INHALE OR INJECT?
How volatile and intravenous anesthetics compare
The practice of anesthesiology has grown by leaps and bounds in the last 60 years, spurred by developments in technology and pharmacology. The science of inducing anesthesia in people undergoing surgery is no longer limited to administering drugs for hypnosis, analgesia, and amnesia. The process has been elevated to an art form, providing quality anesthesia at the least cost while maximizing safety and tolerability.
Recent studies have compared the newer inhalationals and injectables, both of which are characterized by efficacy with fast onset and recovery, but each with unique benefits, costs, and idiosyncracies.
On one side of the divide are the new and improved intravenous agents bolstered by the development of propofol. The other end of the spectrum is also cutting edge, with the versatility and ease of administration provided by the volatile agent sevoflurane. Both were shown to compare favorably in terms of rapid induction, recovery, and tolerability. The kinetic profiles also allow for a more accurate dosing and maintenance concentration using hemodynamic variables.
Where they part ways
The use of total intravenous anesthesia (TIVA) may be more appropriate for certain procedures, like laryngoscopy or bronchoscopy, where use of inhalational anesthesia is challenging although not contraindicated (Milligan et al., 1990). Also, the protective reflex for hypoxic pulmonary vasoconstriction may be affected adversely by volatile agents, especially among patients possibly afflicted with malignant hyperpyrexia. But procedures like abdominal surgeries, where adequate control of the airway is necessary, may require intubation and use of volatile anesthetics instead of TIVA.
Advocates of intravenous agents, however, have expressed some concern over possible pollution in the operating environment associated with volatile agents (Halsey, 1991). Many volatile anesthetics are metabolized to flouride ions and trifluoroacetic acid, which were reported to exhibit cross-reactivity. Other studies showed increased production of flouride ions after the administration of sevoflurane and after prolonged use of isoflurane (Breheny, 1992). Therefore, a separate anesthetic-gas scavenging system must always supplement existing operating room ventilation (i.e., laminar air system) to keep the air clean.
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Postoperative nausea and vomiting (PONV) are also reported more frequently after volatile anesthesia compared with TIVA (Raftery and Sherry, 1992). Studies have noted direct antiemetic properties for propofol even at subhypnotic doses, thus the low incidence for nausea (Borgeat, 1992). Inhalational enthusiasts, however, contend that although PONV is an unpleasant experience for the patient, which may require drug therapy for some and even unplanned admission for a few, any dissatisfaction with inhalation anesthesia does not appear to correlate with the occurrence of PONV (Smith, 2000). And while propofol induction was faster than sevoflurane, it was associated with an increased incidence of oxygen desaturation and respiratory depression.
Problems with TIVA, on the other hand, involve its delivery. While propofol has a favorable pharmacokinetic profile, its maintenance requires a different technique compared with the volatile anesthetics delivered via calibrated vaporizers. The latter is known to maintain a more constant blood concentration of the inhalational, able to respond more accurately and promptly to changing surgical and anesthetic requirements.
Several manual infusion schemes have been developed for propofol to meet this need. Target-controlled infusion delivers variable infusion based on a pharmacokinetic model that describes the elimination and redistribution of the drug. The predicted blood concentration of the drug can be altered in a manner to suit varying levels of surgical stimulation and individual patient requirements. However, no single blood concentration of a TIVA agent will result in satisfactory anesthesia for all patients and all surgical conditions.
Sevoflurane v. propofol
Anesthesiologists Lydia Egay and Merle Odi of the University of the Philippines-Philippine General Hospital recently reviewed studies on sevoflurane and propofol to assess the advantages and disadvantages of each.
They noted that in terms of tolerability, induction was free of complications in 77 to 80 percent of patients for both anesthetics. When induction was made in incremental doses, there were a few airway-related side effects with sevoflurane. On the other hand, there were more non-airway-related side effects (hypotension, hypertension, hypoxia, injection-site pain) with propofol (20 percent as against 14 percent).1
Egay noted that sevoflurane has gained acceptance not only because of its low blood solubility, providing for rapid induction and recovery, but also its nonirritability to the airway, permitting ease of induction. Nonetheless, propofol induction and time-to-tracheal intubation was faster than sevoflurane.
To speed up inhalation induction and lessen side effects, Egay and Odi suggest:
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Using vital-capacity induction with 8% sevoflurane.
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Using nitrous oxide to tap the benefits of the "second-gas effect."2
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Giving Fentanyl 1 mcg/kg and midazolam 1-2 mg IV prior to sevoflurane induction.
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Giving children oral premedication of 0.5 mg/kg midazolam with 10 mg/kg paracetamol syrup 20 to 30 minutes before induction.
On emergence and recovery, however, sevoflurane patients would wake up faster than propofol from the time the anesthetic is discontinued. Odi said quick recovery from anesthesia is particularly crucial in the outpatient setting. To offset and lessen the incidence of PONV, she suggests using steroids intraoperatively, maintaining low concentrations of sevoflurane, and using propofol 1 mg/kg to reduce excitement and agitation on emergence.
Roger Badillo II, MD
References
1. Jellish et al., Anesth Analg, March 1996.
2. Philip BK, et al., Anesth Analg 1999; 69: 623-627
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