Effects of Spontaneous Breathing Activity on Atelectasis Formation During General Anaesthesia
Condition: Atelectasis
MeSH terms: Pulmonary Atelectasis
Study type: Interventional

Summary: 
      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
    

Detailed description: 
      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.
    

Intervention name(s): Spontaneous Breathing; Pressure Controlled Ventilation; Pressure Support Ventilation
Intervention type(s): Other; Other; Other
Allocation: Randomized
Intervention model: Parallel Assignment
Masking: Single (Participant)
Primary purpose: Supportive Care
Keywords: Ventilation; General Anesthesia; electrical impedance tomography; pressure support; spontaneous breathing; laryngeal mask; Patients who are scheduled for elective knee or ankle surgery