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Inhalational Agents

NITROUS OXIDE

From U of Miami School of Medicine

Description

Nitrous oxide is the only inhalational anesthetic agent used that is a not a vapor, but a true gas by definition. For anesthetic purposes, it is supplied in blue cylinders, compressed to its liquid phase.

Physical Constants
Molecular Weight........................................................44
Boiling Point..................................-88.0 deg. C @ 760 mm Hg
Vapor Pressure.................................39,000 mm Hg @ 20 deg. C

Partition Coefficients
Blood:Gas......................................................................0.47
Brain:Blood....................................................................1.1
Oil:Gas..........................................................................1.4

Potency
MAC in 100% oxygen..............................................104% atm


Pharmacokinetics

Mechanism of Action: unknown
Dosage: titrate to effect for analgesia or supplementation of general anesthesia
Onset of Action: dose dependant
Peak Effect: dose dependant
Duration of Action: dose dependant
Elimination: pulmonary, renal, GI(0.04% is reduced to nitrogen in GI tract by anaerobes)
Metabolites: free radicals may be liberated when reduced


Pharmacology

Respiratory / Airway
  • Direct depression of medullary ventilatory center with concentrations greater than 50%
  • Increases respiratory rate
  • Decreases functional residual capacity
  • Can increase pulmonary vascular resistance, especially with preexisting pulmonary hypertension
  • Relaxes bronchial smooth muscle (direct effect, and decreased afferent tone)


Cardiovascular
  • Direct myocardial depression at concentrations >40%
  • Can lead to hypotension and decreased cardiac output when administered alone
  • When combined with a volatile agent, there is usually less circulatory depression than if either agent is used alone
  • Attenuates baroreceptor and vasomotor reflexes
  • Enhances isoflurane-induced coronary artery vasodilation, and can lead to coronary artery steal


CNS
  • Causes cerebral vasodilation and increased cerebral blood flow
  • Increases intracranial pressure due to increased cerebral blood volume


Musculoskeletal / Other
  • Does not cause skeletal muscle relaxation
  • Weak triggering agent for malignant hyperthermia
  • Chronic exposure may interfere with DNA synthesis as a result of reduced methionine synthetase activity



Principal Adverse Effects

Respiratory
Respiratory depression, apnea, diffusion hypoxia

Cardiovascular
Hypotension, arrhythmias

CNS
Dizzyness, euphoria, increased CBF/ICP, neuropathy with chronic exposure

GI
Nausea, vomiting, ileus

Other
Bone marrow depression with chronic exposure, malignant hyperthermia


Other Considerations

  • Diffuses into air-containing spaces 34 times faster than nitrogen can diffuse out, and can lead to potentially dangerous airspace expansion (pneumothorax, bowel obstruction, etc)
  • May cause diffusion hypoxia during emergence if supplemental oxygen is not administered
  • Crosses the placenta, and can cause fetal depression