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Arterial carbon dioxide tension (Paco2) has not previously been studied in detail as a predictor of mortality in patients with community acquired pneumonia. This retrospective Canadian study examined the relationship between Paco2 and in-hospital mortality in patients with community acquired pneumonia.
Of the 2171 study subjects, in-hospital mortality (10%) was greater in those with hypocapnia (Paco2 <32 mm Hg) (OR 1.8 (95% CI 1.0 to 3.2)) and hypercapnia (Paco2 >45 mm Hg) (OR 2.6 (95% CI 1.5 to 4.5)) than in those with normal Paco2 values (40–44 mm Hg). In-hospital mortality rates within these Paco2 bands were similar for patients with and without chronic obstructive pulmonary disease (COPD). However, COPD was more common in those with hypercapnia and bacteraemia was more common in those with hypocapnia. For patients without bacteraemia, mortality was 2.5 times higher in those with hypercapnia but was not greater in those with hypocapnia, raising the possibility that bacteraemia may be the leading cause of death in hypocapnic patients. Respiratory rate was only loosely correlated with Paco2 levels and did not increase in-hospital mortality rates. Surprisingly, markers of renal function, blood pressure, age, and respiratory rate did not correlate with mortality rates.
This shows that Paco2 levels are another marker of in-hospital mortality in community acquired pneumonia and could be used to risk stratify patients on admission. However, the study did not show the mode of death in these patients. Further assessment is required to establish whether Paco2 levels add value over and above routinely assessed clinical and laboratory parameters.