Structural Changes in the Tracheae of Preterm Lambs Induced by Ventilation

Abstract

ABSTRACT. Compliant immature airways sustain significant deformation following positive pressure ventilation. To evaluate the structural changes induced by in vivo positive pressure ventilation, tracheae of preterm lambs (107-116 d gestational age) were studied histologically.Nonventilated (group I: n = 7) and ventilated (group 11: n = 7) tracheal segments were excised and studied by histologic and morphometric techniques. Computerized image analysis was used to measure dimensions of tracheal wall components and of the tracheal section. The circumference, diameter, and cross-sectional area of the section as well as the length of the trachealis muscle were significantly greater; although the thickness of the muscle and cartilage were seen to be significantly lower in group I1 sections compared to group I sections. Also, in comparison to group I, in group I1 sections there was lesser overlap of the posterior free ends of tracheal cartilage and the epithelial layer was flattened and focally abraded. Our findings demonstrate structural changes in the airway of preterm animals and characterize alterations in the geometric arrangement of muscle and cartilage after PPV. These results suggest possible structural mechanisms for the functional changes seen during and subsequent to mechanical ventilation. (Pediatr Res 26: 434-437, 1989) Abbreviation PPV, positive pressure ventilationThe airways of immature animals are known to be very compliant, collapsible structures when compared to those of more mature animals (1-4). PPV further increases airway collapsibility, decreases the inflationary complicance (5), produces chronic volume deformation (6-8), and results in airway epithelial damage (9-14). To date, insufficient data are available to identify the morphologic alterations in the principal components of the tracheal wall (cartilage and smooth muscle) after positive pressure ventilation. Structural changes within the tracheal wall may explain, in part, the mechanical alterations consequent to ventilation. T o test the hypothesis that PPV produces structural changes in the tracheal cartilage and smooth muscle dimensions, both ventilated and unventilated tracheal segments were excised from premature lambs and examined by histologic and morphometric techniques. Potential differences in muscle and cartilage dimensions were analyzed for statistical significance between the ventilated and unventilated groups. MATERIALS AND METHODSThis study used tracheal segments obtained from age-and wtmatched preterm lambs between 107 and 116 d gestational age. The protocols for these studies had been approved by the Institutional Animal Care and Use Committee of Temple University, Philadelphia.Animal preparation. Pregnant ewes of known gestation were restrained in a prone position after sedation (Ketamine HC1, 500 mg intramuscular) and epidural anesthesia (0.5-1 mg/kg, 0.75% bupivacaine HCI). The uterus was exposed ventrolaterally and the fetal head allowed to emerge through a uterotomy. A rubber glove filled with warm saline was promptl...

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