On Respiratory Failure (and wiping the floor with the competition)

Posted on December 3, 2011

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I think most of us studying it would agree that medicine attracts a certain type of person. Competitive, driven, maybe even a little bit ruthless. If you’d asked me a week ago if I fit these criteria I’d probably have said no. Yes, I work hard (although I’m certainly not the hardest worker out there), and sure it’s nice to do better than your peers in exams. But now that the week is over, I’m sure that I am at least two of these things.

I mentioned in my last post (you can read it here) that I’ve spent my week with the paediatric anaesthetic department as part of my SSC project, handing out questionnaires to parents. It’s been going well (with 41 responses so far), but on Friday Lucy and I ventured into foreign territory and stormed the Day Surgery Unit. It went well for about thirty minutes, until we realised that we’d strayed a bit too far and were thoroughly ensconced behind enemy lines.

The DSU, it turns out, has its own student. And she has her own questionnaire.

So for the rest of Friday morning Lucy and I went toe to toe with said student in a battle to get as many questionnaires filled out as possible. To make matters worse, both questionnaires were remarkably similar- probably because they’d both been based on a questionnaire produced by the Association of Paediatric Anaesthetists, so I expect patients and parents who agreed to fill out both were thrilled with the overlapping content. Even so, we got a good few responses, and though I’m not 100% certain I’m pretty sure we won the morning. The competitive monster within me has stirred and, as long as I don’t start slashing people’s tyres to come out on top, I think I’m ok with it.

Anyway, enough of that, because it’s time to learn about…

                                                                                 …(fanfare please)…

RESPIRATORY FAILURE!

Respiratory failure? That’s when you stop breathing, right? Not quite.

The Oxford Handbook of Clinical Medicine says respiratory failure “occurs when gas exchange is inadequate, resulting in hypoxia“.

Specifically, respiratory failure is defined as a partial pressure of oxygen <8kPa, though you can further categorise respiratory failure into 2 types depending on the carbon dioxide partial pressure.

TYPE I RESPIRATORY FAILURE

(Hypoxia with Pa O2 <8kPa and normal/low PaCO2)

Type I respiratory failure is caused by ventilation/perfusion (V/Q) mismatch- that means the amount of oxygen reaching the alveoli isn’t matched by the amount of blood perfusing the alveoli.

  • Pneumonia
  • Pulmonary oedema
  • Pulmonary embolism
  • Asthma
  • Emphysema

TYPE II RESPIRATORY FAILURE

(Hypoxia with PaO2 <8kPa and Hypercapnia with PaCO2 >6kPa)

Type II respiratory failure is caused by alveolar hypoventilation.

  • Pulmonary disease: asthma, COPD, pulmonary fibrosis
  • Reduced respiratory drive: sedative drugs, CNS tumour, trauma
  • Neuromuscular disease
Management, as ever, depends on the underlying cause, which should be treated. With type I respiratory failure, give oxygen by mask to treat the hypoxia. Type II respiratory failure needs to be treated more cautiously. In healthy people, respiration is driven by the hypercapnic drive, but for people with COPD, who may have relatively high PaCO2 levels, they may be relatively insensitive to low levels of CO2 because they retain CO2. Because of this their respiration may be mediated by the hypoxic drive. This poses a problem for treatment of Type II respiratory failure, because (if their breathing is driven by a lack of oxygen rather than an abundance of carbon dioxide) giving high flow oxygen could cause them to stop breathing altogether. That said, you should still give oxygen (but O2 therapy should start at 24% rather than high flow O2). Venturi masks can be used to titrate oxygen therapy. Again, I’ve found a (much posher) wordpress blog that features a post about respiratory failure in much more detail than I’ve covered. Read it here.
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