title: Metabolic basis for inspiratory muscle fatigue in normal humans.
abstract: Inspiratory muscle fatigue, a common event in patients in the intensive care unit, is under multifactorial control. To test the hypothesis that systemic oxygenation is a factor in this event, we subjected five healthy males (age 42 +/- 3 yr) to continuous inspiratory pressure (75% of maximal inspiratory pressure, -95 +/- 5 cmH2O) with the use of a controlled breathing pattern while they breathed normoxic (21% O2), hyperoxic (30% O2), and hypoxic (13% O2) mixtures. Inspiratory muscle endurance (IME; time that pressure could be maintained) and other cardiorespiratory parameters were monitored. Room air IME (3.3 +/- 0.4 min) was shortened (P < 0.05) during 13% O2 breathing (1.6 +/- 0.4 min) but was unaffected during 30% O2 breathing (4.0 +/- 0.6 min). Inspiratory loading lowered the respiratory exchange ratio (RER) during the 21 and 30% O2 trials (1.02 +/- 0.01 to 0.80 +/- 0.03% and 1.05 +/- 0.05 to 0.69 +/- 0.01%, respectively) but not during the 13% O2 trials (1.03 +/- 0.03 to 1.06 +/- 0.07%). At the point of fatigue during the 13% O2 trials, RER was lower compared with the same time point during the 21 and 30% O2 trials. A significant relationship was observed between IME and RER (r = -0.73, P = 0.002) but not between IME and any of the other measured variables. We conclude that 1) hypoxemia impairs the ability of the inspiratory muscles to sustain a mechanical challenge and 2) substrate utilization of the respiratory muscles shifts toward a greater reliance on lipid metabolism when O2 is readily available; this shift was not observed when the O2 supply was reduced.
authors.affiliation: Indiana University Department of Medicine, Indianapolis.
authors.forename: E T
authors.initials: ET
authors.lastname: Mannix
authors.forename: T Y
authors.initials: TY
authors.lastname: Sullivan
authors.forename: P
authors.initials: P
authors.lastname: Palange
authors.forename: I R
authors.initials: IR
authors.lastname: Dowdeswell
authors.forename: F
authors.initials: F
authors.lastname: Manfredi
authors.forename: P
authors.initials: P
authors.lastname: Galassetti
authors.forename: M O
authors.initials: MO
authors.lastname: Farber
mesh_terms: D000328:Adult; D005221:Fatigue / Q000378:metabolism*; D006801:Humans; D000860:Hypoxia / Q000378:metabolism; D050356:Lipid Metabolism; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology; D010807:Physical Endurance / Q000502:physiology; D012129:Respiratory Function Tests; D012132:Respiratory Muscles / Q000378:metabolism*; D014692:Ventilation-Perfusion Ratio