Kinetics of arterial carbon dioxide during veno-venous extracorporeal membrane oxygenation support in an apnoeic porcine model

Extracorporeal membrane oxygenation (ECMO) has been successfully used to support severely hypoxemic patients [1–7]. Despite the worldwide increase in ECMO support use, there are few studies exploring the physiology of polymethylpentene membranes in the veno-venous configuration [8, 9].

After initiation of extracorporeal support, blood flow and sweep gas flow are adjusted to achieve target values of arterial oxygen (PaO₂), pH, and carbon dioxide (PaCO₂) partial pressures. Titrating these ECMO settings can involve multiple steps because, frequently, the initially chosen parameters will not produce the desired result in terms of blood gases, leading to at least one more cycle of parameter change and blood gas analysis. The whole process has to be repeated whenever the patient condition changes. As a consequence of its lower solubility and lower volume of distribution throughout multiple compartments, PaO₂ reaches equilibrium much sooner than PaCO₂ [10]. Therefore, in order to optimize the frequency of changes in ECMO settings, it would be of utmost importance to know how long it takes for CO₂ to equilibrate after ECMO support initiation or any parameter change.

Based on previous unpublished data, we have hypothesized that a 50-min period would be enough for PaCO₂ equilibrium after each change on ECMO settings. Therefore, we conducted this experimental study to analyze the kinetics of arterial partial pressure of carbon dioxide following the changes in ECMO parameters during apnoeic veno-venous ECMO support.

This manuscript is part of a sequence of experiments during porcine respiratory ECMO support, some of which were previously published elsewhere [11]. The study was approved by the Institutional Animal Research Ethics Committee from the Hospital Sírio-Libanês in São Paulo, Brazil, and was done according to the National Institutes of Health guidelines for the use of experimental animals. Instrumentation, surgical preparation, and lung injury were performed as previously described [11].

Instrumentation and surgical preparation

Five domestic female Agroceres pigs (80 [79,81] kg) were anesthetized with thionembutal (10 mg.kg⁻¹, thiopental, Abbott, Brazil) and pancuronium bromide (0.1 mg.kg⁻¹, Pavulon, AKZO Nobel, Brazil) and connected to a mechanical ventilator (Evita XL Dräger, Dräger, Lübeck, Germany) with the following parameters: tidal volume 8 mL/kg, end-expiratory pressure 5 cmH₂O, FiO₂ initially set at 100 % and subsequently adjusted to maintain arterial saturation between 94 and 96 %, and respiratory rate titrated to maintain PaCO₂ between 35 and 45 mmHg or an end-tidal CO₂ (NICO, Dixtal Biomedica Ind. Com, São Paulo, Brazil) between 30 and 40 mmHg. The electrocardiogram, heart rate, oxygen saturation, and systemic pressures of the animals were monitored with a multiparametric monitor (Infinity Delta XL, Dräger, Lübeck, Germany). Central venous pressure (CVP), mean pulmonary artery pressure (PAPm), pulmonary artery occluded pressure (PAOP), and cardiac output (CO) were measured with the use of a pulmonary artery catheter. Anesthesia was maintained with midazolam (1–5 mg.kg⁻¹.h⁻¹) and fentanyl (5–10 mcg.kg⁻¹.h⁻¹), and muscular relaxation was maintained with pancuronium bromide (0.2 mg.kg⁻¹.h⁻¹).

A 25-cm ECMO arterial cannula (Edwards Lifesciences, Irvine, CA, USA) was introduced into the right external jugular vein. A 55-cm ECMO drainage cannula (Edwards Lifesciences, Irvine, CA, USA) was positioned close to the right atrium via the right femoral vein with the aid of transhepatic ultrasonographic visualization. Only the guidewires were inserted until the first baseline measurements, after the stabilization period (Fig. 1). Heparin infusion was then started at 1000 IU/h. After the first baseline, guidewires were replaced by the ECMO cannulas. Cannula diameter was chosen in accordance with vein diameter as measured with the aid of ultrasonography. Four animals had a 20-Fr and one had a 21-Fr drainage cannula. Three animals had a 21-Fr and two had a 20-Fr return cannula. A central venous catheter and an arterial line were placed in the left femoral vein and artery, respectively.

In all conditions tested, carbon dioxide had a long time to equilibrium after changes in ECMO settings. With an ECMO blood flow of 1500 mL/min, increases in sweep gas flow beyond 3.0 L/min did not significantly affect PaCO₂. With an ECMO blood flow of 3500 mL/min, all increments of sweep gas flow up to 12 L/min resulted in additional reductions of PaCO₂. CO₂ equilibrium was faster with higher blood and sweep gas flows.

