Lab Case 168 -Interpretation

64 year old male presents to ED c/o 1 day history of cough and SOB.

Acidaemia –pH 7.20

Respiratory – pCO2 – 72mmHg

Expected HCO3 –patient unlikely a CO2 retainer  as patient has mild COPD with no previous admissions and HCO3 <30 therefore for every 10 increase in CO2 there is an increase of HCO3 by 1 -> expected HCO3 is 27. Therefore adequate compensation.

AG = 5, therefore no existing HAGMA

Expected PAO2= (760-47)XFiO2 – (CO2/0.8)  = 195mmHg

A-a gradient = 103 (expected =20) therefore increased

The ABG shows a fully compensated primary acute respiratory acidosis, with an elevated A-a gradient. There is a mild increase in creatinine, with a normal potassium and AG

In the above context the respiratory acidosis is likely due to an exacerbation of the patients COPD. The increased A-a gradient could be secondary to an underlying pneumonia, but there is no history of infective symptoms. The COPD alone can cause V/Q mismatch and an increased  gradient. However  the  history is acute in nature, therefore a PE should be considered in the DDx.

The second VBG shows worsening CO2 and pH, even taking into account that the initial gas was an ABG. This could be for a number of reasons.

-underlying pneumothorax, pneumonia

-inadequate bronchodilator and steroid treatment

-inadequate BiPAP settings – leading to poor TV, RR and MV

-patient not tolerating the NIV

-mask leaks

This patient had not had a CXR done prior to the second blood gas. An urgent CXR was then ordered which showed a right sided 3cm pneumothorax. The patient was taken off BiPAP and a chest drain was inserted, which rapidly improved the patients work of breathing. He was then placed back on BiPAP and his pH and CO2 rapidly improved.

For revision on NIV see guidelines on Jedo


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