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

Basic Concepts n Absorb oxygen (oxygenation), excrete CO 2 (ventilation) n Breathing is controlled by medullary ventilatory center n MV = RR x TV, PaCO 2

Basic Concepts n Gas exchange is affected by multiple factors (ventilatory drive, muscle function, blood flow to lungs, chest wall compliance, lung disease) n Minimum Alveolar Concentration

Pattern of Breathing n Normal: intermittent deep breaths separated by varying intervals n Anesthetics: dose-dependent increases in RR –Isoflurane - increases RR up to 1 MAC –N 2 O - increases RR more than others at > 1 MAC n MV decreases: TV decrease > RR increase n GA: Rapid, shallow, regular, rhythmic breathing pattern

Pattern of Breathing: RR

PaCO 2 n Measure of adequacy of ventilation n Increases more with enflurane and desflurane than with isoflurane or halothane n N 2 O - no change in PaCO 2 from baseline n Degree of PaCO 2 increase due to volatile anesthetics decreases with time (i.e., there is less ventilatory depression after prolonged exposure)

PaCO 2

Ventilatory Response to CO 2 n Normal (awake) - CO 2 increases MV 1-3 L/min for each 1 mmHg increase in PCO 2 n Inhaled anesthetics - dose-dependent depression of slope (=decreased sensitivity to the ventilatory stimulant effects of CO 2 ) and rightward shift (=attenuated responsiveness to CO 2 ) of CO 2 response curve

Ventilatory Response to CO 2

Ventilatory Response to Arterial Hypoxemia n Awake - PaO 2 below 60 => increase in MV n 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 n Halothane and isoflurane at 1 MAC decrease bronchospasm n Probably due to anesthetic-induced decreases in afferent (vagal) nerve output n Effect is additive with beta-2 agonists

Airway Irritability n Isoflurane and desflurane - modest irritants –coughing –breathholding –production of secretions n Halothane, sevoflurane - well tolerated

Hypoxic Pulmonary Vasoconstriction n A reflex constriction of pulmonary arterioles in areas of atelectasis in attempts to decrease or prevent perfusion of unventilated alveoli n Inhaled anesthetics directly inhibit HPV when studied in isolated lung models n Clinically, no significant effect, presumably due to compensatory mechanisms

Respiratory Muscle Function n Optimal function: descent of diaphragm is coupled with expansion of rib cage due to contraction of intercostal muscles n Inhaled agents produce muscle relaxation (as well as depression of the medullary ventilatory center)

Respiratory Muscle Function n 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/hypercapniainterferes with rib cage expansion in response to hypoxemia/hypercapnia stabilization of the rib cage is decreased during spontaneous ventilationstabilization of the rib cage is decreased during spontaneous ventilation –descent of diaphragm tends to cause chest to collapse, leading to decreased lung volumes n Effects of other volatile agents on intercostal muscle function have not been reported

Summary n dec MV (inc RR, dec TV) => inc. PCO 2 n dec response to CO 2 n dec response to hypoxemia n dec synergy between hypoxemia and hypercapnia as ventilatory stimulants n bronchodilation n airway irritation n inhibition of hypoxic pulmonary vasoconstriction n respiratory muscle function interference (Halothane)