Hospitals often have their own policies, however the BTS Guidelines list magnesium before aminophylline now. In order, the suggested treatment of acute severe asthma in adults is:
High-flow O2
Beta-2 agonists
Steroids
Ipratropium bromide
Magnesium
Aminophylline
However, a lot of how people are treated often depends on their own protocol or what has/hasn't worked for them in the past.
HTH.
In terms of whether one is any better than the other, the evidence for the use of either in an acute exacerbation is quite limited. Studies of emergency treatments are always quite difficult to do. There is slightly more evidence that magnesium is effective than for aminophylline, which is why it's now listed first in the BTS guidelines.
The HDU consultant at my local always says that there is little evidence that magnesium is helpful unless your CO2 is rising, which would be in the end stages of a severe attack for most people. A&E still seem to use it in most people who are not responding to nebs, etc, though.
Personally, I usually get IV aminophylline and magnesium simultaneously, if they are able to get two cannulas into me. If not, I tend to have IV aminophylline for a while before stopping it for 20 minutes to give the magnesium, because time is often of the essense and aminophylline has always worked better for me in the past. As Cathbear says, it is often very much dependent on what your protocol is and what has worked for you in the past.
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