Efficacy and respiratory effects of low-dose spinal morphine for postoperative analgesia following knee arthroplasty

A randomized, double-blind study of 38 patients undergoing total knee replacement was undertaken to compare the efficacy and respiratory effects of low-dose spinal morphine and patient-controlled i.v. morphine against patient-controlled i.v. morphine alone. Patients received either morphine 0.3 mg or saline 0.3 mi with 0.5% heavy spinal bupivacaine 2-2.5 mi. Respiratory effects were measured continuously for 14 h postoperatively with an Edentec 3711 respiratory monitor. There was an improvement in pain relief in the intrathecal morphine group, with significantly lower median VAS pain scores on movement at 4 h (0 (median 0-1.5) vs 5 (1.25-7.75) P<0.01), 12 h (2(1-5) vs 6 (3-8) P<0.01) and 24 h (3 (1-5) vs 5 (3-7) P<0.05) postoperatively, despite using significantly less patient-controlled morphine (20 mg (10.25-26.25) vs 38.5 mg (27-51) P<0.01) in the first 24 h. There was a small but statistically significant reduction in the median oxygen saturation (Sp(O2)) in the intrathecal morphine group 97 (95-99)% compared with the placebo group 99 (97-99)% (P<0.05). Although marked disturbances in respiratory pattern were observed in both groups, none of the patients in the study had severe hypoxaemia (Sp(O2) <85% >6 min h(-1)) and there was no significant difference in the incidence of mild (Sp(O2) <94% >12 min h(-1)) or moderate (Sp(O2) <90% >12 min h(-1)) hypoxaemia or in the incidence of episodes of apnoea or hypopnoea in the two groups.