IN-VIVO DIAPHRAGM METABOLISM - COMPARISON OF PACED AND INSPIRATORY RESISTIVE LOADED BREATHING IN PIGLETS

Objective: We hypothesized that spontaneous, loaded diaphragm contract ions would lead to diaphragm fatigue, which would correlate with inade quate oxidative metabolism as measured by phosphorus-31 nuclear magnet ic resonance spectroscopy. Design: Prospective, randomized, crossover trial. Setting: University hospital research laboratory. Subjects: Eig ht piglets, 4 to 6 wks of age. Interventions: Each animal underwent, i n random order, a 20-min period of diaphragm pacing and a 45-min perio d of loaded spontaneous breathing, separated by a 20-min recovery peri od. Mechanical Ventilation was used during diaphragm pacing to maintai n a Paco(2) of 35 to 45 torr (4.7 to 6.0 kPa) and a Pao(2) of >100 tor r (>13.3 kPa). During spontaneous breathing, inspiratory loading was a chieved with a 2.0-mm inner diameter endotracheal tube in the breathin g circuit. Measurements and Main Results: During pacing, mean transdia phragmatic pressure decreased by 35%, from 23 +/- 5 (SD) to 15 +/- 3 m m Hg (p <.05), and this decrease correlated with a 335% increase in th e ratio of inorganic phosphate to phosphocreatine, from 0.23 +/- 0.1 t o 1.0 +/- 0.7 (p <.05). During loaded spontaneous breathing, arterial pH decreased from 7.42 +/- 0.06 to 7.25 +/- 0.05 (p < .05), secondary to an increase in Paco(2) from 41 +/- 4 to 65 +/- 11 torr (5.3 +/- 0.5 to 8.7 +/- 1.5 kPa) (p <.05). Despite respiratory acidosis, there was no decrease in trandiaphragmatic pressure during the period of loaded breathing, nor was any change in the ratio of inorganic phosphate to phosphocreatine seen. Conclusions: Diaphragm fatigue in a pacing model correlates with inadequate oxidative metabolism. In contrast, severe inspiratory resistive loaded breathing did not result in changes in ox idative metabolism or decreased diaphragm force output, despite hyperc apnia and respiratory acidosis.