Right-to-left shunting decreases the efficiency of carbon dioxide excretion and increases the alveolar dead space (physiological dead space minus anatomical dead space). The theoretical effects of shunting on alveolar dead space were studied, using a human blood nomogram, for different values of inspired oxygen fraction (FIO2), respiratory quotient (R), and arterial-venous oxygen content difference. An equation developed to define the relationship between alveolar dead space and shunt could be closely fitted to the nomogram data. With high levels of shunt, FIO2 and R, alveolar dead space can substantially increase above normal levels. The conditions needed for this increase are unusual but lie within the range observed in critically ill patients. A rise in dead space of this magnitude can be clinically important in the absence of effective compensatory mechanisms.