Modification of pleural fluid pH by local anesthesia

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%). Conclusions: Local 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.