Isoflurane has a slight pungent ethereal odour, which may limit the rate of gas induction but, despite this, induction and particularly recovery are rapid.
The use of isoflurane-specific vaporisers will facilitate accurate control of the administered concentration of anaesthetic.
The MAC (Minimum Alveolar Concentration) the standard measure of potency for anaesthetics, is 1.15% in pure oxygen, decreasing to 0.5% when given with 70% nitrous oxide, for middle-aged humans. There is an age-relationship: the MAC is significantly higher in children and is lower in the elderly: see table.
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Premedication drugs should be selected according to the needs of the patient. The ventilatory depressant effect of isoflurane should be taken into account. Anticholinergic drugs (eg atropine, glycopyrrolate USP) may be used for their effects in drying oral secretions (antisialogogue) at the discretion of the anaesthetist, but they may enhance the weak effects of isoflurane in increasing heart rate.
As isoflurane has a mild pungency, inhalation should usually be preceded by the use of a short acting barbiturate, or other intravenous induction agent, to prevent coughing. Salivation and coughing may be troublesome in small children induced with isoflurane. Alternatively, isoflurane with oxygen or with an oxygen/nitrous oxide mixture may be administered. It is recommended that induction with isoflurane be initiated at a concentration of 0.5%. Concentrations of 1.5-3.0% usually produce surgical anaesthesia in 7-10 minutes. Blood pressure decreases during induction but this may be compensated by surgical stimulation.
Adequate anaesthesia for surgery may be sustained with an inspired isoflurane concentration of 1.0% to 2.5% in an oxygen/70% nitrous oxide mixture. Additional inspired isoflurane (0.5% to 1.0%) will be required with lower nitrous oxide levels, or when isoflurane is given with oxygen alone or with air/oxygen mixtures. Blood pressure decreases during maintenance anaesthesia in relation to the depth of anaesthesia. That is, blood pressure is inversely related to the isoflurane concentration. Provided there are no other complicating factors this is probably due to peripheral vasodilation. Cardiac rhythm remains stable. Excessive falls in blood pressure may be due to the depth of anaesthesia and in such circumstances can be corrected by reducing the inspired isoflurane concentration.
Induced hypotension can be achieved by artificially ventilating patients with isoflurane 2.5-4.0%. Pre-treatment with Clonidine significantly decreases the isoflurane requirement for maintaining induced hypotension.
The concentration of isoflurane can be reduced to 0.5% at the start of closing the operation wound, and then to 0% at the end of surgery, provided that the anaesthetist is satisfied that the effect of any neuromuscular blocking drugs has been reversed and the patient is no longer paralysed. After discontinuation of all anaesthetics, the airways of the patient should be ventilated several times with oxygen 100% until complete recovery.