Nitrous Oxide An Inhalational

N2O (commonly called laughing gas) produces its anesthetic effect without decreasing blood pressure or cardiac output. Although it directly depresses the myocardium, cardiac depression is offset by an N2O-mediated sympathetic stimulation. Likewise, respiration is maintained. Tidal volume falls, but minute ventilation is supported by a centrally mediated increase in respiratory rate. However, since the respiratory depressant effect of N2O are synergistic with drugs such as the opi-

oids and benzodiazepines, N2O should not be considered benign.

Deep levels of anesthesia are unattainable, even when using the highest practical concentrations of N2O (N2O 60-80% with oxygen 40-20%). Although unconsciousness occurs at these inspired levels, patients exhibit signs of CNS excitation, such as physical struggling and vomiting. If the airway is unprotected, vomiting may lead to aspiration pneumonitis, since the protective reflexes of the airway are depressed.

On the other hand, lower inspired concentrations (25-40%) of N2O produce CNS depression without excitatory phenomena and are more safely used clinically. CNS properties of low inspired tension of N2O include periods of waxing and waning consciousness, amnesia, and extraordinarily effective analgesia. N2O 25% produces the gas's maximum analgesic effect. With this concentration, responses to painful surgical manipulations are blocked as effectively as they would be with a therapeutic dose of morphine. Such low inspired concentrations of N2O are used in dentistry and occasionally for selected painful surgical procedures (i.e., to relieve the pain of labor). Since the tissue solubility of N2O is low, the CNS effects are rapid in onset, and recovery is prompt when the patient is returned to room air or oxygen.

The most common use of N2O is in combination with the more potent volatile anesthetics. It decreases the dosage requirement for the other anesthetics, thus lowering their cardiovascular and respiratory toxicities. For example, an appropriate anesthetic maintenance tension for N2O and halothane would be N2O 40% and halothane 0.5%. With this combination in a healthy patient, anesthesia is adequate for major surgery, and the dose-dependent cardiac effects of halothane are reduced.

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