Drug reference

Nitrous oxide

General Anesthetic · General anaesthetic; Analgesic

Also known as Entonox, Equanox

General AnestheticGeneral anaesthetic; Analgesic

CDSCO approvedSchedule H

EXCRETION

—

not curated

INTERACTIONS

7 major

SEVERE in our sources

PREGNANCY

—

not curated

Top interactionssee all 12 →

IsocarboxazidSevereDatabaseDDInter
MethotrexateSevereDatabaseDDInter
MethylcobalaminSevereDatabase
PhenelzineSevereDatabaseDDInter

Mechanism

Nitrous oxide exerts a stabilizing effect on the postjunctional membrane, which potentiates the activity of competitive neuromuscular blocking agents. It also inactivates vitamin B12 with prolonged use. Although unsatisfactory as a sole anaesthetic due to lack of potency, it is useful in combination with other agents to allow for significant dose reduction.

Indications

Maintenance of anaesthesia (as part of a balanced technique in association with other inhalational or intravenous agents)Analgesia (without loss of consciousness)Obstetric practice (for analgesia)Changing painful dressings (for analgesia)Aid to postoperative physiotherapy (for analgesia)Emergency ambulances (for analgesia)adjunct to other inhalational or intravenous anestheticsanalgesia and mild sedation in outpatient dentistry

Dosing

Adult: For anaesthesia, commonly used in a concentration of 50 to 66% in oxygen as part of a balanced technique. For analgesia (without loss of consciousness), a mixture of 50% nitrous oxide and 50% oxygen is used via self-administration with a demand valve.

Pharmacokinetics

Onset

rapid equilibration, rapid induction and emergence

Excretion

almost all (99.9%) eliminated unchanged by the lungs

Contraindications

Patients with an air-containing closed space (e.g., pneumothorax, intracranial air after head injury, entrapped air following recent underwater dive, recent intra-ocular gas injection) due to risk of increased pressure
clinical settings with air-containing cavities (e.g., pneumothorax, air embolus, obstructed middle ear/bowel, intraocular air bubble, pulmonary bulla, intracranial air)
chronic analgesic or sedative in critical care settings (due to potential metabolic effects)
patients with pre-existing pulmonary hypertension

Side effects

Common

Megaloblastosis (with prolonged use, due to vitamin B12 inactivation)modest increases in respiratory ratedecreases in tidal volume

Serious

Increased pressure in air-containing closed spaces, which can lead to dangerous conditions such as pneumothorax enlargement or complications with intracranial air, entrapped air, or intra-ocular gas injection
markedly depresses the ventilatory response to hypoxia
can significantly increase CBF and ICP
increases venous tone in peripheral and pulmonary vasculature
diffusional hypoxia (upon discontinuation)
interaction with cobalt of vitamin B12 (leading to megaloblastic anemia and peripheral neuropathy)
expansion of volume or pressure in air-containing cavities

Drug interactions

Isocarboxazid

Severe

Database

Clinical effect not specified

Source: DDInter

Methotrexate

Severe

Database

Clinical effect not specified

Source: DDInter

Methylcobalamin

Severe

Database

Acute functional B12 deficiency, leading to megaloblastic anemia, myelosuppression, and neurological damage (e.g., subacute combined degeneration of the spinal cord), especially in patients with pre-existing marginal B12 status.

Avoid prolonged exposure to nitrous oxide in patients with B12 deficiency or at risk. Pre-treat with B12 if prolonged exposure is anticipated in at-risk patients. Monitor neurological status.

Phenelzine

Severe

Database

Clinical effect not specified

Source: DDInter

Selegiline

Severe

Database

Clinical effect not specified

Source: DDInter

Sodium Oxybate

Severe

Database

Clinical effect not specified

Source: DDInter

Tranylcypromine

Severe

Database

Clinical effect not specified

Source: DDInter

Methylnaltrexone

Moderate

Textbook

Generally sees decreases in arterial blood pressure and cardiac output.

Not specified, but implies careful monitoring due to hemodynamic effects.

Source: G&G 14e · p480

Nalmefene

Moderate

Textbook

Generally sees decreases in arterial blood pressure and cardiac output.

Not specified, but implies careful monitoring due to hemodynamic effects.

Source: G&G 14e · p480

Naloxone

Moderate

Textbook

Generally sees decreases in arterial blood pressure and cardiac output.

Not specified, but implies careful monitoring due to hemodynamic effects.

Source: G&G 14e · p480

Naltrexone

Moderate

Textbook

Generally sees decreases in arterial blood pressure and cardiac output.

Not specified, but implies careful monitoring due to hemodynamic effects.

Source: G&G 14e · p480

Oxygen

Moderate

Textbook

Diffusion hypoxia.

Prevent diffusion hypoxia by continuing 100% oxygen inhalation for a few minutes after discontinuing nitrous oxide, instead of immediately switching to air.

Source: KDT 7e · p376