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Pulmonary Effects of Volatile Anesthetics. Ravindra Prasad, M.D. Department of Anesthesiology UNC-CH School of Medicine. Basic Concepts. Absorb oxygen (oxygenation), excrete CO 2 (ventilation) Breathing is controlled by medullary ventilatory center MV = RR x TV, PaCO 2. Basic Concepts.

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pulmonary effects of volatile anesthetics

Pulmonary Effects of Volatile Anesthetics

Ravindra Prasad, M.D.

Department of Anesthesiology

UNC-CH School of Medicine

basic concepts
Basic Concepts
  • Absorb oxygen (oxygenation), excrete CO2 (ventilation)
  • Breathing is controlled by medullary ventilatory center
  • MV = RR x TV, PaCO2
basic concepts1
Basic Concepts
  • Gas exchange is affected by multiple factors (ventilatory drive, muscle function, blood flow to lungs, chest wall compliance, lung disease)
  • Minimum Alveolar Concentration
pattern of breathing
Pattern of Breathing
  • Normal: intermittent deep breaths separated by varying intervals
  • Anesthetics: dose-dependent increases in RR
    • Isoflurane - increases RR up to 1 MAC
    • N2O - increases RR more than others at > 1 MAC
  • MV decreases: TV decrease > RR increase
  • GA: Rapid, shallow, regular, rhythmic breathing pattern
paco 2
PaCO2
  • Measure of adequacy of ventilation
  • Increases more with enflurane and desflurane than with isoflurane or halothane
  • N2O - no change in PaCO2 from baseline
  • Degree of PaCO2 increase due to volatile anesthetics decreases with time (i.e., there is less ventilatory depression after prolonged exposure)
ventilatory response to co 2
Ventilatory Response to CO2
  • Normal (awake) - CO2 increases MV 1-3 L/min for each 1 mmHg increase in PCO2
  • Inhaled anesthetics - dose-dependent depression of slope (=decreased sensitivity to the ventilatory stimulant effects of CO2) and rightward shift (=attenuated responsiveness to CO2) of CO2 response curve
ventilatory response to arterial hypoxemia
Ventilatory Response to Arterial Hypoxemia
  • Awake - PaO2 below 60 => increase in MV
  • Inhaled anesthetics
    • subanesthetic doses (0.1 MAC) - greatly attenuate ventilatory response to hypoxemia
    • anesthetic doses (1 MAC) - abolish ventilatory response to hypoxemia
    • also attenuate the usual synergistic effect of hypoxemia and hypercapnia on stimulation of ventilation
bronchodilation
Bronchodilation
  • Halothane and isoflurane at 1 MAC decrease bronchospasm
  • Probably due to anesthetic-induced decreases in afferent (vagal) nerve output
  • Effect is additive with beta-2 agonists
airway irritability
Airway Irritability
  • Isoflurane and desflurane - modest irritants
    • coughing
    • breathholding
    • production of secretions
  • Halothane, sevoflurane - well tolerated
hypoxic pulmonary vasoconstriction
Hypoxic Pulmonary Vasoconstriction
  • A reflex constriction of pulmonary arterioles in areas of atelectasis in attempts to decrease or prevent perfusion of unventilated alveoli
  • Inhaled anesthetics directly inhibit HPV when studied in isolated lung models
  • Clinically, no significant effect, presumably due to compensatory mechanisms
respiratory muscle function
Respiratory Muscle Function
  • Optimal function: descent of diaphragm is coupled with expansion of rib cage due to contraction of intercostal muscles
  • Inhaled agents produce muscle relaxation (as well as depression of the medullary ventilatory center)
respiratory muscle function1
Respiratory Muscle Function
  • Halothane
    • preferential suppression of intercostal muscle function, relative sparing of diaphragm
    • depression of intercostal muscle function
      • interferes with rib cage expansion in response to hypoxemia/hypercapnia
      • stabilization of the rib cage is decreased during spontaneous ventilation
        • descent of diaphragm tends to cause chest to collapse, leading to decreased lung volumes
  • Effects of other volatile agents on intercostal muscle function have not been reported
summary
Summary
  • dec MV (inc RR, dec TV) => inc. PCO2
  • dec response to CO2
  • dec response to hypoxemia
  • dec synergy between hypoxemia and hypercapnia as ventilatory stimulants
  • bronchodilation
  • airway irritation
  • inhibition of hypoxic pulmonary vasoconstriction
  • respiratory muscle function interference (Halothane)