What’s wrong with this picture?
These were prescribed to my daughter after getting a DVT during an emergency medical procedure recently. The initial dose requested by the doctor was 2.2mg Warfaren.
“Crush and disperse THREE tablets upto 12ml of water and give 8.8ml ONCE a day”
We seem to be missing the word “in” before “upto”, okay so I can figure that one out.
What exactly do you mean by “in upto 12ml of water”? From these instruction I may produce a mixture with a concentration of something large (a vanishing small quantity of water) to 1/4 mg/ml.
This is clearly not what they meant and indeed I can’t draw 8.8ml from a powder in the limit where I (legally according to the instructions) added no water.
So assume that we add 8.8ml of water well, that would naively mean that when I draw off 8.8ml and provide the dose my daughter is overdosed by 0.8mg Warfaren
That’s clearly wrong the label is misleading. What I need to do is work with the ratio: If I use less water I need to give correspondingly less mixture to administer the same dose.
Turns out that the advice as received from the medical professionals is to use 3ml water to ‘dissolve’ 3mg of the crushed tablets and take 2.2 ml as the dose.
And so we come to error number 3:
We aren’t dissolving the tablets we’re dispersing them; they’re going into suspension. My humble chemistry education suggests to me that 3 undissolved tables in 3ml of water occupies 3ml + the volume of the tablets.
The tablets are flat-cylindrical in shape having a diameter of 8mm and depth of 2mm. A simple depth times pi r² should give me a volume measurement per tablet:
2x3.14x4x4 = 100mm³ which is 0.1cm³ or 0.1ml
So the total volume as instructed to mix will actually be 3.3ml
and thus the real concentration is 3mg/3.3ml = 0.91mg/ml
So a 2.2 ml dose actually provides 2.2ml * 0.91mg/ml = 2mg
i.e. about a 10% error in dosage
(using the dispenser’s initial maths this is more like a 2.15mg dose, closer but still incorrect)
Apart from potentially overdosing my child (which I avoided). This means that, due to the iterative approach used with Warfaren dose calculations, my daughter will probably need one extra clinic before the Warfaren becomes therapeutic and I will therefore have to keep on with the painful, twice daily, enoxaparin injections.
Which brings me onto error number 4:
Enoxaparin (Heparin to you and me perhaps) is the first line drug you get if the docs find that you have a DVT. It comes into effect quickly and wares off quickly. But is is administered as an injection twice daily.
Before my daughter was allowed to be discharged I had to pass a test on administering the drug. There’s a problem though: she needs 0.17ml per injection but the prepacked jabs carry 0.2ml in an ungraduated, self-administering syringe.
The procedure is therefore to charge a new sterile syringe with a dose of 0.17ml from the self-help syringe.
I performed this twice, once with a pre-prepared dose so that I could learn how to do the injection into the Insuflon (R)* (a subcutaneous injection port, which is like a cannula but under the skin not into a vein or artery). [For some reason they must call their drugs by their generic names not their
familiar names: Enoxaparin not Heparin, Salbutamol not Ventolin, …; but it seems that obscure devices they must call them by their registered trademark names]. The second time I administered the dose, I prepared the syringe and measured it myself with supervision.
It was the dose measuring (again) that was at fault. The Syringe consists of a plunger and a case and to this one attaches a needle which is housed in a plastic tube. I was given a 0.2ml self-help syringe of enoxaparin. I dutifully filled 0.2ml of enoxaparin into the second syringe through its nose before placing the new needle and its housing on the end. I moved the plunger to 0.17ml as instructed but was somewhat perturbed to not see any fluid appear out of the end of the needle. I showed this to the nurse and voiced my concerns. She didn’t seem to be overly concerned but took the syringe and needle to discuss with another nurse. When she came back she told me that what I had done was correct.
I duly made the inoculation. And we were granted permission to leave and be discharged.
7am the next day, it was time to make perform the injection again, at home, measuring the dose myself. Again seeing no liquid come out of the needle during preparation worried me. So when we turned up later at the hospital to get a thumb-prick test from a clinic we were told to attend but which didn’t exist, I found a doctor in the original ward and quizzed her. After I had rambled on for some time I managed to get across that there was in fact something wrong with the picture and she found a nurse to go through the process with me (the doctor also checked my incorrect Warfaren dose calculations — based on a new dose size 2.5mg but using the erroneous method described in error 3 above — deeming them to be correct). Two nurses saw me one who had administered the enoxaparin previously and one who seemed to be more senior. Both confirmed that what I had been taught on the measurement of the injection was incorrect.
That evening when I measured up the dose with 0.4ml of enoxaparin and primed the syringe to 0.17ml I did see the needle squirt. I also noted that it did this when the syringe read 0.3ml meaning that the original dose I was giving my daughter was 0.10ml enoxaparin with 0.07ml of Air. A dose60% the size it should have been, and Air!
Surely it’s not a good idea to inject air. Air is mostly Nitrogen, divers with the bends die when little bubbles of nitrogen form in their blood stream.
I was not very happy about this.