NITROUS OXIDE (N2O)

QUESTION: If a nitrous oxide tank is contaminated with water vapor, ice will form on the cylinder valve as a result of the:
a. latent heat of vaporization
b. specific heat
c. vapor pressure
d. low pressure of the nitrous oxide
e. ambient temperature

HIGHLIGHTS ON N2O:
Inorganic anesthetic gas; nonexplosive and nonflammable (but can combust like O2)
Gas at room temperature and ambient pressure; can be kept as liquid under pressure bc critical temperature lies above room temperature.  Inexpensive.
*analgesic, anxiolysis and anesthetic properties

System Effects/SE:
CNS: increases CBF and cerebral blood volume --> incr ICP
incr CMRO2

CV: no significant net effects/changes in ABP, CO and HR

Pulm: increases RR, decreases tidal volume (Vt) --> minimal change in minute ventilation and ETCO2
hypoxic drive (mediated by peripheral chemoreceptors in carotid bodies) is markedly depressed by N2O

Neuromuscular: unlike VAAs, does not provide muscle relaxation; not a triggering agent for MH

Renal: increase renal vascular resistance, decr renal blood flow
decr GFR and UOP

Hepatic/GI:
minimal decrease in hepatic blood flow
PONV (controversial) d/t activation of CTZ & vomiting center in medulla

Heme:
Bone marrow depression (megaloblastic anemia)
alters immune response to infection
pernicious anemia; peripheral neuropathy

Other:
Inhibits enzymes (cobalt atom) that are Vitamin B12 dependent --> methionine synthetase (myelin formation), thymidylate synthetase (DNA synthesis)
Teratogenic effects

Diffusion into closed air spaces:
N2O is insoluble compared to other VAAs, but it is 35 times more soluble than nitrogen in blood.  Thus, it tends to diffuse into air-containing cavities more rapidly than nitrogen is absorbed by the bloodstream.  Example - pneumothorax: the N2O will diffuse into the cavity more rapidly than the air (nitrogen) diffuses out.  If the walls surrounding the cavity are rigid, pressure rises instead of volume.
**Examples of conditions in which N2O may be hazardous:
air embolism, pneumothorax, acute intestional obstruction, intracranial air, tympanic membrane grafting (tympanoplasty) in middle ear, tracheal tube cuffs, pulm HTN, pulm blebs

Pharmacokinetics/dynamics
Almost all N2O eliminated by exhalation.  Small amount diffuses out through skin
0.004% undergoes reductive metabolism in GI tract by anaerobic bacteria

MAC 105% (low potency)
BG Coefficient: 0.47
OG Coefficient: 225

Clinical Use:
65% N2O decreases MAC of other VAAs by 50%
concentration of N2O flowing through vaporizer can influence concentration of VAA delievered

2nd gas effect: incr vapor pressure and insoluble nature of N2O will enable it to be absorbed from alveolus first resulting in incr alveolar concentration of O2 and VAA

Diffusion Hypoxia: elimination of N2O is so rapid that alveolar oxygen and CO2 are diluted; prevented by admin of 100% during emergence

1845 - Horace Wells disgraces himself with unsuccessful demonstration of N2O in surgery



ANSWER: (a) The nitrous oxide tank contains a liquid, and in order for it to become vaporized, heat must be supplied.  As the cylinder is opened, heat is removed from the cylinder and from the air in the immediate vicinity.  The temperature falls, causing condensation.

REFERENCES:
Morgan, Mikhail. Clinical Anesthesiology (4th ed). pg. 164-166
Dershwitz. McGraw-Hill specialty board review: anesthesiology examination & board review (6th ed)