Registered Respiratory Therapist (RRT-TMC) Practice Exam

Physiologic dead space is quantified using the CO2-based Bohr relationship. It compares the CO2 in the alveolar/arterial compartment with the CO2 present in the mixed expired gas. The dead-space fraction is given by the difference between CO2 in the arterial (or alveolar) compartment and the mixed-expired CO2, divided by the CO2 in the arterial/alveolar compartment: (PaCO2 - PECO2) / PaCO2 (or using PACO2 instead of PaCO2). This means that when mixed-expired CO2 is lower than arterial/alveolar CO2, a portion of ventilation is not contributing to gas exchange, increasing the dead-space fraction. The other formulas point to different concepts: the alveolar-arterial O2 gradient (PAO2 - PaO2) reflects diffusion or V/Q mismatch, not dead space; the difference in oxygen content between arterial and venous blood (CaO2 - CvO2) relates to oxygen delivery/consumption; and the final option does not describe a standard calculation for physiologic dead space.