Nitrous Oxide (Nitrous)

Anesthesia Implications

Classification: Inhaled Anesthetic
Therapeutic Effects: Sedation

Duration: 20-25 minutes (5 minute half-life)

Contraindications

Surgeries:
1. Retinal surgeries – causes gas bubble expansion and increased intraocular pressure (IOP) which can lead to blindness. At minimum, discontinue 15 minutes prior to bubble placement
2. Recent retinal surgery:
—Avoid for 30 days following a perfluoropropane bubble placement
—Avoid for 10 days following an SF6 bubble placement
—Avoid for 5 days following an air bubble placement
—No contraindication if the surgeon used silicone oil
2. Tympanoplasty
3. Pulmonary surgeries – increases pulmonary pressures and atelectasis
4. Surgeries monitoring MEP’s – Nitrous interferes with MEP monitoring
5. Surgeries of the mouth/throat/airway – Nitrous is not flammable but will support combustion
6. Laparoscopic surgeries – This is not an absolute contraindication. Two risks here: If the pneumoperitoneum pressure is not well regulated, nitrous can cause damaging pressures to build. Secondly, if another gas (other than carbon dioxide) is used to inflate the abdomen, nitrous will support combustion if electrocautery is used.

Conditions:
1. Increased CPP – Nitrous oxide will further increase CPP
2. Trauma – can expand free gases in the body, such as tension pneumothorax, pneumocephalus, air embolism
3. Bowel obstruction – again because of expansion of air-filled spaces
4. Pulmonary hypertension – Nitrous increases pulmonary resistance
5. Omphalocele and Gastroschisis
6. Foreign body aspiration – reduces FiO2
7. Bullous disease
8. Marfan’s Syndrome – high incidence of spontaneous pneumothorax

Primary Considerations

The LEAST potent inhaled anesthetic. Provides analgesia as well as sedation.

Sedation onset and offset both are very quick. Since it’s MAC value is 104%, it cannot be used as the sole anesthetic agent. Used as an adjuvant to other anesthetics.

30 times more soluble than nitrogen and will rapidly diffuse into and expand any air-filled space.  Compliant air spaces such as a pneumothorax can double with a 1:1 (50%) nitrous mix.  A 2:1 (67%) nitrous mix can triple the airspace.

Nitrous Oxide, when used as the SOLE agent, actually activates the SNS, which increases SVR, CVP, and arterial pressure

Increases pulmonary vascular resistance (PVR)

There’s still a lot of concern surrounding nitrous oxide causing PONV. It has been shown that if nitrous oxide administration stays under 1 hour, there is very little chance of it causing PONV. See the article here

Nitrous oxide interferes with MEPs.

Can cause diffusion hypoxemia. When discontinuing nitrous oxide, put the patient on 100% O2.  Diffusion hypoxemia happens when the rapid elimination of nitrous oxide from the blood to the alveoli displaces the oxygen. This reduces alveolar O2 concentrations and leads to hypoxemia – especially so if the patient is on room air. The iconic time that this happens is when the nitrous has been discontinued and the patient is emerging.  This is NOT so much a concern in healthy patients, but should be of greater concern in patients that have conditions where oxygen exchange is challenged (COPD, etc).

According to Chu & Fuller, there is no clear reason to omit nitrous oxide from laparoscopic surgeries (contrary to the popular belief that nitrous will expand the gases used to inflate the peritoneum).

Causes myocardial depression in the presence of heart failure, especially when administered with opioids.

Reduces cerebral vascular tone and uncouples CBF and CMRO2.  CBF and CMRO2 are both elevated, but CMRO2 rises disproportionately more than CBF.

Causes decreased renal blood flow secondary to increased renal vascular resistance

Irreversibly inhibits vitamin B12.  B12 inhibits methionine synthase – an enzyme needed for myelin production and folate metabolism.  Below lists risks associated (which are of particular note in patients with pernicious anemia, vegans, and hx of alcohol abuse):

• teratogenicity – there is very little data supporting this
• megaloblastic anemia via bone marrow suppression
• possibility of spontaneous abortion – for this reason many providers will wait until the third trimester before using nitrous
• homocysteine build-up
• Immunocompromise

OB patients: Nitrous oxide is gaining traction as an analgesic alternative during labor. The common prescription is a 50/50 mix of oxygen and Nitrous. The patient self-administers via face-mask. When combined with opioids, this mixture is NOT associated with hypoxia, loss of airway reflexes, or unconsciousness. The biggest problem with this is the levels of free nitrous oxide to healthcare workers is most often in excess of occupational exposure limits. Nitrous is generally avoided in parturients during the first two trimesters. There is lacking evidence in human trials (because human trials are not performed), but in animals it has been shown to cause birth defects.

Pediatric Patients – when combined with sevoflurane, nitrous markedly reduces induction time, but also reduces the preoxygenation reserve. This is generally NOT a big concern, but should be tailored to the patient. Chu and Fuller recommend induction using this combination.

Metabolism

Excreted unchanged from the lungs