Modeling acid‐base balance for in‐series extracorporeal carbon dioxide removal and continuous venovenous hemofiltration devices

Abstract

Patients with acute respiratory distress syndrome and acute kidney injury (AKI) treated by kidney replacement therapy may also require treatment with extracorporeal carbon dioxide removal (ECCO2R) devices to permit protective or ultraprotective mechanical ventilation. We developed a mathematical model of acid‐base balance during extracorporeal therapy using ECCO2R and continuous venovenous hemofiltration (CVVH) devices applied in series for the treatment of mechanically ventilated AKI patients. Published data from clinical studies of mechanically ventilated AKI patients treated by CVVH at known infusion rates of substitution fluid without ECCO2R were used to adjust the model parameters to fit plasma levels of arterial partial pressure of carbon dioxide (PaCO2), arterial plasma bicarbonate concentration ([HCO3]), and plasma pH (as well as certain other unmeasured physiological variables). The effects of applying ECCO2R at an unchanged and a reduced tidal volume on PaCO2, [HCO3] and plasma pH were then simulated assuming carbon dioxide removal rates from the ECCO2R device measured in the clinical studies. Agreement of such model predictions with clinical data was good whether the ECCO2R device was positioned proximal or distal to the CVVH device in the extracorporeal circuit. Although carbon dioxide removal rates from the ECCO2R device measured in one previous clinical study were higher when it was placed proximal to the CVVH device, suggesting that such in‐series positioning was optimal, the current mathematical model demonstrates that proximal positioning of the ECCO2R device also results in lower bicarbonate (and, therefore, total carbon dioxide) removal from the distal CVVH device. Thus, the removal of total carbon dioxide by such extracorporeal circuits is relatively independent of the position of the in‐series devices. It is concluded that the described mathematical model has quantitative accuracy; these results suggest that the overall acid‐base balance when using ECCO2R and CVVH devices in a single extracorporeal circuit will be similar, independent of their in‐series position.

Targeting arterial partial pressure of carbon dioxide in acute respiratory distress syndrome patients using extracorporeal carbon dioxide removal

“…These results assumed the average CO 2 removal rate from the PrismaLung device as reported 13 ; otherwise, all other model parameters remain unchanged. These latter model predictions were previously reported 12 …”

“…A notable strength of the current model is that it includes all potential sources of total CO 2 generation and removal in critically ill, mechanically ventilated ARDS patients. As highlighted elsewhere when treating in-series ECCO 2 R and CVVH circuits, 12 the importance of CO 2 removal by both the ECCO 2 R and CVVH devices is included. The clinical significance of CO 2 removal by CVVH devices was first demonstrated by Jonckheer et al 18 It should be noted that the current model predictions are based on an average patient from clinical studies and several parameters of the model were assumed a priori; thus, individual patient responses may vary from the predictions in this work.…”

“…Modeling total CO 2 removal by the ECCO 2 R and CVVH devices was identical to that described previously 9,10,12 . Removal of dissolved CO 2 from the blood by the ECCO 2 R device at a rate equal to JCO 2 (see Figure 1) is assumed to be governed by the following relationship describing the change in total CO 2 concentration in blood across the device (Δ[ t CO 2,ECCO2R ]):where Q b denotes the blood flow rate through the device.…”

“…As previously described using a whole‐body model, 8,9,12 the arterial partial pressure of CO 2 in lung capillaries can be calculated by assuming that: (1) The partial pressures of CO 2 in alveoli and lung capillary blood are in equilibrium and (2) CO 2 loss from the lung is altered by CO 2 removal via ECCO 2 R, infusion of bicarbonate‐containing solutions and ultrafiltration via CVVH. With these assumptions, the arterial partial pressure of CO 2 in lung capillaries is directly proportional to the overall rate of CO 2 removal by the lungs via the rates of tissue CO 2 production, CO 2 removal via ECCO 2 R, and net administration of total CO 2 via the infusion of substitution fluid minus that lost due to ultrafiltration via CVVH.…”

“…11 Once the total CO 2 concentration in lung blood has been thusly determined, the total CO 2 concentrations in other compartments, i.e., arterial blood, venous blood, and venous blood distal to the application of ECCO 2 R and CVVH devices can be calculated assuming the patient is in steady state. 9,12 These relationships can subsequently be used to calculate the complete acid-base composition of arterial, venous, and post-device venous blood by applying the model of acid-base chemistry of blood. 11 Modeling total CO 2 removal by the ECCO 2 R and CVVH devices was identical to that described previously.…”