80 year old female presents complaining of increased fatigue. The patient has had a recent admission for hyponatraemia and has a background of COPD

ABG below on FiO2 0.32

pH 7.305                                            Na 125mmol/l

pCO2 93 mmHg                               K 4 mmol/l

pO2 62mmHg                                  Cl 76mmol/l

HCO3 44 mmHg                               Cr 37 umol/l

B/E 15                                                Glu  8 mmol/l

Lactate 0.5mmol/l

Describe and interpret the VBG

Answer:

Mild acidaemia pH 7.305

Respiratory Acidosis pCO2 93mmHg – likely chronic with history of COPD and HCO3 >30

Compensation Expected HCO3 – for every 10 increase in PCO2 4 increase in HCO3 = 44 mmol/l

A-a gradient:

PAO2 = (713xFIO2) – PCO2 x 1.25 = 228.16 – 116.25 = 112

Moderately low Na and Chloride

The ABG shows a compensated mild acute on chronic respiratory acidosis with a raised A-a gradient, with a moderately low sodium and chloride. Such a high PCO2 with a near normal pH raises the question of an underlying metabolic alkalosis.  It is likely that the patient is a chronic CO2 retainer but it is unlikely that they run such a high PCO2 when they are well. Diuretic use, excess steroid use in the context of COPD, or Cushings as part of a paraneoplastic phenomena  are most likely causes for a metabolic alkalosis. The low sodium and chloride could be due to diuretic use or part of SIADH due to underlying cancer