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Weighing your options

The problem with critical care is that the usual standards and guidance have a tendency to disappear out of the window. Take weights. All patients should be weighed on admission to hospital, which is fine for elective patients and those who are reasonably fit. But this is impracticable for many patients we admit, such as those undergoing resuscitation.

This is the situation we found ourselves in the other day with a 64-year-old man who had collapsed with myocardial infarction. He had subsequently been admitted to the intensive care unit and, unsurprisingly, his admission clerking didn’t include a weight.

With no previous notes to go on, we did the usual ICU thing and “guesstimated” a weight. This is usually not a problem with your average-sized patient where you’re not so worried about drug doses, such as for this chap. However, we were having trouble sedating him to assess his brain function. We wanted to use a short-acting sedative — the drug of choice in this case was remifentanil.

Now I don’t know what it is, but as soon as you introduce the nanogram into a calculation everyone starts to sweat. It should not be any different from the conversion, say, between grams and milligrams but there must be a psychological barrier. So, I know what the dose should be (in ng/kg/min), I know the strength (5mg in 50ml), I have the patient’s guesstimated weight (that’s 95kg) and all I need to do is calculate the rate in ml per hour.

Several minutes, one bit of paper and a calculator later and I triumphantly produce a dose range of 5.7–8.4ml/h. A nurse colleague had done his own calculation and we compared results. Shock horror — he had calculated a dose range of 6–9ml/h. What was going on? Was he better at maths than me? (Surely not.) I rechecked my calculations and I looked at his.

The answer was simple: I had calculated on the basis of a weight of 95kg and he was working with a weight of 100kg.

Would this have made a difference? Not in this case, because we were titrating the dose according to response and we ended up giving him a far higher dose, and adding in a second drug, to sedate him (and, strictly speaking, we should really have used ideal body weight). Nonetheless, there are some weight-crucial medicines with which you could not take such an ad hoc approach.

Are there solutions? Yes. Some emergency departments have a type of weighbridge (where you can wheel the patient on and get a weight) and some beds have inbuilt scales. In our ICU we have access to some blocks that go under each wheel of the bed that can measure a patient’s weight. It’s just a matter of remembering that these are available — and knowing where to find them.

Citation: Clinical Pharmacist DOI: 10.1211/CP.2013.11120714

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