FIGURE 6 Ventilatory response to increases in inspired CO2 plotted as a function of Paco2. Solid line is average for 33 normal subjects with standard deviation error bars. Dashed lines show the responses for subgroups of the whole population to demonstrate the large range of individual variability in normal subjects. (After Kellogg, Chap. 20 in Fenn and Rahn, eds., Handbook of physiology, Respiration. Bethesda, MD: American Physiological Society, 1964.)

co2 for two subgroups of the population used to establish the average curve. The range of differences between these groups illustrates the large variability that can occur in physiologic control systems between individuals, which may be under genetic control.

The hypercapnic ventilatory response can be measured in patients more easily by the rebreathing method. A person breathes in and out of a bag filled with 7% co2 and 93% o2, while continuously measuring ventilation and end-expired Pco2 (« Paco2 or Paco2). Metabolic co2, which is expired into the bag, progressively increases Pco2 inspired from the bag, causing a progressive increase in Paco2 and ventilation. High o2 levels in the bag ensure high Pao2 to eliminate any potential effects of hypoxia on ventilation. The slope of the ventilation versus Pco2 plot quantifies the hyper-capnic ventilatory response.

Both tidal volume and frequency increase with Paco2, but tidal volume reaches a maximum before frequency. Further increases in ventilation at high Paco2 levels depend on increases in respiratory frequency and this is less effective than increasing tidal volume at increasing alveolar ventilation (because of dead space; see chapter 21). Hence, the hypercapnic ventilatory response is most efficient at low levels of Paco2, and larger increases in total expired ventilation are necessary at high levels of

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