Respiratory Muscle Fatigue and Ventilatory Failure

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Respiratory Muscle Fatigue and Ventilatory Failure

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Respiratory Failure
Lung Failure
Pump Failure
Gas exchange failure
Ventilatory failure
manifested by
Hypoxemia
manifested by
Hypercapnia
Central
Fatigue
Mechanical
depression
defect
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Physical Signs of Respiratory Muscle Fatigue

• Tachypnea
• Abdominal paradox
• Respiratory alternans

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Pathophysiology of Respiratory Muscle Fatigue

• Muscle contractile failure
• Neuromuscular transmission failure
• Fatigue of central nervous system

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Muscle Contractile Failure

• Slow muscle, a high oxidative potential, is less
  susceptible to developing fatigue than fast
  muscle
• MgATP H2O ? MgADP Pi H
• Glycogen depletion, lactic acid accumulation,
  inability to utilize blood-borne substances, and
  decrease in the rate of ATP hydrolysis are merged
  to explain loss of force.

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Neuromuscular Transmission Failure

• Action potential fatigue minimized when
  contraction was inhibited by hypertonic solution
  and abolished completely in noncontracting fibers
  poisoned with cyanide or iodoacetate
• Neuromuscular transmission failure in man during
  diaphragmatic fatigue not clearly understood

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Determinants of Critical Task (Pressure, Work)

• The threshold of fatigue--level of exercise which
  cannot be sustained indefinitely, expressed as a
  percentage of the maximum performance
• Diaphragm--40 of maximum, all the inspiratory
  muscle--50 to 70 of the maximum, mean
  inspiratory flow--0.6-0.9 L/s, the ratio of
  inspiratory time--0.3-0.5
• Largely affected by other factors (pressure-time
  index, velocity of contraction, operational
  length, energy supply, efficiency of the muscle)

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Fatigue of The Central Nervous System

• Fatigue characterized not only by loss of force
  but also by slowing of the muscle contractile
  speed
• Afferent information via large (types I and II)
  and small (types III and IV) fibers affects the
  central respiratory controllers discharge in
  terms of firing rate, firing time, and frequency
  of breathing

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Respiratory Muscle Fatigue and Ventilatory
Failure in Clinical

• Fatigue during loaded breathing
• Fatigue during weaning
• Fatigue as a cause of respiratory failure

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Fatigue Druing Loaded Breathing

• Stage of infinite possibilities (task of limited
  duration)
• Stage of alternative strategies (partial resting
  of one group of muscles)
• Stage of exhaustion (no longer sustain the
  breathing task, and gives up)

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Fatigue During Weaning

• Tachypnea
• Bradypnea
• Central apnea

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Fatigue as Cause of Respiratoy Failure

• CO2 retainers in chronic state of fatigue or that
  the CNS sets a pattern of breathing in an attemp
  to avoid exhaustion
• VT ?, VD/VT ?, PaCO2?

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THE END

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