Question

George arrives in the emergency department by ambulance with acute dyspnea, tachypnea, and cyanosis. His arterial blood gases are: pH 7.232, PaO2 54 mmHg, PaCO2 56 mmHg. How would you interpret these, and what do these values indicate?

Answer 1

Arterial blood gas (ABG) test measures amounts of certain gases (typically oxygen O₂ and carbon dioxide CO₂) dissolved in blood withdrawn from an artery. ABG represents amount of the gases in partial pressure values (of arterial oxygen in P_aO₂, of arterial carbon dioxide in P_aCO₂) as well as acidity of arterial blood in pH.

The normal arterial blood P_aO₂ is 75-100 mmHg. George’s P_aO₂ is 54 mmHg which indicates that his blood is not properly oxygenated and George is hypoxemic (hypoxia is a condition wherein the patient is deprived of proper oxygen supply at a local or global level). It should be noted that P_aO₂ below 60 mmHg requires supplemental oxygen administration and if it further falls below 25 mmHg, the patient risks death. Hence, while George is not at immediate risk of death, he should be immediately supplemented with exogenous oxygen supply. Low P_aO₂ also explains George’s condition of cyanosis (bluish coloration of skin caused by inadequate oxygenation of blood).

The normal arterial blood P_aCO₂ is 35-45 mmHg. George’s P_aCO₂ is 56 mmHg which indicates that his levels of CO₂ in his arterial blood are higher and he is suffering from respiratory acidosis or hypercapnia which occurs when production of CO₂ exceeds its elimination. This explains George’s conditions of dyspnea (difficult or labored breathing leading to reduced elimination of CO₂) and tachypnea (rapid and shallow breathing caused when lungs try to increase elimination of CO₂).

Normally, the arterial blood pH ranges from 7.33 to 7.44. In George’s case, pH of his arterial blood is 7.232 which means that his blood is acidemic or more acidic than normal. In George’s case of compromised respiratory ventilation (reduced elimination of CO₂ by lungs), pH of blood also decreases. This is because CO₂ is dissolved in blood as carbonic acid. Hence, any increase in CO₂ would lead to acidosis of blood (seen as reduced pH of George’s arterial blood to a value of 7.232).