Magnesium inhibits the release of acetylcholine from the motor nerve terminal and thus potentiates the action of the non-depolarizing neuromuscular blocking drugs. We have examined the possibility that this effect might enhance the speed of onset of non-depolarizing block with pancuronium. Following the administration of pancuronium 100 micrograms kg-1, 95% depression of thumb twitch occurred in 68.3 (SD 25.9) s in magnesium-pretreated subjects and in 73.7 (19.5) s in a group given a priming dose (10 micrograms kg-1) of pancuronium. Tracheal intubation was performed after 97.8 (22.5) s in the magnesium group and in 121.0 (37.5) s in the control group (ns). It is concluded that pretreatment with magnesium does not usefully increase the speed of onset of action of pancuronium.

The anaesthetic management of ~'o patients with phaeochromocylomo compticating pregnancy is presentedThe perioperative management of a patient with a phaeochromocytoma requires adequate adrenergic blockade and the minimizing of stimuli that may provoke cateeholamlne release. The pregnant patient with a phaeochromocytoma poses additional problems, in that anaesthetic techniques advocated for the non-pregnant patient may be hazardous to the fetus. Magnesium (Mg) has been shown to inhibit the release of catecholamine~ from both the adrenal medulla ~,z and peripheral adrenergie nerve terminals) It also blocks catecholamine receptors Key wordstONS: magnesium; SURGERY; phaeochromocytoma; PREGNANCY; hypertension, delivery; COMPLICATIONS; hypertension. directly in addition to a direct dilator effect on vessel walls. 4 Its successful use in phaeochromocytoma has been previously described, s and the agent has particular appeal in the obstetric field as its actions in pregnancy are well known. We present two cases in whom MgSO~ was used in ~onjunction with other agents to control cardiovascular disturbances during delivery. In one case the delivery and tumour excision were handled on separate occasions, allowing for a comparison of the pregnant and non-pregnant state. Case reports Case 1A 26-year-old pregnant woman presented with unstable hypertension at 32 weeks gestation. The presence of a phaeochmmocytoma was demonstrated by a raised urinary VMA excretion and a markedly elevated plasma norepinephrine level of 1166 pg.m1-1. Over the next five days, blood pressure was stabilized in (he range 140/80-150/100 mmHg from an admission level of 180/120 mmHg with oral prazosin 4 mg twice daily and atenolol 50 mg daily. No significant changes occurred in haematocrit or electrolyte levels during (his period. Onec fetal maturity was confirmed it was decided to attempt vaginal delivery under epidural anaesthesia Insertion of monitoring lines and induction of labour provoked severe cardiovascular instability (BP 220/I I0 mmHg) and fetal distress which made immediate operative delivery imperative.In view of the failure of the epidural anaesthetic to control symptoms, general anaesthesia was chosen. Prior to induction, oral antacids were given and a 4 g bolus of MgSO4 was administered IV over 15 minutes, followed by a continuous intusion of 1.5 g'hr -1. This produced little improvement in the control of blood pressure. Blood pressure was reduced to 105/60 mmHg with an infusion CAN I ANAESTH 1988 / 35:2 fpp 178-82

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