1. Department of Cardiac Surgery and Cardiology, State University Hospital, Catharijnesingel 101, PO Box 16250, NL-3500 CG Utrecht, The Netherlands
2. Department of Physiology, University of Groningen, Bloemsingel 10, NL-9712 KZ Groningen, The Netherlands
3. Department of Clinical Chemistry, Herlev Hospital, University of Copenhagen, Herlev Ringvej, DK-2730 Herlev, Denmark