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 chronically hypercapnic patients.

Objectives

To describe the dynamics of CO₂ stores Methods: Fifteen pigs (61.7±4.3 kg) were randomized to: 48 hours hyperventilation (group “Hyper”, n=4); 48 hours hypoventilation (group “Hypo”, n=4), 24 hours hypoventilation plus 24 hours normoventilation (group “Hypo-Baseline”, n=4); 24 hours hypoventilation + 24 hours hypoventilation+ECCO₂R (group “Hypo-ECCO₂R”, n=3). Forty-eight hours after randomization the current minute ventilation was reduced by 50% in every pig.

Measurements

CO₂ elimination (V̇CO₂), oxygen consumption (V̇O₂), metabolic V̇CO₂ (V̇O₂ times the metabolic respiratory quotient). Changes in the CO₂ stores were calculated as VO₂-metabolic VCO₂.

Main results

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 while in group “Hypo-ECCO₂R” they decreased by 0.32±0.24 l⸱kg-1 (p=0.197). In the second 24 hours period, in group “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 minute ventilation caused a PaCO₂ rise of 9.3±1.1, 32.0±5.0, 16.9±1.2 and 11.7±2.0 mmHg⸱hr-1 in group “Hyper”, “Hypo”, “Hypo-Baseline” and “Hypo-ECCO₂R”, respectively (p<0.001). The PaCO₂ 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 chronically hypercapnic patients.