Isoflurane Therapy for Status Asthmaticus in Children and Adults: DISCUSSION

Status Asthmaticus

Isoflurane produces bronchodilation through (3-ad- renergic receptor stimulation, direct relaxation of bronchial smooth muscle, antagonism of the action of acetylcholine and histamine, and interference with hypocapnic bronchoconstriction. Thus, a patient who is already receiving maximum doses of standard bron- chodilators may show an additional response. As our case reports suggest, isoflurane acts rapidly and may be lifesaving while high-dose corticosteroids take effect. In contrast, ketamine, an intravenous anes­thetic agent that has also been used in asthma, acts by adrenergic stimulation. Little response is seen in patients receiving large doses of P-agonists and theo­phylline.

There are several advantages of isoflurane over other inhalational anesthetic agents. Historically, diethyl ether and cyclopropane were used, but their extreme flammability precluded their use in electrically active environments. Isoflurane is the least fat soluble of the anesthetic vapors and has the lowest blood gas solu­bility coefficient. Consequently, depth of anesthesia can be most rapidly adjusted with isoflurane, and time to recovery of consciousness is short, despite pro­longed use.

All halogenated vapors are proarrhythmic, espe­cially in the presence of adrenergic stimulants. Nev­ertheless, isoflurane is the least likely to cause arrhyth­mias. In dogs, at least four times the dose of epinephrine is needed to produce ventricular extra- systoles in the presence of isoflurane as compared with halothane (in equi-anesthetic doses). revatio 20 mg

Halogenated hydrocarbons have been implicated in severe hepatic and renal toxicity. While halothane and enflurane are not directly toxic, their metabolic prod­ucts appear to be responsible for a rare form of hepatic necrosis. Enflurane is also responsible for fluoride- induced renal failure. Isoflurane undergoes minimal metabolism (<0.2 percent), and to our knowledge, there are no documented cases of isoflurane-induced hepatic or renal injury.

Isoflurane causes dose-dependent hypotension by peripheral vasodilation. Cardiac output is unchanged, while heart rate increases by 5 to 20 percent as the drug concentration is increased to 2 percent. Hypo­tension is usually responsive to plasma volume expan­sion, but occasionally vasopressors are needed. All patients in our series responded to volume expansion.

Isoflurane causes amnesia in low concentrations and general anesthesia as the concentration is increased. Above 1.5 percent, other sedatives and muscle relax­ants are rarely needed. Narcotics ought to be given for painful procedures, since isoflurane is a poor analgesic. If long-acting sedatives are avoided, as in our patients, awakening is rapid when isoflurane treatment is discontinued.

Until recently, it was very difficult to administer isoflurane in the ICU. Standard anesthetic ventilators do not achieve sufficiently high inspiratory pressures, nor do they have the necessary alarms and safety features for prolonged use outside the operating room. A standard ICU ventilator, (Siemens 900C), can be fitted with an isoflurane vaporizer. This ventilator generates inspiratory pressures up to 120 cm H20 and provides essential alarms. canadian discount pharmacy

Scavenging exhaust gases in the ICU is necessary for preventing contamination with waste gases that can cause sedation. There is also concern about the effects of long-term anesthetic exposure for health care personnel.

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