Thursday, 6 June 2013

Value for money post: the vagus nerve, mathematics, systems theory and bowel movements

To quote Gray's, the anatomy bible:
the vagus nerve is composed of both motor and sensory fibers, and has a more extensive course and distribution than any of the other cranial nerves, since it passes through the neck and thorax to the abdomen.

The spelling of "fibers" and Oxford comma belong to the author, although I do love a good Oxford comma.

For the uninitiated, it carries parasympathetic fibres to the thoracic and abdominal organs. These provide the opposite of the sympathetic 'fight or flight' response; instead, they encourage the body to perform functions suitable for a nice relaxing afternoon. The heart slows down and its muscles contract less forcefully when vagal tone increases, whilst the gut blood flow increases and muscle activity increases to digest food, absorb nutrients from it and pass waste in the form of faeces.

Unfortunately, if you are a youngster in heart failure and you go to the bathroom and try very hard to open your bowels, your vagal tone can increase much more than is good for you, and can instantly put you into a state called cardiogenic shock; your heart is no longer pumping out enough blood to oxygenate you adequately, and you become shockingly unwell unbelievably quickly.

This, it goes without saying, can break up the gentle routine of an afternoon on the ward. Trying to engage your insulin-saddled post-lunch brain into quickly prescribing and preparing a 10 microgram/kg/minute infusion of dobutamine, a task which should be reasonably straightforward (although inevitably double or triple-checked) at the best of times, can suddenly become a feat of mathematical impossibility.

Obviously, as Murphy's law would have it, the protocol for prescribing a dobutamine infusion would disappear at the exact time it was most urgently required. Reason's 'Swiss Cheese Model' suggests defects in multiple layers of protective measures must all line up to lead to disrupt the system; meanwhile, a speaker at a study day I recently attended suggested that "the hallmark of an unsafe medical system is a person at the end of any process who has to perform flawlessly."

Luckily, we have people like that. We call them nurses. "Put x mg into 50ml of saline and infuse at 2ml/hr to run at 10," she told me. I double-checked the maths. It worked.

After all that, the PICU team decided they'd start dobutamine on the unit. Typical.

No comments:

Post a Comment