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Hello! It’s time for #MCQMonday. The Respiratory system is endlessly fascinating with lots to learn. Last week was #lungcancer themed. This week is airways disease. 1/n
Ok so here it is - today’s #MCQMonday 2/n
Which one of the following is the single most important mechanism responsible for the increase in pCO2 following administration of oxygen? NB HPV = hypoxic pulmonary vasoconstriction 3/n
Ok, it's Wednesday so it's answer time! There is a split in the votes with the most people going for lost chemreceptor drive, then relieif of hypoxic vasoconstriction, then the Haldane effect. So who is right...? #RespEd
As with all great #SCE exam questions all the answers are correct but some are more correct than others... The single best answer is relief of hypoxic vasoconstriction #RespEd 5/n
In COPD, patients optimise their gas exchange by hypoxic vasoconstriction leading to altered alveolar ventilation-perfusion (V/Q) ratios. This paper from @accpchest explains it better than we can! https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5310129/ 6/n
Oxygen administration overcomes hypoxic pulmonary vasoconstriction, leading to increased blood flow to poorly ventilated alveoli. This leads to increased V/Q mismatch & physiological deadspace. 7/n
You may be very mad right now if you were taught that it was a loss of central chemoreceptor ‘hypoxic drive’ that was the mechanism for pCO2 rise after O2 administration... You should be mad! 8/n
We've actually known that VQ mismatch was the primary mechanism for ages! This paper is from @ATScommunitywas back in the 1960s https://www.atsjournals.org/doi/abs/10.1164/arrd.1967.96.6.1173?journalCode=arrd #RespEd #physiology 9/n
Here's one of many papers which disprove the idea that a reduction in respiratory drive is a major contributor to oxygen induced hypercapnia in #AECOPD. @atscommunity https://www.atsjournals.org/doi/10.1164/arrd.1980.122.2.191?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%20%200pubmed #RespEd 10/n
Aubier's classic paper also looked at the contribution of the Haldane effect. Do you remember what that is? Eponymous names are a nightmare....openanaesthesia to the rescue: https://www.openanesthesia.org/haldane_effect/ #RespEd 11/n
Deoxygenated Hb binds CO2 with greater affinity than oxygenated hemoglobin. So O2 induces a rightward shift of the CO2 dissociation curve, which is called the Haldane effect. In #COPD patients who cannot increase minute volume, it accounts for around 25% of the pCO2 rise 12/n
