Mobilizing carbon dioxide stores: An experimental study

Rationale: Understanding the physiology of CO2 stores mobilization is a prerequisite for intermittent extracorporeal CO2 removal (ECCO2R) in patients with chronic hypercapnia.Objectives: To describe the dynamics of CO2 stores.Methods: Fifteen pigs (61.7 +/- 4.3 kg) were randomized to 48 hours of hyperventilation (group "Hyper," n = 4); 48 hours of hypoventilation (group "Hypo," n = 4); 24 hours of hypoventilation plus 24 hours of normoventilation (group "Hypo-Baseline," n = 4); or 24 hours of hypoventilation plus 24 hours of hypoventilation plus ECCO2R (group "Hypo-ECCO2R," n = 3). Forty-eight hours after randomization, the current (V)over dot(E) was reduced by 50% in every pig.Measurements and Main Results: We evaluated (V)over dot(CO2,) (V)over dot(O2) , and metabolic (V)over dot(CO2) ((V)over dot(O2) times the metabolic respiratory quotient). Changes in the CO2 stores were calculated as (V)over dot(CO2) - metabolic (V)over dot(CO2). After 48 hours, the CO2 stores decreased by 0.77 +/- 0.171 kg(-1) in group Hyper and increased by 0.32 0.271 kg(-1) in group Hypo (P = 0.030). In group Hypo-Baseline, they increased by 0.08 +/- 0.191 kg(-1), whereas in group Hypo-ECCO2R, they decreased by 0.32 +/- 0.241 kg(-1) (P = 0.197). In the second 24-hour period, in groups Hypo-Baseline and Hypo-ECCO2R, the CO2 stores decreased by 0.15 +/- 0.091 kg(-1) and 0.51 +/- 0.061 kg(-1), respectively (P = 0.002). At the end of the experiment, the 50% reduction of (V)over dot(E) caused a Pa-CO2, rise of 9.3 +/- 1.1, 32.0 +/- 5.0, 16.9 +/- 1.2, and 11.7 +/- 2.0 mm Hg h(-1) in groups Hyper, Hypo, Hypo-Baseline, and Hypo-ECCO2R, respectively (P < 0.001). The Pa-CO2 rise was inversely related to the previous CO2 stores mobilization (P < 0.001).Conclusions: CO2 from body stores can be mobilized over 48 hours without reaching a steady state. This provides a physiological rationale for intermittent ECCO2R in patients with chronic hypercapnia.