Mechanical ventilation effects on distal organs: preliminary reports

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In order to test systemic effects of mechanical ventilation, we studied organs `distal' to the lung by means of an in-vivo animal model. A total of 22 Sprague Dawley rats were anesthetized, curarized and mechanically ventilated (Harvard Rodent, mod. 683). Once total lung capacity was estimated (8 rats), a total of 7+7 rats were randomized to receive tidal volume of 25% and 75% of inspiratory capacity (CI), respectively. Ventilation strategies were: a) 25%CI (9.8 ± 0.7 ml/kg), respiratory rate (RR) 57.8 ± 5.9 bpm, positive end expiratory pressure (PEEP) 4.29 ± 0.64 cmH₂O, mean airway pressure (Paw_m) 6.96 ± 0.61 cmH₂O, peak inspiratory airway pressure (Paw_p) 13.4 ± 2.12; b) 75%CI (31.3 ± 3 ml/kg), RR 18.3 ± 3.3, PEEP zero, Paw_m 6.13 ± 0.53, Paw_p 28.7 ± 2.43. Arterial pressure (invasively monitored), paO₂ pH_a, and paCO₂ were not statistically different between groups throughout the experiment. After 1 h of ventilation animals were sacrificed, liver and kidney isolated and fixed in 4% formalin, cut and H&H stained for optic microscopy. Organs from rats ventilated with 75%CI were consistently different from those ventilated with edema, and liver more homogeneously edematous. From a morphometric assay conducted on 5+5 rats, 75%CI liver resulted represented by more empty zone (index of edema) as compared to 25%CI liver (18.17 ± 5.02 vs 11.89 ± 2.69, respectively; P < 0.001). We conclude that rats ventilated for an hour with a tidal volume equal to 75%CI are characterized by different liver and kidney morphology from those of rats ventilated with 25%CI, at the same (Paw_m), arterial pressure, acid-base status and oxygenation.