Impairment of diaphragm muscle force and neuromuscular transmission after normothermic cardiopulmonary bypass: effect of low-dose inhaled CO

Abstract

Cardiopulmonary bypass (CPB) is associated with significant postoperative morbidity, but its effects on the neuromuscular system are unclear. Recent studies indicate that even relatively short periods of mechanical ventilation result in significant neuromuscular effects. Carbon monoxide (CO) has gained recent attention as therapy to reduce the deleterious effects of CPB. We hypothesized that 1) CPB results in impaired neuromuscular transmission and reduced diaphragm force generation; and 2) CO treatment during CPB will mitigate these effects. In adult male Sprague-Dawley rats, diaphragm muscle-specific force and neuromuscular transmission properties were measured 90 min after weaning from normothermic CPB (1 h). During CPB, either low-dose inhaled CO (250 ppm) or air was administered. The short period of mechanical ventilation used in the present study ( approximately 3 h) did not adversely affect diaphragm muscle contractile properties or neuromuscular transmission. CPB elicited a significant decrease in isometric diaphragm muscle-specific force compared with time-matched, mechanically ventilated rats ( approximately 25% decline in both twitch and tetanic force). Diaphragm muscle fatigability to 40-Hz repetitive stimulation did not change significantly. Neuromuscular transmission failure during repetitive activation was 60 +/- 2% in CPB animals compared with 76 +/- 4% in mechanically ventilated rats (P < 0.05). CO treatment during CPB abrogated the neuromuscular effects of CPB, such that diaphragm isometric twitch force and neuromuscular transmission were no longer significantly different from mechanically ventilated rats. Thus, CPB has important detrimental effects on diaphragm muscle contractility and neuromuscular transmission that are largely mitigated by CO treatment. Further studies are needed to ascertain the underlying mechanisms of CPB-induced neuromuscular dysfunction and to establish the potential role of CO therapy.

Structure–activity relationships in rodent diaphragm muscle fibers vs. neuromuscular junctions

“…In the rat DIAm, differences in NMJ morphology are associated with differences in susceptibility to neuromuscular transmission failure across fiber types. Larger and more complex motor end-plates at type IIx and/or IIb fibers consistently show greater failure rates (Ermilov et al, 2010; Mantilla et al, 2004a; Rowley et al, 2007). However, in the mouse DIAm differences in NMJ morphology were far less pronounced than in the rat.…”

“…Although small differences in planar area were reported between NMJs at type I and IIa fibers in rat DIAm (Prakash et al, 1996b; Sieck et al, 1999b), subsequent studies did not verify these differences (Mantilla et al, 2004a; Prakash et al, 1996a; Prakash et al, 1999; Prakash et al, 1995; Sieck and Prakash, 1997). However, significant differences in NMJ planar area and branching were consistently observed between NMJs at type I and IIa fibers (expressing MyHC Slow or MyHC 2A , respectively) compared to type IIx and/or IIb fibers in the rat DIAm (expressing MyHC 2X and/or MyHC 2B ) (Ermilov et al, 2007; Mantilla et al, 2004a; Mantilla et al, 2007; Prakash et al, 1996a; Prakash et al, 1996b; Prakash et al, 1999; Prakash et al, 1995; Rowley et al, 2007; Sieck and Prakash, 1997; Sieck et al, 1999b). Thus, in the present study we compared morphological measurements in two groups: NMJs at type I and IIa vs. NMJs at type IIx and/or IIb fibers.…”

“…The extent of DIAm neuromuscular transmission failure was estimated in a subset of rats (n=6) and mice (n=8) using a previously described method (Aldrich et al, 1986; Ermilov et al, 2010; Fournier et al, 1991; Johnson and Sieck, 1993; Kuei et al, 1990; Mantilla et al, 2004b; Miyata et al, 1995; Prakash et al, 1999). Briefly, a segment of the midcostal region of the DIAm that included the phrenic nerve was mounted in a tissue bath containing Rees-Simpson solution (with the following composition in mM: Na + 135, K + 5, Ca 2+ 2, Mg 2+ 1, Cl − 120, HCO 3 − 25 at pH 7.4) maintained at 26°C and oxygenated with 95% O 2 and 5% CO 2 .…”

“…Repetitive stimulation of the DIAm (40 Hz, 330-ms trains, repeated every s) was used to calculate the ratio of force generated after 2 min of stimulation compared to the initial force (Aldrich et al, 1986; Ermilov et al, 2010; Mantilla et al, 2004b; Miyata et al, 1995; Prakash et al, 1999). …”

Impact of diaphragm muscle fiber atrophy on neuromotor control

“…Moreover, with assist-mode ventilation where a small degree of neural activation triggers the mechanical ventilator, DIAm fiber atrophy is largely mitigated (Sassoon et al, 2002; Sassoon et al, 2004). Although studies examining the effects of controlled mechanical ventilation on DIAm structure and function have attempted to exclude the potential confounding influence of inflammation and sepsis, it is during such conditions that such rapid catabolic effects are observed (Ermilov et al, 2010; Supinski et al, 1999; Supinski and Callahan, 2006). …”