PO2 or pO₂ or PO2 or .....?

Acid-base physiology and significance of blood gas results are topics that many students find daunting. Their difficulty is not aided by the non-standard abbreviations used to indicate partial pressure of a gas. The issue is highlighted in a recent letter to the journal Clinical Chemistry. 

The authors cite the different abbreviations used for partial pressure (p, p, P or P) and the various ways in which these are used in the literature to describe the partial pressure of carbon dioxide and oxygen: pco2, PCO2, pCO₂, Po₂, PO2, PO2 etc. They make a plea for standardization and specifically recommend the use of uppercase P rather than p for partial pressure. 

It seems that the non-standardized nomenclature that has evolved over the years can only needlessly contribute to the mystification of a difficult topic – why not a standard P?