The effect of opioid analgesia on exercise test performance in chronic low back pain

The effect of opioid analgesia on tests of muscular function in chronic low back pain (CLBP) is unknown. Twenty-eight subjects with CLBP of at least moderate intensity performed the Sorensen isokinetic exercise test once after receiving 1 microg/kg fentanyl intravenously and once after placebo in a randomized-order double-blind crossover design. Naloxone 3 microg/kg was administered after the fentanyl phase. Fentanyl reduced mean+/-SD pain from 4.0+/-2.1 to 3.1+/-2.2 on a 0-10 verbal rating scale (P<0.05). Mean+/-SD Sorensen test performance was 77+/-49 s in the fentanyl arm and 60+/-42 s in the placebo arm. This represents an increased performance with fentanyl of 28% (P<0.001). We conclude that in addition to relieving pain in CLBP, the administration of 1 microg/kg fentanyl is associated with an improvement in lumbar exercise test performance. We presume that the pain relief resulted in increased test performance. Our result is at odds with those of randomized trials which have failed to demonstrate increased function following the treatment of pain with opioid analgesics. This highlights the complexity of the interaction between pain, analgesia and changes in function.