Background Rectus sheath block can provide analgesia following umbilical hernia repair. However, conflicting reports on its analgesic effectiveness exist. No study has investigated plasma local anesthetic concentration following ultrasound-guided rectus sheath block (USGRSB) in children. Objectives Compare the effectiveness and bupivacaine absorption following USGRSB or wound infiltration (WI) for umbilical hernia repair in children. Methods A randomized blinded study comparing WI to USGRSB in 40 children undergoing umbilical hernia repair was performed. Group WI (n=20) received wound infiltration 1mg/kg 0.25% bupivacaine. Group RS (n=20) received USGRSB 0.5mg/kg 0.25% bupivacaine per side in the posterior rectus sheath compartment. Pain scores and rescue analgesia were recorded. Blood samples were drawn at 0, 10, 20, 30, 45 and 60 minutes. Results Patients in the WI group had a 2-fold increased risk of requiring morphine (Hazard ratio 2.06, 95% CI 1.01, 4.20, p=0.05). When required, median time to first morphine dose was longer in the USGRSB group (65.5 min vs 47.5 min, p=0.049). Peak plasma bupivacaine concentration was higher following USGRSB than WI (median: 631.9 ng/ml IQR: 553.9 – 784.1 vs 389.7 ng/ml IQR: 250.5-502.7, p= 0.002). Tmax was longer in the USGRSB group (median 45 min IQR: 30 - 60 vs 20 min IQR: 20 – 45, p= 0.006). Conclusions USGRSB provides more effective analgesia than WI for umbilical hernia repair. USGRSB with 1mg/kg 0.25% bupivacaine is associated with safe plasma bupivacaine concentration that peaks higher and later than WI. Caution against using larger volumes of higher concentration local anesthetic for USGRSB is advised.

Morphine distribution is very limited during chronic intrathecal delivery in ambulatory pigs, and there are significant spinal cord drug concentration gradients as a function of distance from the infusion point. Consequently, catheter tip position may be critical, particularly when infusing isobaric solutions. These data also support the hypothesis that inflammatory masses complicating chronic intrathecal opioid delivery occur at the catheter tip because limited drug distribution results in extremely high drug concentrations at that point.

Even at very slow infusion rates, drug distribution within the cerebral spinal fluid and spinal cord are affected by baricity/posture. These findings suggest that patient position and solution baricity may be important clinical factors determining the distribution and ultimate efficacy of chronic intrathecal drug infusions.