Rational Testing

Interpreting arterial blood gas results

BMJ 2013; 346 doi: http://dx.doi.org/10.1136/bmj.f16 (Published 16 January 2013)
Cite this as: BMJ 2013;346:f16

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  1. Nicholas J Cowley, research registrar, anaesthesia and intensive care medicine1,
  2. Andrew Owen, academic clinical fellow1,
  3. Julian F Bion, professor of critical care medicine2
  1. 1Department of Anaesthesia and Critical Care, Queen Elizabeth Hospital Birmingham, Birmingham B15 2WB, UK
  2. 2University Department of Anaesthesia and Intensive Care Medicine, Queen Elizabeth Hospital, Birmingham B15 2TH
  1. Correspondence to: N J Cowley n.j.cowley{at}bham.ac.uk

Learning points

  • Interpretation of arterial blood gases requires a systematic assessment of oxygenation, pH, standard bicarbonate (sHCO3−) and base excess, partial pressure of carbon dioxide (PaCO2), and additional analytes

  • The P/F ratio (ratio between the PaO2 and the inspired oxygen concentration expressed as a fraction) is a useful guide to the presence and severity of impaired alveolar gas exchange

  • Reassess all acutely ill patients regularly, and consider repeat arterial blood gas analysis

  • Errors in blood gas analysis are dependent more on the clinician than on the analyser

You have been called to see a 69 year old man on a surgical ward because he has become drowsy and short of breath. He had a large bowel resection the previous day, has a background of type 2 diabetes, and is a current smoker. On examination his arterial blood pressure is 104/65 mm Hg, his heart rate 132 beats/min and irregular, and his respiratory rate 22 breaths/min; his oxygen saturations with pulse oximetry are 94% on supplemental oxygen via a 40% Venturi-type mask. He is slightly confused and is complaining of abdominal pain despite using patient controlled analgesia with morphine. His chest is clear on auscultation.

What is the next investigation?

You take a blood specimen for analysis of arterial blood gases for rapid biochemical evaluation to guide diagnosis and initial management. Table 1 shows the results. It is important to adopt a systematic approach to interpreting results of arterial blood gases, as outlined in table 2, preceded by a brief history and focused clinical examination.

View this table:
Table 1

 Report of arterial blood gases for the hypothetical patient described

View this table:
Table 2

 Guide to systematic approach to analysis of a report of arterial blood gases

Step 1: Assess oxygenation

Arterial oxygen tension (PaO2) is the partial pressure of oxygen in arterial blood. The main determinants of PaO2 are the inspired …

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