Anaesthesia nitrous oxide

Q72 In anaesthesia, nitrous oxide may be used up to a concentration of 66% in oxygen. Nitrous oxide is unsatisfactory as a sole anaesthetic. 

Dosage and administration. For the maintenance of anaesthesia, nitrous oxide must always be mixed with at least 30% oxygen. For analgesia, a concentration of 50% nitrous oxide with 50% oxygen usually suffices. 

Adverse effects. The incidence of nausea and vomiting increases with the duration of anaesthesia. Nitrous oxide interferes with the synthesis of methionine, deoxythymidine and DNA. Exposure of to nitrous oxide for more than 4 hours can cause megaloblastic changes in the bone marrow. Because prolonged and repeated exposure of staff as well as of patients may be associated with bone-marrow depression and teratogenic risk, scavenging systems are used to minimise ambient concentrations in operating theatres. 

Nitrous oxide has a low potency and must be used in combination with other inhalation anaesthetics for general anaesthesia. Nitrous oxide provides rapid induction and recovery. It also has an analgesic action and is used as a 50% mixture with oxygen to provide analgesia without loss of consciousness during labour and manipulations of injured body parts. 

Two methods of anaesthesia are currently in use, the application of inhaled gaseous or volatile anaesthetics such as halothane, sevoflurane and isoflurane to maintain a level of anaesthesia. Older compounds in this category include nitrous oxide and chloroform. 

Nitrous oxide is used for the maintenance of anaesthesia as part of a combination of drugs because, owing to its lack of potency, it cannot be used... 

Fig. 15. Relationship between the alfentanil plasma concentrations and the probability of needing naloxone to restore adequate spontaneous ventilation. The diagram at the upper part shows the alfentanil plasma concentrations of the patients who required naloxone (upward deflection) or did not require naloxone (downward deflection). The plasma concentration-effect curve for this clinical endpoint (lower part) was defined from the quantal data shown in the upper diagram using logistic regression. Bars indicate SE of C5o%. (From Ausems ME, Hug CC, Stanski DR, Burm AGE. Plasma concentrations of alfentanil required to supplement nitrous oxide anaesthesia for general surgery. Anaesthesiology 1986 65 362-73, reproduced by permission.)...

Nitrous oxide is used for induction and maintenance of anaesthesia. It is widely used as carrier gas for other volatile agents in general anaesthesia. The usual concentra-... 

It may be used to produce light sedation and amnesia for unpleasant procedures by intravenous injection. It has also been used for induction and to supplement nitrous oxide anaesthesia. The benzodiazepines are further discussed in chapter Sedative and hypnotics in detail. 

Sevoflurane has a dose-dependent effect on cerebral blood flow and intracranial pressure cerebral autoregulation is preserved (this is not the case with isoflurane). During hypocarbia, in the absence of nitrous oxide, 1 MAC does not increase intracranial pressure (ICP). It reduces the cerebral metabolic rate for oxygen (CMR02) by approximately 50% at concentrations approaching 2 MAC. This is similar to the reduction observed during isoflurane anaesthesia. 

For inhalational induction of anaesthesia in children, 6% sevoflurane in 50% nitrous oxide and oxygen is probably optimal. However, some anaesthetists consider it to be inferior to halothane for the management of the irritable or constricted airway and for anaesthesia for bronchoscopy. Sevoflurane is preferred for dental procedures as there is a lower risk of cardiac arrhythmias than with halothane, especially in children. In children with congenital heart disease, whereas the cardiac index is reduced by halothane it is preserved with sevoflurane. In adults, 8% sevoflurane is well tolerated and, provides rapid induction of anaesthesia without adversely affecting haemodynamic stability. 

Nitrous oxide has both a direct depressant and sympthomimetic effect on the myocardium. In healthy patients these tend to counterbalance each other, the resultant effect being minimal cardiovascular depression. In patients with car-diovascular disease or who are taking conconcurrent medication with, e.g. 3 blockers, its depressant effect may be more obvious. Nitrous oxide supplementation of high-dose opioid-based anaesthesia may result in a reduction in cardiac output and heart rate although the mechanism of this is unclear. Nitrous oxide may have a venoconstrictor effect resulting in increased pulmonary vascular resistance, particularly in the presence of pulmonary hypertension. 

Diffusion hypoxia describes the rapid movement of nitrous oxide from the blood into the alveoli in the early recovery phase. It is usual to circumvent this problem by administering 100% oxygen for a few minutes at the end of anaesthesia. 

Nitrous oxide is approximately 34 times more soluble in blood than nitrogen. It will diffuse into, and from, air-containing cavities more rapidly than nitrogen. Thus, during nitrous oxide anaesthesia, air-or gas-filled cavities will tend to expand with the risk of rupture and pneumothorax, e.g. lung cyst, bullae. Similarly, chronic inflammation in the middle ear may result in blockage of the Eustachian tube. In these circumstances, nitrous oxide may induce barotrauma and pain. 

Rapidity of recovery has been one of the most consistent and compelling features of anaesthesia with xenon. After 2 hours of xenon anaesthesia recovery is two to three times as fast as recovery from equi-MAC mixtures of N20/sevoflurane and N20/isoflurane. Marked emetic effects after both nitrous oxide and xenon were reported in a volunteer study, but this was conducted under highly artificial conditions. 

