Adding ketamine to morphine for patient-controlled analgesia after thoracic surgery: influence on morphine consumption, respiratory function, and nocturnal desaturation.

BACKGROUND: I.V. patient-controlled analgesia (PCA) with morphine is often used for postoperative analgesia after thoracic surgery, but the required doses may increase postoperative respiratory disorders. Adjunction of ketamine could reduce both doses and related respiratory side-effects.

METHODS: The main objective of this prospective, randomized double-blinded study was to evaluate the influence of adding ketamine to PCA on morphine consumption and postoperative respiratory disorders. Consecutive patients undergoing lobectomy (n = 50) were randomly assigned to receive, during the postoperative period, either i.v. morphine 1 mg ml(-1) or morphine with ketamine 1 mg ml(-1) for each. Morphine consumption was evaluated by cumulative doses every 12 h for the three postoperative days. Postoperative respiratory disorders were assessed by spirometric evaluation and recording of nocturnal desaturation.

RESULTS: The adjunction of ketamine resulted in a significant reduction in cumulative morphine consumption as early as the 36th postoperative hour [43 (SD 18) vs 32 (14) mg, P = 0.03] with a similar visual analogue scale. In the morphine group, the percentage of time with desaturation < 90% was higher during the three nights [1.80 (0.21-6.37) vs 0.02 (0-0.13), P < 0.001; 2.15 (0.35-8.65) vs 0.50 (0.01-1.30), P = 0.02; 2.46 (0.57-5.51) vs 0.55 (0.21-1.00), P = 0.02]. The decrease in forced expiratory volume in 1 s was less marked in the ketamine group at the first postoperative day [1.04 (0.68-1.22) litre vs 1.21 (1.10-0.70) litre, P = 0.039].

CONCLUSIONS: Adding small doses of ketamine to morphine in PCA devices decreases the morphine consumption and may improve respiratory disorders after thoracic surgery.