Investigation of a simple, retrospective test for in-flight hyperventilation

The experiment was designed to study the feasibility of using a single rebreathing estimate of mixed venous carbon dioxide tension (PvCO2) was a simple field test for hyperventilation in pilots. The results confirmed that the fall of end tidal carbon dioxide tension (P(ET)CO2) during hyperventilation and rise during recovery was exponential. The results also showed that the relationships between PvCO2 and P(ET)CO2 values during the unsteady states of carbon dioxide washout and accumulation may be described as a loop which encloses the theoretically derived line for the steady-state relationships. The deviation from the steady-state line appears on theoretical consideration to be directly proportional to carbon dioxide elimination rate, and indirectly proportional to cardiac output. Because of the exponential recovery, and because one value of PvCO2 could correspond to a range of values of P(ET)CO2, it is concluded that a field test for hyperventilation based on a single rebreathing estimate of PvCO2 would not be of value. The finding of a low value of PvCO2 would, however, be an indication that hyperventilation had taken place.