Are there any session-to-session changes in ventilation during a weekly hemodialysis cycle?

Significant changes in pre-dialytic partial pressure of CO₂ (pCO₂) during a week-long cycle of hemodialysis (HD) can be an effect of the intermittent supplementation of bicarbonate to correct chronic acidosis in patients. Mathematical modeling efforts carried out using the same parameters before each HD session might fail to produce accurate predictions of pCO₂ and plasma bicarbonate concentration (C_Bic) because of this variability. A numerical model describing acid-base equilibrium changes during HD was applied to predict pCO₂, pH, and C_Bic in 24 chronic HD patients, using both fixed parameters for the whole week and estimating a new value of minute ventilation (V_E) and net acid generation rate (G_H) for each interdialytic interval. Dialysances of bicarbonate and dissolved CO₂ were also estimated independently for each HD session. The error of the model compared to the pre-dialytic data of C_Bic and pCO₂ significantly decreased when V_E and G_H were estimated piecewise throughout the week. To fit the data, V_E changed from 3.9 ± 1.0 mL/min before HD1, to 3.8e1 mL/min after HD1, 3.6 ± 1.0 mL/min after HD2, and 3.9 ± 1.1 mL/min after HD3 (p < 0.05). G_H changes after each session were not statistically significant. V_E values strongly correlated with pre-dialytic pCO₂ (Spearman's ρ = -0.97), but G_H only weakly correlated with pre-dialytic C_Bic (ρ = -0.30). Acid-base equilibrium is extremely sensitive to respiratory regulation. When attempting to predict the evolution of pCO₂ a C_Bic during the HD cycle, changes in the respiration parameters must be accounted for by the model, at the risk of a significant loss of prediction accuracy.