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. 2010 Feb 25;108(5):1084–1096. doi: 10.1152/japplphysiol.90963.2008

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Fig. 3. — Composition of respired gas during each of the 3 main protocols. Top: hypercapnia protocol consisting of a 25-min period of euoxic hypercapnia preceded and followed by a 5-min period of euoxic eucapnia. Ventilation was spontaneous throughout. Middle: hypocapnia protocol consisting of a 25-min period of euoxic hypocapnia preceded and followed by a 5-min period of euoxic eucapnia. The volunteers undertook voluntary hyperventilation throughout. Bottom: control protocol consisting of a 35-min period of euoxic eucapnia. Ventilation was spontaneous throughout. Left: values during the determination of the maximum pressure difference across the tricuspid valve (ΔPmax). Right: values during the determination of cardiac output. Results show inspired Pco₂ (Pi_CO₂; triangles), inspired Po₂ (Pi_O₂; diamonds), end-tidal Pco₂ (Pet_CO₂; circles), and end-tidal Po₂ (Pet_O₂; squares). Closed symbols indicate periods of either hypercapnia or hypocapnia; open symbols indicate periods of eucapnia. Data are means for 12 subjects; error bars are ± SE. Note that, in the hypercapnia protocol, Pi_CO₂ is close to 40 Torr during euoxic hypercapnia, which coincidentally is very close to Pet_CO₂ before and after this period.