Flow of participants through study. GCP=good clinical practice; PCA=patient controlled analgesia; VAS=visual analogue scale. *8 participants transferred between treatment groups during 12 hour study period: 2 patients discontinued PCA and went into treatment as usual; 6 went the other way. †Following advice from the Data Monitoring Committee’s independent statistician, participants recruited at site 5 were excluded and replaced with newly recruited participants from other sites