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Extracorporeal Membrane Oxygenator to Support Diseased Lungs
Monitoring the gas exchange capacity of hollow fibers with VisiSens
HTTG / LEBAO - Hanover Medical School, Germany
The function of a diseased lung can be supported by the application of an extracorporeal membrane oxygenator (ECMO). Aim of this study is to improve the hemocompatibility of the ECMO´s gas exchange hollow fiber surface by coating with distinct bioactive components. The coating must not negatively affect the gas permeability so the patient´s blood is oxygenated properly. The VisiSens system was tested for its applicability to serve as a monitoring tool and quantify the gas exchange capacity of coated hollow fibers. The system allowed to measure oxygen saturation directly at the site of diffusion and occluded fibers not taking part in the oxygenation could be identified.
Patients suffering from end-stage lung disease are confronted with a lack of medical options for the restoration of the organ function. In most cases lung transplantation remains the only treatment. Due to an increasing mismatch between potential organ donors and organ recipients, there is a high mortality rate of patients on the waiting list. For bridging the time to lung transplantation the function of the diseased lung can be supported by the application of an extracorporeal membrane oxygenator. This system consists of a multitude of hollow fibers, delivering oxygen by diffusion through the fiber membrane to the patient´s blood. Since the artificial fiber surface thereby gets into direct contact with the blood, thrombus formation and infection can occur over time, resulting in the failure of this device. In most of the cases such devices cannot be applied longer than about 30 days. In order to extent the time period of application the hemocompatibility of the ECMO´s hollow fiber surface by coating with bioactive components is tested. These coatings must not affect the gas permeability of the hollow fibers. Therefore, the VisiSens system was applied for oxygen monitoring right at the site of diffusion. A mesh consisting of gas exchange hollow fibers (fiber diameter: 380 µm) was mounted between two glass slides. The VisiSens sensor film attached to the top glass slide with silicone glue was in direct contact with the fibers. Silicone duplicating material was used to insulate the mesh sample from ambient atmosphere. Embedding a silicone tube allowed for the perfusion of the hollow fibers with a gas mixture.
Materials & Methods
The hollow fiber mesh and VisiSens sensor were set up as described above. An oxygen mixer (Sechrist) and an ECG patient monitor (Datex, Cardiocap II) were connected to enable adjustment of the oxygen saturation and the applied gas pressure. The VisiSens camera (Detector Unit DU01) was placed over the fiber mesh construct to monitor the oxygen saturation at the fiber surface. Uncoated gas exchange hollow fiber membranes were used to proof the approach. Before each measurement 99 % CO2 was applied through the hollow fibers, with a pressure of 20 mmHg for 30 min to eliminate the remaining oxygen in the sample system. For measuring the oxygen saturation over time, a time drive measurement was set in the VisiSens AnalytiCal 1 software, with pictures taken every 5 seconds. 60 images were recorded in each measurement (n= 3). The measurement was performed with 21 % O2 applied at a pressure of 22 mmHg. As a control half of the monitored fibers in a freshly mounted mesh were deliberately occluded with silicone glue. For this experiment the same settings were applied as before, except for the duration of the measurement (20 pictures each 5 s only). The pictures were processed and evaluated with the VisiSens AnalytiCal 1 software. The 0 % oxygen saturation value was set with a picture taken directly after the O2 elimination by CO2 perfusion. For the 100 % value, one area of the last picture from the time drive measurement with oxygen perfusion was chosen. For measuring the mean saturation of oxygen per time, over the whole monitored area, the VisiSens AnalytiCal 1 z-axis profile function was applied.
Monitoring Hollow Fibers
Following the elimination of residual O2 in the system by CO2 perfusion, 21 % O2 was applied through the gas exchange hollow fiber with a pressure of 22 mmHg for 300 seconds with images taken every 5 seconds. The measurement was repeated 3 times. After calibration and evaluation with the z-profile function a graph was generated comprising the three different data sets (Fig. 3), showing the mean oxygen saturation per measured time point. It could be observed that the increase in oxygen started after 50 seconds in each measurement. The increase in the oxygen saturation was almost linear and the slopes of the 3 different regression lines were not deviating much from each other.
The second experimental set-up was meant to secure that the previously monitored increase in oxygen saturation really arose from diffusion through the hollow fibers, as it could have been possible that there were small leaks within the chamber. For that reason the oxygen saturation over the mesh with half of its fibers occluded was assessed. The evaluation images taken after 50 seconds of O2 perfusion showed two separate areas over the fiber mesh with significantly different oxygen saturation perfusion (Fig. 4, bottom). In the top area a saturation of 43.8 % O2 was measured, whereas in the lower area only 26.1 % O2 saturation could be detected. The area with less saturation correlated with the occluded fibers´ location.
With the VisiSens system the oxygen saturation could be directly monitored at the site of diffusion, i. e. at the hollow fiber membrane surface. Three different measurements with the same experimental set-up led to comparable results. With the sensor system it was possible to detect the occluded fibers, not taking part in the oxygenation. Hence, the VisiSens might become an essential tool for the characterization and the evaluation of differently coated oxygenator hollow fiber membranes regarding their oxygenation capacity.