Effect of opioid administration on cardiorespiratory and muscle oxygenation during lifting in chronic back pain patients.

The objectives of this study were to examine the effects of opioid administration on the acute cardiorespiratory and muscle oxygenation responses during a repetitive lifting and lowering test (RLL) to voluntary fatigue in participants with chronic low back pain (LBP). Written informed consent was obtained from 27 LBP participants (mean age 50.9 +/- 16.4 years) who completed one testing session during which they were administered a saline placebo and opioid (1 microg/kg of fentanyl intravenously) in random order. The participants performed the RLL at a rate that they felt that they could sustain for an 8-h working day. Acute opioid administration increased the total lifting time and total work done during RLL by 35 and 48%, respectively (p < 0.05). However, the increased work capacity was accompanied by a significant (p < 0.05) increase in oxygen cost of 22% per unit amount of work done and significant (p < 0.05) increases in heart rate (7%) and ventilation rate (10%). Near infrared spectroscopic analysis of erector spinae oxygenation and blood volume responses during RLL indicated no significant (p > 0.05) differences between the opioid and placebo phases. These findings suggest that the increased energy cost of lifting as a result of opioid administration was due to enhanced central oxygen transport and not peripheral muscle oxygen extraction.