Rapid ventricular pacing (RVP) is a technique to obtain flow arrest for short periods of time during dissection or rupture of the aneurysm. RVP results in an adequate fall in blood pressure which presents as an on-off phenomenon. The technique facilitates the dissection and manipulation of cerebral aneurysms and arteriovenous malformations (AVMs) and can be lifesaving in the case of an intraoperative bleeding or rupture. In a former study blood pressure and clinical outcome were used as study parameters. However it is not clear whether repetitive periods of pacing are harmless for the patient. Silent cardiac and cerebral infarcts may be undetected if only clinical outcome is taken as a study parameter. The investigators will study the safety of RVP, particularly for the heart and the brain, using magnetic resonance imaging, brain oxygenation (Sct O₂) evaluated by near-infrared spectroscopy and troponin levels. The purpose of this study is to evaluate the effect of repetitive periods of RVP on the oxygenation of the heart and brain using magnetic resonance imaging, Sct O2 (3) and troponin levels (4) both markers for ischemia damage.