Static and dynamic characteristics of sheep chest wall during mechanical ventilation

Background: Sheep are increasingly used as experimental animal model for respiratory diseases; however, to date, mechanics of sheep chest wall remains incompletely characterized. Aim: To assess elastic, resistive and viscoelastic characteristics of the chest wall in the whole inspiratory capacity range, in healthy mechanically-ventilated sheep. Methods: After anaesthesia, rumen evacuation, and paralysis, 10 healthy sheep (36±8 kg) were studied supine during mechanical ventilation with constant tidal volume (8 ml/kg) and variable inspiratory flow (V̇insp) at 0, 5, 10 and 15 cmH2O end-expiratory pressure (ZEEP, P5, 10 and 15). Oesophageal pressure recording during constant-flow inflations followed by rapid airway occlusions and inspiratory pauses allowed assessment of chest wall quasi-static elastance (Est,cw) and interrupter resistance (Rint,cw). Chest wall viscoelastic properties were indexed by the slow pressure decay following end-inspiratory interruptions (ΔPvisc,cw). Data are expressed as median(IQR). Results: Est,cw decreased modestly from ZEEP (9.74 (1.91) cmH2O/L) to P10 (7.30 (1.32) cmH2O/L, p 0.006, Fig.1), as Rint,cw did (from ZEEP 0.88 (0.38) to P15 0.52 (0.26) cmH2Os/L, p<0.001, Fig.2). Conversely, ΔPvisc,cw did not vary significantly with PEEP (p 0.706). Overall, the relation between ΔRvisc,cw/V̇insp (ΔRvisc,cw) and inspiratory duration (Ti) was at all PEEP levels an exponential rise to maximum, as previously reported for different species (Fig.3). Conclusions: Elastic and viscoelastic properties of sheep chest wall are relatively invariant in the inspiratory capacity range, while interrupter resistance tends to decrease with increasing volume. This implies that, in this species, elastic or viscoelastic modifications of lung mechanics, induced by an experimental disease affecting the lung only, can be monitored in terms of mechanical variations of the whole respiratory system, even in presence of a concomitant change in the operating volume; conversely, only if lung volume changes greatly, small variations of airway resistance may be masked by changes of Rint,cw, when respiratory system Rint alone is measured.