Instability of the carbon dioxide stimulus under the "mixed venous isocapnic" conditions advocated for testing the ventilatory response to hypoxia.

Meaningful and reproducible ventilatory responses to hypoxia cannot be obtained from the mixed venous isocapnic test reported by Rebuck and Campbell if it is not recognized that the CO2 drive increases progressively for at least 6 to 10 min after the PCO2 step.