Dynamics of re-expansion of atelectasis during general anaesthesia |
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Authors: | Rothen H U; Neumann P; Berglund J E; Valtysson J; Magnusson A; Hedenstierna G |
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Institution: | Department of Anaesthesiology and Intensive Care Medicine, University Hospital, CH-3010 Bern, Switzerland; Department of Anaesthesiology and Intensive Care, Department of Radiology and Department of Clinical Physiology, University Hospital, Uppsala, Sweden |
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Abstract: | A major cause of impaired gas exchange during general anaesthesia is
atelectasis, causing pulmonary shunt. A 'vital capacity' (VC) manoeuvre
(i.e. inflation of the lungs up to 40 cm H2O, maintained for 15 s) may
re-expand atelectasis and improve oxygenation. However, such a manoeuvre
may cause adverse cardiovascular effects. Reducing the time of maximal
inflation may improve the margin of safety. The aim of this study was to
analyse the change over time in the amount of atelectasis during a VC
manoeuvre in 12 anaesthetized adults with healthy lungs. I.v. anaesthesia
with controlled mechanical ventilation (VT 9 (SD 1) ml kg-1) was used. For
the VC manoeuvre, the lungs were inflated up to an airway pressure (Paw) of
40 cm H2O. This pressure was maintained for 26 s. Atelectasis was assessed
by analysis of computed x-ray tomography. The amount of atelectasis,
measured at the base of the lungs, was 4.0 (SD 2.7) cm2 after induction of
anaesthesia. The decrease in the amount of atelectasis over time during the
VC manoeuvre was described by a negative exponential function with a time
constant of 2.6 s. At an inspired oxygen concentration of 40%, PaO2
increased from 17.2 (4.0) kPa before to 22.2 (6.0) kPa (P = 0.013) after
the VC manoeuvre. Thus in anaesthetized adults undergoing mechanical
ventilation with healthy lungs, inflation of the lungs to a Paw of 40 cm
H2O, maintained for 7-8 s only, may re-expand all previously collapsed lung
tissue, as detected by lung computed tomography, and improve oxygenation.
We conclude that the previously proposed time for a VC manoeuvre may be
halved in such subjects.
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