Electrical impedance tomography (EIT) is a novel method to monitor regional lung function. For this purpose, 32 surface electrodes are placed around the human thorax. Weak alternating currents are applied via two of these electrodes and the resulting potentials are measured at the remaining electrodes. From the measured voltages, real-time images are calculated which show the distribution of electrical impedance within the body representing functions rather than structures. Using EIT these lungs can be analysed on a regional basis with respect to the following risk factors: collapse, at risk of becoming atelectatic or overdistension. Identifying these lung areas of particular clinical relevance by EIT may help to find the best individual PEEP level for each patient.