Describe the effects of V/Q inequality on the partial pressure of oxygen (PaO2) and carbon dioxide (PaCO2) in arterial blood.
Very few candidates demonstrated understanding of this core topic. Candidates did not
accurately define V/Q inequality and the physiological factors causing this phenomenon. V/Q
scatter as well as true shunt (V/Q=0) and dead space (V/Q=∞) needed to be considered. The
inability of high V/Q areas to compensate for low V/Q zones owing to the relatively small
contribution of blood flow from these high V/Q units was not discussed. The differential effect of
FiO2 on true shunt versus V/Q scatter was seldom explained. The shape of the oxy-Hb
dissociation curve and CO2-dissociation curve were sometimes mentioned but their effect on
arterial gas tensions not well explained.
Often graphs were reproduced inaccurately and contradictory statements made, leaving the
impression that candidates did not understand the basic concepts. It is core knowledge for Intensive Care Specialists managing respiratory failure. A sophisticated knowledge based on the
chapter in Nunn is a minimum standard expected for this topic