Background: Weaning difficulties from mechanical ventilation are associated with diaphragm fatigue and reduced respiratory muscle endurance capacity. Often the work of breathing is increased during the weaning process as a result of inspiratory resistance loading (IRL). IRL produces increased free radical formation that contributes to deoxyribonucleic acid (DNA) damage. The purpose of this study was to determine whether dopamine reduced nuclei DNA damage when the work of breathing was increased. We hypothesized that the administration of low-dose dopamine (2 microg/kg/min) during IRL decreases myonuclei DNA damage associated with free radical formation.
Methods: In this in vivo study, 30 male Sprague-Dawley rats were divided into three groups: (1) the sham group receiving no IRL or no intravenous fluids, (2) IRL with administration intravenous saline, and (3) IRL with intravenous low-dose dopamine (2 microg/kg/min). All rats from the same breed and similar colonies were purchased from one laboratory facility to ensure homogeneity. The animals were anesthetized and tracheotomized, and an ultrasonic sensor was attached to the right hemidiaphragm to measure diaphragm shortening. Diaphragm fatigue was produced by IRL. Dopamine (2 microg/kg/min) was infused intravenously before and during loading. The diaphragms were excised, and myonuclei DNA damage was measured using the fluorescent dyes ethidium bromide and acridine orange and comet analyses as indices of free radical injury.
Results: In rats receiving saline, diaphragm shortening decreased by 37% after 45 minutes of IRL (P = .002) compared with baseline. In contrast, rats infused with dopamine exhibited a 31% increase in diaphragm shortening after 45 minutes of IRL (P = .037). With the use of differential dye uptake, in the saline group 59% of the nuclei were apoptotic, and 18% were necrotic. However, in the dopamine group there was significantly less apoptotic nuclei (16%, P < .001) and necrotic nuclei (7%, P = .005). Myonuclei DNA damage, measured by comet analyses, was associated with tail length and tail olive moment, which were 37% and 60% greater, respectively, in the saline group than in the dopamine group (P < .05).
Conclusion: These data support the hypothesis that low-dose dopamine during IRL reduced myonuclei DNA damage as measured by the fluorescent dyes and comet analysis. In addition, diaphragm fatigue was prevented by the administration of dopamine during IRL.