Intervention (if applicable): All measurements and data collection will take place around administration of drugs that are given according to routine clinical practice (cefazolin 2 grams for prophylaxis of infections, morphine 10 mg intravenously at the end of surgery, a patient-controlled analgesia (PCA) pump with morphine for postoperative pain relief and nadroparin 0,6 ml for trombo-embolic prophylaxis). For muscle relaxation, patients are randomised to receive one of two generally accepted dosing regimen of atracurium (0.5 mg/kg based on ideal body weight or 0.5 mg/kg based on total body weight). Peri- and postoperatively, a maximum amount of 70 ml of blood will be collected from an indwelling arterial line for measurement of concentrations of cefazolin, morphine, and anti-Xa (nadroparin). One week after surgery the patient will be checked for thrombosis using ultrasonography.