 5.3 kPa • Temp 37°C • Blood fully oxygenated • At sea level JRG Hope Hospital April 2004

The Basics - Base Excess (Deficit) • The amount of base that needs to be added to or subtracted from each litre of blood (ecf) to return the pH to a value of 7.4 at: • pCO2 5.3 kPa • Temp 37°C JRG Hope Hospital April 2004

Ivor Lewis Oesophagectomy • 54 year old lady • No significant PMH • 7 hour procedure • Initial post op period stable JRG Hope Hospital April 2004

Cellulitis (?) • 75 year old lady • Established atrial flutter • Admitted with spreading cellulitis right calf • Hypotension unresponsive to fluids (and dobutamine!) JRG Hope Hospital April 2004

Initial ABGs JRG Hope Hospital April 2004

Hyperchloraemic acidosis • Base deficit traditionally used as marker for metabolic acidosis • Appropriate fluid resuscitation should decrease base deficit • Chloride rich solutions (0.9% NaCl) can potentiate metabolic acidosis www.anaesthetist.com/icu/elec/ionz/Stewart • If base deficit persists despite ‘adequate’ fluids in an otherwise well patient, check [Cl-]! JRG Hope Hospital April 2004

Summary • Arterial blood gases can be used to guide therapy • Must be interpreted in the light of the clinical setting • Look at acid–base disturbances, then oygenation with FiO2 • Remember the possibility that treatment may cause problems! JRG Hope Hospital April 2004

Using Blood Gases