Modification of pleural fluid pH by local anesthesia

Background: It is a common practice to anesthetize patients before performi ng a thoracentesis, We postulated that this technique may cause a clinicall y significant difference in the pH of the pleural fluid. Methods: We compared two methods of determining pleural fluid pH, Fifty pat ients 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 (gro up B) anesthetizing the patient with 5 mt of 2% mepivacaine. pH was then de termined on both samples using an arterial blood gas machine. Agreement ana lysis was performed overall and in subcategories of pH used to define compl icated (< 7.1), borderline (7.1 to 7.3), or uncomplicated (> 7.3) parapneum onic effusions, We analyzed these same data stratified by the volume of ple ural fluid in relationship to the size of the hemithorax (< 15% and > 15%), Results: There was a statistical difference between the mean pH in both gro ups (group A, 7.32; group B, 7.28; p < 0.0001). There was a significant cor relation between the two measures (r = 0.97; p < 0.0001). Using the pH subc ategories, there was 45% discordance in classification for patients with pa rapneumonic effusions, The pH values obtained in group B wrongly predicted whether the patient required a chest tube in two of four cases (50%), In pa tients with effusions that occupied < 15% of the affected hemithorax, there was an 80% discordance in classification for patients with parapneumonic e ffusions, and the pH values obtained in group B wrongly predicted whether t he patient required a chest tube in two of two cases (100%), Conclusions: Local anesthesia is typically used before thoracentesis is per formed. However, in cases of suspected parapneumonic effusions that occupy < 15% of the affected hemithorax, pH results may be significantly altered b y use of local mepivacaine anesthesia.