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In response
- Nolan J McDonnell (12 October 2009)
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Cardiopulmonary arrest in pregnancy
- Michael F James (2 October 2009)
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Nolan J McDonnell, Staff Specialist Department of Anaesthesia and Pain Medicine, Kind Edward Memorial Hospital for Women
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We would like to thank Dr James for his expert comments in regards to this case report. Firstly, to address his comment in regards to the underlying cause of the arrest, we would hypothesise that the underlying pathophysiology was a combination of respiratory and cardiovascular affects of the acute magnesium toxicity. I am of the personal opinion that the loss of cardiac output (as demonstrated by the lack of a palpable pulse and a bloodless surgical field) was likely secondary to marked peripheral vasodilatation from the magenesium, in combination with some direct myocardial depression and the dramatic response that was associated with the delivery of the neonate by perimortem caesarean would support this. Secondly, we are in agreement in regards to the inherent risks associated with magnesium infusion, especially when it is administered from an infusion bag that contains large quantities of magnesium. After a second recent adverse event at this institution, which is currently accepted but awaiting publication in the literature, we took definitive steps to avoid a potential future occurence. We examined a number of different options that included administration via a syringe driver, but were reluctant to implement a hospital wide change in both the presentation and delivery system for our magnesium infusions. Hence, after discussions with the manufacturer of our 8% solutions, we now have this solution provided in 100 ml bags rather than a 500 ml bag. This reduces the total dose that could be administered in the event of any error to 8 grams, as opposed to 40 grams which could be administered with the 500 ml bags. We accept that other institutions may prefer to switch to administration via a syringe driver. We would advise that whatever system is utilised, it must be ensured that the magnesium solution itself does not contain a large amount of magnesium, as programming and device errors may still occur no matter what system is used for administration and also that calcium is readily availably in locations where magnesium is administered and all staff are familiar with the management of acute magnesium toxicity. Conflict of Interest:None declared |
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Michael F James
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Dr McDonnell is to be congratulated on the excellent management and successful outcome of the two cases of perimortem caesarean delivery described in his report (Br J Anaesth 103(3): 406–9 (2009)). The second of these cases relates to magnesium toxicity and contains some important messages. The serum magnesium concentration of 10.1 mmol L-1 is the highest blood concentration of magnesium ever reported in a human subject and it is interesting that this patient and her baby made a full recovery. The most likely cause of the cardiac arrest seems to be hypoxic as a consequence of the respiratory paralysis that would be inevitable with this concentration of magnesium in the plasma, but the possibility of direct myocardial depression remains. It is a pity that the clinicians could not decide whether this was a primary respiratory or cardiac arrest as this information would have significantly extended our knowledge of the safety of magnesium infusions. The return of palpable pulses within one minute of the caesarean delivery suggests that the arrest was probably respiratory, as the plasma concentration would not have fallen significantly during this brief interval before the restoration of the circulation. Nevertheless, this report of such an exceptionally high magnesium concentration emphasises the inherent cardiovascular safety of this drug. However, despite this large cardiovascular safety margin, magnesium sulphate remains a potentially lethal drug because of its ability to produce neuromuscular blockade. The delivery of large quantities of magnesium from an intravenous infusion bag is potentially extremely dangerous, as this case report illustrates. All of the recent reports of magnesium toxicity have related to exactly this type of administration, where the electronic infusion device intended to be used was either not connected or malfunctioned. In my view, continuous infusions of magnesium should be administered from a syringe pump with appropriate precautions against overdosage, and not from an infusion bag, whatever instructions and precautions are in place to minimise the risk of inadvertent overdose. As magnesium sulphate is currently the drug of choice for the control of eclamptic convulsions, I strongly recommend that intravenous infusions of this drug administered in a safe, controlled fashion and not through an intravenous infusion bag. Conflict of Interest:None declared |
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