Use of magnesium in moderating tachycardia in acute severe asthma in pregnancy

Editor—We would like to support the observation that magnesium may be used to reduce the tachycardia associated with beta-1 adrenoreceptor agonism from i.v. boluses of salbutamol as reported in a section of your asthma review¹ by reporting a case of a 20 week gestation woman suffering life-threatening acute asthma.

A chronic asthmatic 22 yr primiparous female of 20 week gestation presented to hospital with breathlessness. Her asthma was controlled by inhaled salbutamol and beclomethasone. She had not been hospitalized before. Examination revealed scattered bilateral respiratory wheeze, and the oxygen saturation on 4 litre min⁻¹ flow of oxygen was 86%. After initial assessment, sequential nebulized 5 mg salbutamol was administered with i.v. steroids, but she failed to improve clinically. She was unable to complete sentences and complained of feeling tired. A decision was made to administer i.v. salbutamol, but this was limited to only a 60 μg (1 μg kg⁻¹) bolus by a sinus tachycardia of 160–170 bpm associated with chest pain and ST depression on a 12-lead ECG. The call for anaesthetic assistance had been for intubation and ventilation of the patient. However, 2 g of i.v. magnesium was administered over 10 min, with resolution of the chest pain, ST depression and the heart rate fell to 110–120 bpm. This allowed completion of the administration of a full dose of 5 μg kg⁻¹ i.v. salbutamol and this was followed by a salbutamol infusion. There was a marked improvement in her breathlessness, and she was soon able to complete full sentences. Intubation and ventilation in an asthmatic, late second trimester parturient with all the attendant risks was avoided. The patient improved rapidly, and was discharged home after 2 days in a general medical ward. Her pregnancy continued uneventfully.

Magnesium sulphate has been used to treat the tachycardia and haemodynamic instability associated with phaechromocytoma.² Previous publications^3,4,5 have reported the use of magnesium sulphate to avoid the tachycardia associated with beta-2 adrenoreceptor agonism used to treat acute asthma. To our knowledge, this is the first time that magnesium has been reported for this use in a pregnant woman. Magnesium clearly has multiple desirable effects in this situation, such as tocolysis, and allowing the administration of i.v. beta-1 agonists with additional tocolytic properties of their own. This case supports the extension of the use of magnesium to the pregnant woman with severe or life-threatening acute severe asthma for the purpose of administering i.v. salbutamol.

Declaration of interest

None declared.