hi, recently we have an experence of dry tap in spinal anesthesia. even after repeated attempts of spinal, we were not able to locate space. i can feel the resistence, but csf was not coming out. after few attempts, i gave spinal with bupivacaine after feeling resistence even though there was no csf (dry tap). surprisingly there was full effect of spinal.
   what can be the cause of it? does anybody have similar experince? 

hi,
   we are doing total knee arthroplasy. we routinely use inj. clexane for prevention of D.V.T.
  We usually give spinal/epidural.
   my query is," when should we start clexane? preop or postop? if postop, then after how much time of spinal anesthesia? if we have traumatic spinal tap, then what to do?
   regards.