1 Intensive and Respiratory Care Unit, Department of Internal Medicine, Groningen University Medical Center, P.O. Box 30001, 9700 RB, Groningen, The Netherlands. t.s.van.der.werf@int.azg.nl.
2 Intensive and Respiratory Care Unit, Department of Internal Medicine, Groningen University Medical Center, P.O. Box 30001, 9700 RB, Groningen, The Netherlands.