Background: It is a common practice to anesthetize
patients before performing a thoracentesis. We postulated that this
technique may cause a clinically significant difference in the pH of
the pleural fluid.
Methods: We compared two methods of
determining pleural fluid pH. Fifty patients undergoing diagnostic or
therapeutic thoracentesis were enrolled. Two 4-mL aliquots of pleural
fluid were anaerobically collected into blood gas syringes containing
heparin, one before (group A) and the other after (group B)
anesthetizing the patient with 5 mL of 2% mepivacaine. pH was then
determined on both samples using an arterial blood gas machine.
Agreement analysis was performed overall and in subcategories of pH
used to define complicated (< 7.1), borderline (7.1 to 7.3), or
uncomplicated (> 7.3) parapneumonic effusions. We analyzed these same
data stratified by the volume of pleural fluid in relationship to the
size of the hemithorax (< 15% and > 15%).
Results: There was a statistical difference between
the mean pH in both groups (group A, 7.32; group B, 7.28;
p < 0.0001). There was a significant correlation between the two
measures (r = 0.97; p < 0.0001). Using the pH subcategories, there
was 45% discordance in classification for patients with parapneumonic
effusions. The pH values obtained in group B wrongly predicted whether
the patient required a chest tube in two of four cases (50%). In
patients with effusions that occupied < 15% of the affected
hemithorax, there was an 80% discordance in classification for
patients with parapneumonic effusions, and the pH values obtained in
group B wrongly predicted whether the patient required a chest tube in
two of two cases (100%).
anesthesia is typically used before thoracentesis is performed.
However, in cases of suspected parapneumonic effusions that occupy< 15% of the affected hemithorax, pH results may be significantly
altered by use of local mepivacaine anesthesia.