Lab Case 231 Interpretation

75 year old male is BIBA after taking an odd turn. According to his wife he became acutely confused with the inability to talk and unable to walk.


Acidaemia – pH 7.09

Respiratory acidosis PCO2 83mmHg

Compensation – Expected HCO3 – acute process – for every 10 increase in PCO2 there is a 1 increase in HCO3 = 28mmol/l

A-a gradient:

A-a gradient for age = 22

Expected PAO2 = (FIO2 x713) – (pCO2 x1.25) = 182

PaO2 = 91mmHg

Raised A-a gradient = 91


The above ABG shows an acute respiratory acidosis with adequate compensation. There is markedly raised A-a gradient. Electrolytes are within normal limits with a low normal sodium and a normal lactate level.  We most commonly see respiratory acidosis in the context of COPD, but there are numerous other causes of respiratory acidosis including CNS, neuromuscular, lung and chest wall pathology. In this clinical context the respiratory acidosis could be due to multiple underlying issues – acute CVA, recent trauma resulting in brain bleed and/or fractured ribs leading to hypoventilation, accumulation of drugs including opioids, diazepam and phenytoin.

The raised A-a gradient will be a combination of hypoventilation as well as some other underlying lung pathology for example pneumonia, atelectasis secondary to underlying rib fractures in this case. A-a gradient will be normal when hypoxaemia is secondary to hypoventilation from hypercapnia alone. In the presence of hypercapnia, the raised A-a gradient suggests there is another mechanism for example V/Q mismatch or shunting that causes the raised A-a gradient, not hypoventilation alone.


Bedside – BSL – as a cause of altered conscious state

Pathology – Phenytoin level

Radiology – CXR looking for underlying trauma and pneumonia. CT brain – looking for traumatic bleed or CVA