Nonhalogenated Inhalational Anesthetics
In contemporary surgical settings, the only useful non-halogenated inhalational anesthetic is N2O. Earlier agents, ether and cyclopropane, have fallen out of favor, since they present a serious safety hazard due to their flammability and explosiveness. They remain interesting from a historical point of view, since they were among the first developed.
N2O (commonly called laughing gas) produces its anes-thetic effect without decreasing blood pressure or cardiac output. Although it directly depresses the myo-cardium, 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 respi-ratory rate. However, since the respiratory depressant effect of N2O are synergistic with drugs such as the opi-oids and benzodiazepines, N2O should not be consid-ered benign.
Deep levels of anesthesia are unattainable, even when using the highest practical concentrations of N2O (N2O 60–80% with oxygen 40–20%). Although uncon-sciousness occurs at these inspired levels, patients ex-hibit 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 exci-tatory 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% pro-duces the gas’s maximum analgesic effect. With this con-centration, responses to painful surgical manipulations are blocked as effectively as they would be with a thera-peutic dose of morphine. Such low inspired concentra-tions of N2O are used in dentistry and occasionally for se-lected 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 lower-ing their cardiovascular and respiratory toxicities. For ex-ample, an appropriate anesthetic maintenance tension for N2O and halothane would be N2O 40% and halothane 0.5%. With this combination in a healthy pa-tient, anesthesia is adequate for major surgery, and the dose-dependent cardiac effects of halothane are reduced.