Comparative Assessment of Membrane Oxygenator Efficiency: Pressure Drop and Oxygenation Performance during Cardiopulmonary Bypass

INTRODUCTION

One of the key technologies in modern heart surgery is cardiopulmonary bypass (CPB). The introduction of CPB revolutionized cardiac surgery by providing the ability to stop the heart while maintaining blood flow and oxygenation to vital organs.^[1] This advancement allows surgeons to perform intricate procedures, such as congenital heart defect repair, valve replacements and repair, aneurysm repair and coronary artery bypass surgeries, in a controlled, bloodless, and motionless field. Annually, around 2 million heart surgeries are performed globally, with a significant portion involving CPB and oxygenators.^[2]

The CPB circuit mimics the natural functions of the heart and lungs, utilizing an oxygenator for gas exchange and a pump to ensure blood circulation throughout the body. Since its introduction in the mid-20^th century, CPB has been indispensable for conducting complex heart surgeries.^[3,4] Early oxygenation systems included animal lungs, rotating cylinder or disc oxygenators, and bubble oxygenators, all of which effectively oxygenated blood but were prone to complications. Over time, advancements in design and materials have significantly improved these systems.^[5,6]

A significant breakthrough in oxygenator technology occurred in the late 1950s with the development of membrane oxygenators, marking a major milestone in artificial oxygenation.^[7] The introduction of the “Azygos principle” by Lillehei’s team led to the creation of the first membrane oxygenators, which allowed for efficient gas exchange by facilitating the diffusion of oxygen and carbon dioxide through a microporous fiber bundle. These specialized oxygenators now support or replace the lungs during CPB and extracorporeal life support procedures, ensuring gas exchange and carbon dioxide removal.^[8]

Modern oxygenators are designed to minimize microemboli passage, enhance biocompatibility, and optimize gas transfer efficiency. Manufacturers have developed various oxygenators, each featuring different blood flow pathways and performance characteristics.^[9] Despite the widespread use of CPB, ongoing innovation aims to enhance the safety, efficiency, and user-friendliness of oxygenators. Given the critical role of oxygenators in maintaining oxygen supply, their design must ensure effective oxygenation and carbon dioxide removal to avoid adverse patient outcomes.^[10]

Key performance indicators for oxygenator effectiveness include pressure drops across the membrane and oxygenation efficiency. The pressure drops that reflect blood flow resistance should be kept low to reduce the strain on both the heart and the pump. Oxygenation efficiency, which measures the ability of the oxygenator to remove carbon dioxide and provide oxygen, directly impacts patient safety and the overall success of the procedure.^[11]

At our institution, we utilize several oxygenators, including the SORIN (Inspire) and NIPRO (Vital) models, both of which have shown extensive use and promise in cardiac surgery. The Nipro Vital Membrane Oxygenator is known for its minimal pressure drop and effective gas exchange, which ensures smooth blood flow and reduces cardiac strain. Its biocompatible coating is also effective in reducing inflammation and enhancing patient safety.^[12] On the other hand, the Sorin Inspire Membrane Oxygenator utilizes advanced fiber technology to improve oxygenation efficiency while maintaining low resistance. Its optimized blood distribution design minimizes the risk of clot formation and enhances overall performance during CPB procedures.^[13] Both oxygenators are widely regarded for their reliability and performance in clinical settings.

This study seeks to compare the performance of the Nipro Vital and Sorin Inspire oxygenators by evaluating key parameters, including pressure drop and oxygenation efficiency – essential factors in determining their effectiveness in supporting patients during CPB.

MATERIAL AND METHODS

This prospective observational study was carried out in the Department of Cardiovascular Thoracic Surgery at Kasturba Medical College, Manipal, including 25 patients per group who underwent CPB for cardiac surgery between 2023 and 2024. The study received approval from the Institutional Ethics Committee (IEC2: 682/2022), and all participants provided written informed consent. The inclusion criteria comprised patients aged 18 years or older and scheduled for elective heart surgery involving CPB. Eligible participants were either inpatients or patients attending the pre-operative assessment clinic. Exclusion criteria included patients requiring emergency surgery or off-pump procedures.

The patients were assigned to one of two groups: the Sorin (Inspire) group (Group S) or the Nipro (Vital) group (Group N) in a 1:1 ratio [Figure 1]. The primary outcomes of the study were pressure drop and oxygenation efficiency.

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Figure 1:

(a) Nipro Vital oxygenator; (b) Sorin Inspire oxygenator.

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RESULTS

The study included 25 patients in each group, with Group N (Nipro Vital) having an average age of 59.73 ± 10.67 years and Group S (Sorin Inspire) having an average age of 54.13 ± 12.63 years. A comparison of the ages between the two groups revealed no statistically significant difference (P = 0.2, unpaired t-test), indicating that both groups were comparable in terms of age. The gender distribution was also similar between the groups, with Group N consisting of 18 males and 7 females, and Group S consisting of 20 males and 5 females.

Regarding the total duration of CPB time, there was no significant difference between the two groups (P = 0.4, unpaired t-test). The mean CPB time in Group N was 148.33 ± 46.88 min, while in Group S, it was 134.33 ± 54.09 min, suggesting that the surgical duration was comparable across both groups.

The primary outcome measures, the pre-membrane and post-membrane pressures, were assessed to determine the pressure drops across the oxygenators [Figure 2]. These pressure measurements were recorded at three different flow stages: flow-0, flow-1, and flow-2, which are baseline flows, during hypothermia, and during rewarming, respectively. The results indicated that the pre-membrane and post-membrane pressures were similar between the two groups at all flow stages (P > 0.05 for each stage on unpaired t-test), demonstrating that there was no significant difference in pressure readings between Group N and Group S. As a result, the pressure drops across the oxygenators were also found to be comparable between the two groups. These findings are summarized in Table 1.

