The Influence of Body-mass Index on the Outcome of Spinal Anesthesia for Total Knee Replacement Arthroplasty

• spinal anesthetic success/ failure [ Time Frame: at 2 hour after the anesthetic induction (at the time of patient's report of pain) ] [ Designated as safety issue: No ]
  spinal anesthetic success/ failure as determined by the patient's self-report of pain (NRS >= 2) before the end of surgery
  
  

• Time to patient's first report of pain [ Time Frame: at 3 hour after anesthetic induction (at the time of patient's report of pain) ] [ Designated as safety issue: No ]
  Time to patient's first report of pain from successful anesthetic induction
  
  
• Spinal anesthesia block level [ Time Frame: 2, 5, 10, 15 min after anesthetic induction, at the surgery end ] [ Designated as safety issue: No ]

  Spinal anesthesia block level as determined by Pinprick sensory test with Bromage scale.

  Sensory 0 = an ability to appreciate a pinprick as sharp; 1 = perception of a pinprick as less sharp than in unblocked areas; 2 = perception of a pinprick as touch but not sharp (analgesia); 3 = an inability to feel pinprick(anesthesia).

  Bromage Scale 0 = ability to lift an extended knee at the hip; 1 = ability to flex the knee but not to lift an extended leg; 2 = ability to flex toes only; 3 = inability to move.

  
  
• the incidence of tourniquet pain [ Time Frame: at 90 min afer anesthetic induction ] [ Designated as safety issue: No ]
  the incidence of tourniquet pain during the tourniquet time reported by the patient (NRS >= 2)
  
  

Though the spread of spinal anesthetic drug is unpredictable, patients factors (age, gender, height,weight, body-mass index), spinal anatomy, anesthetic drug, additive (epinephrine, opioid) and lumbosacral cerebrospinal fluid volume are known to be the determinants of sensory block level. Among these determinants, the influence of body-mass index (BMI) on spinal anesthesia is controversial, and there is no specific guideline showing the relative priority of these determinants.

Therefore, in these prospective observational study, the investigators are trying to evaluate (1) the influence of body-mass index on spinal anesthetic outcome by survival analysis and (2) the relative influence of these determinants on spinal anesthetic outcome by logistic regression analysis.