The best anaesthetic vapour was found to be a 1 2 3 mixture of alcohol, chloroform, and ether, known as ACE. Despite being widely used, it was always known that ACE presented a risk, and yet it was the anaesthetic of choice for more than 100 years. Ether caused serious fires and explosions in operating theatres and while the risk of this was small - about one serious incident every 100,000 operations - it was alarming when it happened. Chloroform was not a fire risk but it could be deadly to some patients, killing them within minutes in certain tragic cases, and seriously damaging the liver of others. (Nitrous oxide was less risky, and continues to be used even today, but it does not produce deep anaesthesia.)... 

Lantos J., Temes G., andTorokB. (1986) Changes during ischaemia in extracellular potassium ion concentration of the brain under nitrous oxide or hexobarbital-sodium anaesthesia and moderate hypothermia. Acta Physiol. Hung. 67,141-153. 

METHOTREXATE ANAESTHETICS-NITROUS OXIDE t antifolate effect of methotrexate t toxicity of methotrexate Nitrous oxide is usually used for relatively brief durations when patients are anaesthetized, and hence this risk during anaesthesia is minimal. However, nitrous oxide may be used for analgesia for longer durations, and this should be avoided... 

Newaz MA, Yousefipour Z, Nawa NN (2005) Modulation of nitric oxide synthase activity in brain, liver, and blood vessels of spontaneously hypertensive rats by ascorbic acid protection from free radical injury. Qin Exp Hypertens 27(6) 497-508 Nunn JF, Chanarin I, Taimer AG, Owen ER (1986) Megaloblastic bone marrow changes after repeated nitrous oxide anaesthesia. Reversal with fofinicacid. Br J Anaesth 58(12) 1469-1470 Parker WD Jr, Hass R, Stumpf DA, Parks J, Eguren LA, Jackson C (1984) Brain mitochondrial metabolism in experimental thiamine deficiency. Neurology 34(11) 1477-1481... 

Fyman R4, Triner L, Scfaranz H, Hartung J, Casthely PA, Abrams LM, Keaney AE, Cottrell IE. Effect of volatile anaesthetics and nitrous oxide-fentanyl anaesthesia oa bleeding time. Br J Anaesih 1984 S6 1197-200... 

Horace Wells, a dentist in Hartford, Connecticut, introduced nitrous oxide to produce anaesthesia during dental extraction. 

Agents that have low solubility in blood, i.e., a low blood/gas partition coefficient (nitrous oxide, sevoflurane), provide a rapid induction of anaesthesia because the blood reservoir is small and agent is available to pass into the brain sooner. 

Nitrous oxide (1844) is a gas with a slightly sweetish smell. It is neither flammable nor explosive. It produces light anaesthesia without demonstrably depressing the respiratory or vasomotor centre provided that normal oxygen tension is maintained. 

Disadvantages. Nitrous oxide is expensive to buy and to transport. It must be used in conjuction with more potent anaesthetics to produce full surgical anaesthesia. 

Uses. Nitrous oxide is used to maintain surgical anaesthesia in combination with other anaesthetic agents, e.g., isoflurane or propofol, and, if required, muscle relaxants. Entonox provides analgesia for obstetric practice, for emergency management of injuries, and during postoperative physiotherapy. 

Rupp SM, Fahey MR, Miller RD. Neuromuscular and cardiovascular effects of atracurium during nitrous oxide-fentanyl and nitrous oxide-isoflurane anaesthesia. Br J Anaesth 1983 55(Suppl l) S67-70. 

Gray WM. Occupational exposure to nitrous oxide in four hospitals. Anaesthesia 1989 44(6) 511-14. 

Crawford MW, Lerman J, Sloan MH, Sikich N, Halpern L, Bissonnette B. Recovery characteristics of propofol anaesthesia, with and without nitrous oxide a comparison with halothane/nitrous oxide anaesthesia in children. Paediatr Anaesth 1998 8(l) 49-54. 

Nelskyla K, Korttila K, Yli-Hankala A. Comparison of sevoflurane-nitrous oxide and propofol-alfentanil-nitrous oxide anaesthesia for minor gynaecological surgery. Br J Anaesth 1999 83(4) 576-9. 

Peric M, Vranes Z, Marusic M. Immunological disturbances in anaesthetic personnel chronically exposed to high occupational concentrations of nitrous oxide and halothane. Anaesthesia 1991 46(7) 531-7. 

Murat I, Beydon L, Chaussain M, Levy J, Saint-Maurice JP. Ventilatory changes during nitrous oxide isoflurane anaesthesia in children. Eur J Anaesthesiol 1986 3(5) 403-11. 

Choi WW, Mehta MP, Murray DJ, Sokoll MD, Forbes RB, Gergis SD, Abou-Donia M, Kirchner J. Neuromuscular and cardiovascular effects of mivacurium chloride in surgical patients receiving nitrous oxide-narcotic or nitrous oxide-isoflurane anaesthesia. Can J Anaesth 1989 36(6) 641-50. 

From RP, Pearson KS, Choi WW, Abou-Donia M, Sokoll MD. Neuromuscular and cardiovascular effects of mivacurium chloride (BW B1090U) during nitrous oxide-fentanyl-thiopentone and nitrous oxide-halothane anaesthesia. Br J Anaesth 1990 64(2) 193-8. 

Nestor PJ, Stark RJ. Vitamin B12 myelonenropathy precipitated by nitrous oxide anaesthesia. Med J Anst 1996 165(3) 174. 

Yang YF, Herbert L, Ruschen H, Cooling RJ. Nitrous oxide anaesthesia in the presence of intraocular gas can cause irreversible blindness. BMJ 2002 325(7363) 532-3. 

Crawford JS, Lewis M. Nitrous oxide in early human pregnancy. Anaesthesia 1986 41(9) 900-5. 

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