General anesthesia is a state of unconsciousness in which pharmacological agents produce hypnosis, amnesia, and analgesia. Other endpoints met during most general anesthetics include muscle relaxation, immobility, and attenuation of sympathetic and somatic reflexes. The induction of general anesthesia is achieved by either intravenous or inhalation routes. The “maintenance” phase begins when the amnestic patient is not only unconscious, but also unable to produce movements in response to surgery. At this point, there are several techniques available for the anesthesiologist to maintain general anesthesia during a given operation or procedure.
TOTAL INHALATION ANESTHESIA
This technique involves the sole administration of potent volatile agents such as sevoflurane to maintain general anesthesia. Advantages of this approach include the ability to maintain spontaneous ventilation and satisfactory blunting of sympathetic responses to noxious stimulation. Modern inhalation agents are easier to titrate to the patient’s blood pressure, pulse, minute ventilation, and movements. The major disadvantage of this technique is significant dose-dependent cardiovascular depression. In addition, volatile anesthetics do not provide any degree of analgesia. This approach is most amenable for short procedures for which intraoperative and postoperative pain is expected to be minimal, such as myringotomy, cystoscopy, and examinations under anesthesia.
TOTAL INTRAVENOUS ANESTHESIA
The technique of “total intravenous anesthesia” (TIVA) can be used for the complete maintenance of general anesthesia or for the administration of deep sedation. TIVA utilizes continuous infusions or repeated doses of a short-acting sedative-hypnotic drug. Opioids, either in bolus form or through an infusion, are often added for these procedures that may produce more than minimal stimulation.
There are several advantages to TIVA:
Decreased incidence of postoperative nausea and vomiting.
Rapid induction and easy titration.
Rapid emergence even after long infusions due to favorable context-sensitive half-times.
No risk of malignant hyperthermia.
Minimal suppression of neurophysiologic-evoked potentials.
Avoidance of occupational exposure or environmental pollution by volatile agents.
No need for gas delivery or scavenging systems.
No expansion of gas cavities.
May reduce intracranial pressure (propofol).
TIVA is used quite extensively for deep sedation and maintenance in ambulatory surgery. It is a simple technique that leads to rapid and clear emergence with minimal postoperative nausea and vomiting. TIVA is especially useful for maintaining general anesthesia in patients for whom delivery of inhalation anesthetics may be compromised or difficult. For example, pulmonary diseases that impair ventilation and perfusion to the lung can lead to inconsistent drug uptake. TIVA allows for a much more rapid onset of action that does not depend on the adequacy of alveolar ventilation. TIVA is also suitable for operations in which ventilation is interrupted, such as laser airway surgery or bronchoscopy.
There are several disadvantages to TIVA for maintenance or deep sedation:
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