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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. 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

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  1. Pulmonary Effects of Volatile Anesthetics Ravindra Prasad, M.D. Department of Anesthesiology UNC-CH School of Medicine

  2. Basic Concepts • Absorb oxygen (oxygenation), excrete CO2 (ventilation) • Breathing is controlled by medullary ventilatory center • MV = RR x TV, PaCO2

  3. 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

  4. 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

  5. Pattern of Breathing: RR

  6. 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)

  7. PaCO2

  8. 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

  9. Ventilatory Response to CO2

  10. 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

  11. 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

  12. Airway Irritability • Isoflurane and desflurane - modest irritants • coughing • breathholding • production of secretions • Halothane, sevoflurane - well tolerated

  13. 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

  14. 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)

  15. 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

  16. 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)

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