Atelectasis and redistribution of ventilation towards non-dependent lung zones are a common side effects of general anesthesia. Spontaneous breathing activity (SBA) during mechanical ventilation may avoid or reduce atelectasis, improving arterial oxygenation; however, it is unclear whether these effects play a significant role during general anesthesia in patients with healthy lungs. Earlier studies on ventilation during general anesthesia had to rely on computed tomography (CT) findings. Recent advances in lung imaging technology allow to assess the regional aeration of the lungs continuously and non-invasive by electrical impedance technology (EIT). In this work, we will use the EIT to assess ventilation changes from the time before induction of anesthesia until discharge from the post-anesthesia care unit. Our main focus is the difference caused by pure positive pressure ventilation (PCV) and assisted spontaneous breathing (pressure support ventilation, PSV). Our findings would improve our understanding of the physiology of the lungs during general anesthesia and would help to improve the standards of respiratory care during anesthesia

Atelectasis formation is a common phenomenon during general anaesthesia, occurring in almost 90% of patients (Lundquist, Hedenstierna et al. 1995). In patients in supine position, atelectasis of dorsal lung zones is usually accompanied by redistribution of ventilation towards ventral areas (Hedenstierna 2003; Victorino, Borges et al. 2004).The main mechanisms which contribute to the formation of atelectasis are compression (e.g. in obese patients or during laparoscopic surgery), absorption (e.g. when high concentrations of inspired oxygen are used) and reduced surfactant action(Magnusson and Spahn 2003). Atelectasis impairs oxygenation by reducing the functional residual capacity and by causing right-to-left-shunts. Consecutively, hypoxemia after extubation is common in daily practice: 20% of patients in a study experienced desaturations below 92% (Mathes, Conaway et al. 2001), and the risk is even higher in patients with risk-factors such as obesity or thoraco-abdominal procedures (Russell and Graybeal 1993; Xue, Li et al. 1999). Hypoxemic events prolong the stay in PACU, cause more ICU admissions and increase the incidence of cardiac complications (Rosenberg, Rasmussen et al. 1990; Gill, Wright et al. 1992).

Several measures to prevent or treat atelectasis in ventilated patients have been investigated, such as PEEP (Brismar, Hedenstierna et al. 1985; Tokics, Hedenstierna et al. 1987; Neumann, Rothen et al. 1999), recruitment maneuvers (Neumann, Rothen et al. 1999) and spontaneous breathing during mechanical ventilation (Putensen, Rasanen et al. 1994; Putensen, Mutz et al. 1999). The laryngeal mask airway (LMA) is ideally suited for spontaneous breathing during general anaesthesia because of its low resistance. A large survey found that more than half of the routine cases with an LMA are performed under spontaneous ventilation (Verghese and Brimacombe 1996), while positive pressure ventilation is equally acceptable. With regard to the prevention of atelectasis, spontaneous ventilation could be advantageous.

Most works on atelectasis formation during general anaesthesia used CT. Although CT is a gold standard for quantification of lung aeration, it can only provide data on single time points and is not suitable for measurements during routine cases. In recent years, the electrical impedance tomography (EIT) has evolved into a versatile tool, which allows detailed insights into ventilation and perfusion conditions of the lung (Bodenstein, David et al. 2009). EIT allows continuous assessment of lung aeration, is non-invasive and can easily be used as a research and monitoring tool during routine cases.

We hypothesize that compared with positive pressure ventilation (PPV), pressure support ventilation (PSV) during general anaesthesia reduces the extent of redistribution as detected by EIT during and after the procedure.

Inclusion Criteria:

• Patients (age 18-65) scheduled for elective knee or ankle surgery under general anaesthesia with an LMA with an expected duration of at least 60 minutes.

Exclusion Criteria:

• Pregnancy,
• Pulmonary diseases (e.g. Asthma, COPD),
• Implanted pacemaker or AICD,
• Inability to communicate or understand the risks of the study,
• Contraindications for an LMA (e.g. obesity, reflux),
• Deformities of the thorax,
• Failure to place an LMA.