At the base, pO2 is lower and pCO2 greater.ħ) Airway resistance is lowest at the apices because transmural pressure is greatest there (i.e. If V/Q = oo (infinity), then PaO2 = 150 mm and PaCO2 = 0 mm Hg.Ħ) Since PA is proportionally greater at the apices and pa is proportionally lower, pO2 is also greatest there and pCO2 the lowest. This is because haemorrhage would cause Pa to fall below PA with PEEP, PAwould be forced to exceed Pa.ĥ) If V/Q = 0, then PaO2 = 40 mm Hg and PaCO2 = 46 mm Hg. I’ve seen this in several questions.Ĥ) Zone I lung can be “created” if haemorrhage occurs or if PEEP is utilized. Ventilation increases because the alveoli at the base are closer to the diaphragm, so they experience a greater compliance than those at the apices, so ventilation is enhanced per unit change in pressure.Ģ) Heavy exercise allows the V/Q distribution to approach 1 throughout the lung.ģ) At the base, since Pa > Pv > PA, this is the zone (III) where flow is determined by the arterial-venous pressure gradient. If Pa > PA, the pressure within the vasculature exceeds that of the alveoli, so the vessels can stay open.Ī few big things to know (what I’ve encountered through 9500 practice questions so far ):ġ) Ventilation AND perfusion increase while going apices –> base, but perfusion increases proportionally greater than ventilation, so we get V/Q = 3 at the apices and 0.6 at the base. So if PA > Pa, the vasculature is not as patent because the alveoli are compressing the vessels.
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