Abstract
IntroductionAtelectasis and distal airway closure are common clinical entities of general anaesthesia. These two phenomena are expected to reduce the ventilation of dependent lung regions and represent major causes of arterial oxygenation impairment in anaesthetic conditions. The behaviour of the elastance of the respiratory system (Ers), as well as the lung aeration assessed by computed tomography (CT) scan, was evaluated during a descendent positive end-expiratory pressure (PEEP) titration. This work sought to evaluate the potential usefulness of Ers monitoring to set the PEEP in order to prevent tidal recruitment and hyperinflation of healthy lungs under general anaesthesia.MethodsPEEP titration (from 16 to 0 cmH2O, tidal volume of 8 ml/kg) was performed, and at each PEEP, CT scans were obtained during end-expiratory and end-inspiratory pauses in six healthy, anaesthetized and paralyzed piglets. The distribution of lung aeration was determined and the tidal re-aeration was calculated as the difference between end-expiratory and end-inspiratory poorly aerated and normally aerated areas. Similarly, tidal hyperinflation was obtained as the difference between end-inspiratory and end-expiratory hyperinflated areas. Ers was estimated from the equation of motion of the respiratory system during all PEEP titration with the least-squares method.ResultsHyperinflated areas decreased from PEEP 16 to 0 cmH2O (ranges decreased from 24–62% to 1–7% at end-expiratory pauses and from 44–73% to 4–17% at end-inspiratory pauses) whereas normally aerated areas increased (from 30–66% to 72–83% at end-expiratory pauses and from 19–48% to 73–77% at end-inspiratory pauses). From 16 to 8 cmH2O, Ers decreased with a corresponding reduction in tidal hyperinflation. A flat minimum of Ers was observed from 8 to 4 cmH2O. For PEEP below 4 cmH2O, Ers increased in association with a rise in tidal re-aeration and a flat maximum of the normally aerated areas.ConclusionIn healthy piglets under a descending PEEP protocol, the PEEP at minimum Ers presented a compromise between maximizing normally aerated areas and minimizing tidal re-aeration and hyperinflation. High levels of PEEP, greater than 8 cmH2O, reduced tidal re-aeration but increased hyperinflation with a concomitant decrease in normally aerated areas.
Highlights
Atelectasis and distal airway closure are common clinical entities of general anaesthesia
Hyperinflated areas decreased from positive end-expiratory pressure (PEEP) 16 to 0 cmH2O whereas normally aerated areas increased
computed tomography (CT)-scan morphological analyses and respiratory mechanics during PEEP titration The reduction of PEEP from 16 cmH2O to zero end-expiratory pressure (ZEEP) resulted in a decrease in the hyperinflated areas while an increase in normally aerated areas was observed
Summary
Atelectasis and distal airway closure are common clinical entities of general anaesthesia These two phenomena are expected to reduce the ventilation of dependent lung regions and represent major causes of arterial oxygenation impairment in anaesthetic conditions. The behaviour of the elastance of the respiratory system (Ers), as well as the lung aeration assessed by computed tomography (CT) scan, was evaluated during a descendent positive end-expiratory pressure (PEEP) titration. Muscle paralysis, which reduces the displacement of the diaphragm in dependent lung, results in atelectasis and airway closure in anaesthetized patients [3,4]. This effect is enhanced when large inspiratory fractions of oxygen are used during anaesthesia [2,5]. Performing a descending PEEP titration may have similar effects in healthy lungs, because lower pressures may be needed to open ventilatory units than those in diseased lungs
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