Mobilizing Carbon Dioxide Stores. An Experimental Study

Abstract

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 [Formula: see text]e was reduced by 50% in every pig.Measurements and Main Results: We evaluated [Formula: see text]co₂, [Formula: see text]o₂, and metabolic [Formula: see text]co₂ ([Formula: see text]o₂ times the metabolic respiratory quotient). Changes in the CO₂ stores were calculated as [Formula: see text]co₂ - metabolic V̇co₂. After 48 hours, the CO₂ stores decreased by 0.77 ± 0.17 l kg^-1 in group Hyper and increased by 0.32 ± 0.27 l kg^-1 in group Hypo (P = 0.030). In group Hypo-Baseline, they increased by 0.08 ± 0.19 l kg^-1, whereas in group Hypo-ECCO₂R, they decreased by 0.32 ± 0.24 l 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 ± 0.09 l kg^-1 and 0.51 ± 0.06 l kg^-1, respectively (P = 0.002). At the end of the experiment, the 50% reduction of [Formula: see text]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.

Keywords: CO2; acid–base balance; chronic obstructive pulmonary disease; extracorporeal CO2 removal; stores.