Unlike oxygen, which has a fast time to equilibrium and can be monitored by pulse oximetry at the bedside [16], CO₂ requires serial arterial blood gases measurements. Differently from our initial expectative, we found long time constants for CO₂ equilibrium with a blood flow of 1500 mL/min, suggesting that late (>50 min) assessments of arterial blood gases are usually necessary to capture the full impact of new ECMO settings in low ECMO blood flow states. Furthermore, this finding is also explained by the large volume of distribution of CO₂ [17], also stored as bicarbonate, bound to hemoglobin, and dissolved in the peripheral tissues [14, 18, 19]. The plasma dissolved fraction (PaCO₂) actually accounts for less than 5 % of the total body storage of CO₂. Therefore, small variations in PaCO₂ represent large total CO₂ variations [20]. Because the changes in CO₂ elimination are small in relation to its volume of distribution, the time to equilibrium is expected to be long. Moreover, after the elevation of effective ventilation, the reduction of the PaCO₂ will depend mainly of the effective ventilation and volume of CO₂ distribution; and after the reduction of effective ventilation, the elevation of PaCO₂ will depend of the volume of distribution and of the CO₂ production. The last depends on other variables such as temperature, sedation, inflammatory status, etc. Additionally, the interchange among the different molecular forms of CO₂ depends on other modulators, such as temperature, local pH, carbonic anhydrase activity, and others [19, 21]. Thus, the time to PaCO₂ equilibrium is a complex function derived from the combination of many variables. In this study, a 50-min period was sufficient to achieve the PaCO₂ equilibrium in all sweep gas flow tested only with a blood flow of 3500 mL/min, otherwise in using 1500 mL/minute of blood flow, it may take as long as 90 min to reach the equilibrium.

We kept the animals apnoeic to restrict the elimination of CO₂ to the ECMO system. With the subjects ventilating, even with a constant minute alveolar volume, changes in alveolar elimination of CO₂ are important, because alveolar elimination depends on both alveolar CO₂ concentration and alveolar minute ventilation [11]. We reasoned that the apnoeic model would better reflect the isolated effects of changes in ECMO settings. In Table 1, it is possible to note that CO₂ transfer and production varied importantly between study steps and, more importantly, with an ECMO blood flow of 1500 mL/min. We attribute this finding mainly to the absence of CO₂ stabilization throughout the phase 1 study period, instead of marked change on CO₂ production between steps. Moreover, a variation on cardiac output and unstable animal conditions may have contributed to this finding.

At low ECMO blood flow (1500 mL/min), we found that CO₂ elimination had a ceiling effect at sweep gas flow of 3.0 L/min with higher gas flows exerting no significant impact on PaCO₂. This finding suggests that post-membrane CO₂ was already very low with the sweep gas flow of 3.0 L/min, and that CO₂ transfer was thus perfusion limited. This implies that, due to its high diffusibility, a very low CO₂ partial pressure (its dissolved fraction) can be reached early during the passage of blood through the ECMO membrane, limiting CO₂ elimination independently of the sweep gas flow. This finding is of great interest in situations in which ECMO is used mainly for hypercapnia or in low flow devices, such as ECCO₂R removal technologies.

Our study has several limitations: (1) The low number of animals could lead to type-II errors, attenuating the validity of our results. However, such a limitation would not affect our positive findings. (2) We did not study animals with multi-organ failure syndrome, a condition likely closer to a clinical scenario. However, many positive associations will be intuitively the same. (3) As presented in Table 1, we observed a progressive decrease in hemoglobin concentration throughout the experiment. Considering that hemoglobin acts as a CO₂ reservoir, this may have influenced the results. (4) We did the analysis with a static 50-min period to gas equilibrium. With low ECMO blood flows, it was not enough for PaCO₂ to reach the equilibrium and, therefore, definitive time to equilibrium for such situation remains unknown. Moreover, it is possible that late inter-compartment distributions may occur, affecting our results.

In conclusion, our results suggest that 50 min was enough to reach the equilibrium of PaCO₂ after ECMO initiation or after changes in blood and sweep gas flow with an ECMO blood flow of 3500 mL/min. Longer periods are necessary to reach the equilibrium with an ECMO blood flow of 1500 mL/min. This may help bedside decisions and facilitate daily management of patients during ECMO support.