[Development of core/targetoid fibers in the diaphragm of chronically hypercapnic rats]

We tested the hypothesis that chronic hypercapnia contributes to the development of core/targetoid fibers in ventilatory muscles. Five Wistar rats were kept in 5% CO2 in air for 13 weeks. Arterial blood gases (mean +/- SE) were pH 7.402 +/- 0.017, PaCO2 47.4 +/- 2.4 Torr, and PaO2 105.2 +/- 13.2 Torr. While the rats were chronically hypercapnic, their mean minute ventilation was about 1.7 times higher than that of rats that breathed air. This level was maintained throughout the exposure to CO2. Approximately 6% to 12% of the diaphragmatic type-I fibers were replaced by core/targetoid fibers in 3 of 5 chronically hypercapnic rats but not in the 3 control rats. Chronic hypercapnia may have continuously stimulated the respiratory center and resulted in more work done by the ventilatory muscles. Long-term overuse of the ventilatory muscles, which may occur in patients with chronic respiratory failure, can cause morphological changes in type-I fibers in the diaphragm.