5. Drugs acting on central nervous system

5. Nitrous Oxide and Cyclopropane

Chemical and Physical Properties

a.       Chemically it is dinitrogen monoxide (N2O)

b.      Odourless

c.       Colourless

d.      Heavier than air

e.  Non-explosive

f.   Non-inflammable

g.  Like oxygen it supports combustion

h.  causes euphoria when inhaled in small amounts, so named as laughing gas. In past it was abused for recreational purposes.


a. MAC – has a large value of more than 100

b. It is least potent anesthetic agent

c. Has low Blood: Gas partition coefficient – 0.47, as is less soluble in blood.

d. Induction & recovery –rapid

e. Elimination –after action 99.9% nitrous oxide is exhaled unchanged without any metabolism, mostly by lungs, little through skin. Minute quantity is degraded by intestinal bacteria.

Pharmacological Effects

1.      CVS

Has cardiovascular depressant effects but countered by sympathetic stimulation. Itself does not have much significant effect. Effect shown is that of agent given along with nitrous oxide. When given with halothane, increased cardiac output occurs. When opioids are given heart rate and blood pressure are decreased.

2.      Respiratory system

Not significant. Causes increase in respiratory rate and decrease in tidal volume.

3.      CNS

Increased cerebral blood flow leading to increased intracranial pressure.

4.  GIT / Muscles

Has no significant effect on GIT and skeletal muscles.


1. Analgesia

As it is least potent anesthetic, it is commonly used for analgesic property in concentration of 20%.

2. Sedation

For sedation concentration range is between 30-80%. Not more than 80% as leads to hypoxia.


Most common ratio of 70% nitrous oxide and 30oxygen is used.

–  adjunct; used along with others as an adjunct most commonly, so called second gas effect.

–  for short surgical procedures

Like dental extraction, post operation pain, in painful dressings, fracture manipulations, during child birth.

Adverse effects

1. Diffusional hypoxia / anoxia

When nitrous oxide administration is stopped immediately after body, nitrous oxide from body tissues and blood rapidly diffuses into alveoli. These alveoli are filled with nitrous oxide and the patient suffers from hypoxia (even death might occur). R  emedy is to stop administration of nitrous oxide, but not of oxygen.

2. Vitamin B12 deficiency

As inhibits methionine synthetase, it is required for vitamin B12 synthesis. By inhibitory action causes B12 deficiency, leading to megaloblastic anemia and peripheral neuropathy.

3.Megaloblastic anemia

On prolonged exposure, decrease in methionine synthase occurs, which interferes with vitamin B12 metabolism, leading to megaloblastic anemia.

4.Peripheral neuropathy

5.  Effect on air-containing cavities

In body where air containing cavity (esp. obstructed middle ear, air embolus, pneumothorax, head injury, leading to formation of air containing lesion, majority of gas being nitrogen. Nitrous oxide replaces this nitrogen in cavity. It is more than 30 times soluble than nitrogen. Nitrous oxide enters rapidly while nitrogen exits slowly, so cavity may enlarge. When patient is suspected, nitrous oxide is not used.

6.  Effect of N2O & O2 in same cylinder

Two gases are present in the same cylinder. When temperature is lowered, to fill the cylinder, nitrous oxide liquefies earlier than oxygen. When patient is given anesthesia through these cylinders, first he gets pure oxygen and insufficient anesthesia. Later on he starts having pure nitrous oxide, leading to hypoxia.

To avoid this, it is advised to keep cylinders at a bit higher temperature, keeping them horizontally. During use they are shaken well before administration.

Strong analgesicLess potent
Rapid inductionTransportation difficult
Recovery rarely exceeds 1-4 minSpecial equipment for administration
Non irritantPenetrates into cavities
Nausea/ vomiting uncommonCo2 accumulation and hypoxia on prolonged administration
Little effects on circulation, respiration, liver, kidneyMegaloblastic anemia on prolonged administration
Diffusional anoxia


  1. It was previously used and was a potent general anesthetic.
  2. Non-irritant
  3. Explosive
  4. Flammable

Not used now.

Cyclopropane shock

Many a times patients complained of severe cardiovascular collapse known as cyclopropane shock. To avoid it, patient is administered small amounts of carbon dioxide.

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