title: Reliability of Waveform Analysis as an Adjunct to Loss of Resistance for Thoracic Epidural Blocks.
abstract: BACKGROUND
  The epidural space is most commonly identified with loss of resistance (LOR). Although sensitive, LOR lacks specificity, as cysts in interspinous ligaments, gaps in ligamentum flavum, paravertebral muscles, thoracic paravertebral spaces, and intermuscular planes can yield nonepidural LOR. Epidural waveform analysis (EWA) provides a simple confirmatory adjunct for LOR. When the needle is correctly positioned inside the epidural space, measurement of the pressure at its tip results in a pulsatile waveform. In this observational study, we set out to assess the sensitivity, specificity, as well as positive and negative predictive values of EWA for thoracic epidural blocks.
  
  
  METHODS
  We enrolled a convenience sample of 160 patients undergoing thoracic epidural blocks for thoracic surgery, abdominal surgery, or rib fractures. The choice of patient position (sitting or lateral decubitus), approach (midline or paramedian), and LOR medium (air or normal saline) was left to the operator (attending anesthesiologist, fellow, or resident). After obtaining a satisfactory LOR, the operator injected 5 mL of normal saline through the epidural needle. A sterile tubing, connected to a pressure transducer, was attached to the needle to measure the pressure at the needle tip. A 4-mL bolus of lidocaine 2% with epinephrine 5 μg/mL was then administered and, after 10 minutes, the patient was assessed for sensory blockade to ice.
  
  
  RESULTS
  The failure rate (incorrect identification of the epidural space with LOR) was 23.1%. Of these 37 failed epidural blocks, 27 provided no sensory anesthesia at 10 minutes. In 10 subjects, the operator was unable to thread the catheter through the needle. When compared with the ice test, the sensitivity, specificity, and positive and negative predictive values of EWA were 91.1%, 83.8%, 94.9%, and 73.8%, respectively.
  
  
  CONCLUSIONS
  Epidural waveform analysis (with pressure transduction through the needle) provides a simple adjunct to LOR for thoracic epidural blocks. Although its use was devoid of complications, further confirmatory studies are required before its routine implementation in clinical practice.
authors.affiliation: From the *Montreal General Hospital, Department of Anesthesia, McGill University, Montreal, Quebec, Canada; and †Ramathibodi Hospital, Department of Anesthesia, Mahidol University, Bangkok; and ‡Maharaj Nakorn Chiang Mai Hospital, Department of Anesthesia, Chiang Mai University, Chiang Mai, Thailand.
authors.forename: Prangmalee
authors.initials: P
authors.lastname: Leurcharusmee
authors.forename: Vanlapa
authors.initials: V
authors.lastname: Arnuntasupakul
authors.forename: Daniel
authors.initials: D
authors.lastname: Chora De La Garza
authors.forename: Amorn
authors.initials: A
authors.lastname: Vijitpavan
authors.forename: Sonia
authors.initials: S
authors.lastname: Ah-Kye
authors.forename: Abhidej
authors.initials: A
authors.lastname: Saelao
authors.forename: Worakamol
authors.initials: W
authors.lastname: Tiyaprasertkul
authors.forename: Roderick J
authors.initials: RJ
authors.lastname: Finlayson
authors.forename: De Q H
authors.initials: DQ
authors.lastname: Tran
mesh_terms: D000328:Adult; D000368:Aged; D000767:Anesthesia, Epidural / Q000379:methods / Q000592:standards*; D002983:Clinical Competence / Q000592:standards*; D004824:Epidural Space; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009407:Nerve Block / Q000379:methods / Q000592:standards*; D015203:Reproducibility of Results; D013904:Thoracic Vertebrae