title: Modification of pleural fluid pH by local anesthesia.
abstract: 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.
authors.affiliation: Respiratory Disease Service, Hospital Universitario Ramón y Cajal, Madrid, Spain.
authors.forename: D
authors.initials: D
authors.lastname: Jiménez Castro
authors.forename: G
authors.initials: G
authors.lastname: Díaz
authors.forename: E
authors.initials: E
authors.lastname: Pérez-Rodríguez
authors.forename: E
authors.initials: E
authors.lastname: Prieto
authors.forename: R D
authors.initials: RD
authors.lastname: Yusen
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000779:Anesthetics, Local / Q000494:pharmacology*; D004322:Drainage; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D008297:Male; D008875:Middle Aged; D010996:Pleural Effusion / Q000378:metabolism* / Q000628:therapy; D011446:Prospective Studies
chemical_list: D000779:Anesthetics, Local