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Figure 2:

Comparison of pressure drop across membranes

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In addition, we documented flow and fraction of inspired oxygen (FiO₂) between the two groups [Table 2].

In addition, we also recorded and compared the flow rates and FiO₂ between the two groups.

Perfusate partial pressure of oxygen (PpO₂) [Figures 3-5] of two oxygenators was documented and compared. However, no statistically significant difference was noted between the two groups (P > 0.05 on unpaired t-test).

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Figure 3:

ROC curves for Blood flow and FiO2 parameters across flows. FiO2: Fraction of inspired oxygen, AUC: Area under curve. Comparison of perfusate partial pressure of oxygen (PpO₂), ROC: Receiver operating characteristic curve.

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Figure 4:

Comparison of perfusate partial pressure of oxygen (PpO₂).

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Figure 5:

ROC curves for pressure drops across flows. AUC: Area under the curve, ROC: Receiver operating characteristic curve.

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The PpO₂ for both oxygenators was measured and compared between the two groups. The PpO₂ values, which reflect the level of oxygen in the blood passing through the oxygenator, were assessed at various stages of the procedure. However, statistical analysis revealed no significant difference in PpO₂ between Group N (Nipro Vital) and Group S (Sorin Inspire) (P > 0.05, unpaired t-test). This indicates that both oxygenators provided similar oxygenation levels during the CPB procedure.

DISCUSSION

Our findings showed that there was no statistically significant difference in the pressure drop across the membranes of the two oxygenators. The pre- and post-oxygenator pressures were consistently maintained during both normothermic and hypothermic CPB, with continuous monitoring of input and output pressures revealing no significant differences between the groups. This suggests that both oxygenators performed similarly in terms of pressure management throughout the surgery.

Pressure drop is influenced by various factors, including the patient’s blood parameters such as hematocrit, viscosity, temperature, and blood flow rate, as well as the oxygenator’s internal flow dynamics.^[14] A lower pressure drop is vital in preventing complications such as metabolic acidosis, hemolysis, and other forms of blood damage. A significant pressure gradient within the extracorporeal circuit can cause thrombosis and obstruct blood flow, potentially compromising oxygenation and heat exchange. Although both oxygenators performed well, it is important to note that an excessively high pressure drop can lead to catastrophic failure, requiring the replacement of the oxygenator during CPB or extracorporeal membrane oxygenation (ECMO) therapy. This scenario poses a risk to the patient, highlighting the importance of monitoring and maintaining a safe pressure drop.^[15] Fortunately, our study found that neither of the two oxygenators experienced dangerously high pressure drops, ensuring their safety and reliability during CPB.

Another key finding in our study was the lack of significant difference in oxygenation efficiency, as measured by the delta pressures, between the two oxygenators at any point during the surgery. Oxygenation efficiency refers to the ability of the oxygenator to facilitate gas exchange – specifically, the delivery of oxygen to the blood and the removal of carbon dioxide. Several factors contribute to oxygenation efficiency, including membrane surface area, blood flow rate, hemoglobin concentration, and the permeability of the membrane. An optimal blood flow rate ensures adequate exposure to the oxygenation membrane, while a larger membrane surface area increases the capacity for gas exchange. Sufficient hemoglobin levels further enhance the oxygen transport capacity. When oxygenation efficiency is optimal, the oxygenator ensures adequate arterial oxygenation and prevents hypercarbia, maintaining stable metabolic conditions during CPB.

Inefficient oxygenation can lead to tissue hypoxia, organ dysfunction, and acidosis, while poor removal of carbon dioxide can cause hypercapnia, respiratory acidosis, and hemodynamic instability.^[16,17] Inadequate gas exchange can also trigger systemic inflammation, which may negatively affect post-operative recovery. Maintaining high oxygenation efficiency is essential to ensuring proper oxygen delivery, stable blood gas levels, and better patient outcomes during CPB. Our study demonstrated that both the Nipro Vital and Sorin Inspire oxygenators were equally effective in maintaining oxygenation, thus supporting their use in routine CPB procedures.

Despite these promising results, there are limitations to our study. The oxygenators were evaluated in an elective surgical setting with routine surgery durations, and their performance in more prolonged or complex surgeries was not assessed. In addition, the study did not examine the performance of the oxygenators in ECMO circuits nor did it explore any potential differences in the inflammatory responses triggered by each oxygenator. These areas could be explored further in future research to provide a more comprehensive understanding of the performance of these oxygenators under varying clinical conditions.

CONCLUSION

Both the Nipro Vital and Sorin Inspire membrane oxygenators demonstrate effective gas exchange during CPB, ensuring adequate oxygenation and carbon dioxide removal. This comparative study found no significant difference in key performance parameters between the two oxygenators, indicating that both are equally capable of maintaining stable blood gas levels throughout the procedure. Given their comparable efficiency, the selection of an oxygenator can be based on institutional preferences, availability, and the surgeon’s discretion.

However, a notable distinction between the two is the cost factor. Nipro Vital provides a more economical alternative without compromising performance, making it a cost-effective option for healthcare facilities seeking to optimize resource utilization. This cost advantage may be particularly beneficial in settings with budget constraints or where reducing procedural expenses is a priority. Ultimately, both oxygenators remain reliable choices for routine cardiac surgeries, and the decision should align with institutional policies and financial considerations.