Nitrous Oxide
Nitrous Oxide
physical properties
effects on organ system
biotransformation and toxicity
contraindications
drug interactions
-MAC value: 105 %
-only inorganic gas used in the clincal practice of anesthesia
-colorless and nearly odorless
-nonexplosive, nonflammable (although may support combustion)
-gas at room temperature
-gas at ambient pressure
-critical temperature is above room temperature therefore can be kept liquid while under pressure
-relatively inexpensive anesthetic agent
Central Nervous System
-increases CMR02
-increases CBF
-increases ICP (mild)
Cardiovascular System
-in vivo: direct myocardial contractility depressant
-in vitro: stimulates sympathetic nervous system therefore increases cardiac function
-overall: minimal or slight change in cardiac function due to the two opposing forces explained above
patients with CAD or severe hypovolemia:
-may have unmasked myocardial depression
-resultant drop in arterial blood pressure may lead to myocardial ischemia and further compromise on a already failing heart
patients with preexisting pulmonary HTN or Right sided CHF:
-should avoid nitrous oxide due to increased pulmonary vascular resistance via vasoconstriction of pulmonary artery
Respiratory System
-increases respiratory rate
-decreases tidal volume
-minimal change on minute ventilation
-minimal change on resting PaC02 levels
-hypoxic drive depressed by even low concentrations of nitrous oxide
Hepatic System
-decreases hepatic blood flow but to a lesser extent than volatile anesthetics
Renal System
-decreases renal blood flow secondary to increased renal vascular resistance
-decreased GFR and therefore decreased urinary output
Gastrointestinal System
-may cause postoperative nausea and vomitting
-may stimulate the chemoreceptor trigger zone (CTZ) and vomitting center in medulla
Biotransformation and toxicity
-biotransformation limited < 0.01 % which undergoes reductive metabolism within the GI anaerobic metabolism
-nearly all the nitrous oxide is eliminated through exhalation during the emergence of anesthesia
-toxicity may occur by irreversibly oxidizing the cobalt atom of vitamin B12
-therefore nitrous oxide inhibits enzymes which are vitamin B12 dependant (ex. methionine synthetase, thymidylate synthetase)
methionine synthetase: involved in myelin formation
thymidylate synthetase: involved in DNA synthesis
-prolonged exposure of nitrous oxide may lead to bone marrow suppresion and neurological deficiencies
bone marrow suppresion: megaloblastic anemia
neurologic deficiencies: peripheral neuropathies, pernicious anemia
-possible teratogenic effects: therefore avoid in pregnant women
contraindications
-nitrous oxide tends to diffuse into air-containing cavaties more rapidly than nitrogen absorbed into the blood stream
therefore has an expanding capacity in air-containing cavaties such as:
-air embolism
-pneumothorax
-acute intestinal obstruction
-intracranial air
-pulmonary air cysts
-intraocular air bubbles
-tempanic membrane grafting
Drug Interactions
-prolongs muscle relaxation from nondepolarzing muscle relaxants (NDMR)
-attenuates the circulatory and respiratory effects caused by volatile anesthetics
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