Pulmonary physiology and interpretation of blood gas (PO2, PCO2) results

http://erj.ersjournals.com/content/early/2014/10/16/09031936.00039214.full.pdf+html. Accessed Oct 2014.

Arterial blood gas (ABG) analysis is clinically useful because it allows assessment of two vital interrelated physiological functions: pulmonary gas exchange and maintenance of blood pH (acid-base homeostasis). 

Disturbance of these two functions (either singly or in combination) with consequent abnormal blood gas results is a feature of many respiratory and non-respiratory diseases as well as traumatic injury; disturbance of either of the two functions can also have a solely iatrogenic origin. 

As a recent expert review article nicely demonstrates, understanding and interpretation of patient ABG results depends crucially on a sound understanding of the two physiological functions. The author, a physician with a long and distinguished translational research interest in respiratory physiology, chose to focus only on the pulmonary gas exchange function and explains how disturbance of this function affects blood gas (pO₂ and pCO₂) results. 

But much more, he explains how detailed examination of blood gas parameters and supplementary derived parameters can be used to provide the most detailed assessment of the severity and underlying causative mechanism(s) of disturbed pulmonary gas exchange function that are operating in a particular patient with abnormal pO₂. 

In so doing he defines and explains key concepts, including: ventilation, perfusion and the compensatory physiological response to disturbed pulmonary gas exchange. The article includes discussion of the five mechanisms that can give rise to hypoxemia: reduced inspired pO₂; hypoventilation; ventilation/perfusion inequality; diffusion limitation; and shunting.  

Having provided a comprehensive account of the pathophysiology of pulmonary gas exchange, the author provides the detail of a systematic, multi-level approach for interpretation pO₂, pCO₂ and other derived (calculated) parameters that are affected by disturbed pulmonary gas exchange.

Detailed discussion of the pathophysiology of pulmonary gas exchange inevitably involves mathematical relationships and symbols that have the capacity to mystify the topic for the non-expert. This article has the great merit of accessibility. The aim of the article is clearly stated at the outset and information is presented in a helpfully logical sequence.  

Difficult concepts are explained from first principles with the non-expert in mind, well exemplified by the opening sentence of the article which reads:  “The reason we have a lung is well known: to allow the exchange of gases between the air we breathe and the pulmonary capillary blood.”   

In all, this article provides a valuable educational resource for students of respiratory physiology as well as clinicians whose work involves the interpretation of blood gas results and treatment of patients who are suffering conditions associated with disturbance of pulmonary gas exchange.