Effects of hollow fiber packing fraction on blood flow pattern and gas transfer rate of an intravascular oxygenator (IVOX)

Supplemental oxygenation and carbon dioxide removal through an intravenous respiratory assist catheter can be used as a means of treating patients with acute respiratory failure. We are beginning development efforts toward a new respiratory assist catheter with an insertional size <25F, which can be inserted percutaneously. In this study, we evaluated fiber bundle rotation as an improved mechanism for active mixing and enhanced gas exchange in intravenous respiratory assist catheters. Using a simple test apparatus of a rotating densely packed bundle of hollow fiber membranes, water and blood gas exchange levels were evaluated at various rotation speeds in a mock vena cava. At 12,000 RPM, maximum CO2 gas exchange rates were 449 and 523 mL/min per m 2 , water and blood, respectively, but the rate of increase with increasing rotation rate diminished beyond 7500 RPM. These levels of gas exchange efficiency are two- to threefold greater than achieved in our previous respiratory catheters using balloon pulsation for active mixing. In preliminary hemolysis tests, which monitored plasma-free hemoglobin levels in vitro over a period of 6 hours, we established that the rotating fiber bundle per se did not cause significant blood hemolysis compared with an intra-aortic balloon pump. Accordingly, fiber bundle rotation appears to be a potential mechanism for increasing gas exchange and reducing insertional size in respiratory catheters. ASAIO Journal 2007; 53:368‐373.

/pdf/evaluation-of-fiber-bundle-rotation-for-enhancing-gas-2cetb6bmw4.pdf

Evaluation of fiber bundle rotation for enhancing gas exchange in a respiratory assist catheter.

Patients experiencing acute respiratory failure or acute exacerbations of chronic respiratory failure (e.g., chronic obstructive pulmonary disease [COPD]) can potentially be supported through intravenous respiratory assist devices.1–4 This next generation of artificial lung support, under development but not in clinical use, provides supplemental oxygenation or removal of carbon dioxide through bundles of microporous hollow fiber membranes (HFMs). This technique involves placing the HFM bundle into the vena cava through a peripheral vein (e.g., femoral vein) and connecting the fibers to an external oxygen source.5 Diffusion gradients are created when pure oxygen flows through the fibers, causing O2 diffusion into the blood stream and CO2 diffusion out of the blood (through the gas permeable fiber wall) and into the exhaust gas (mixture of O2 and CO2) exiting the device. Placement within the vena cava provides time for the natural lungs to heal because the oxygenation and CO2 removal is independent of the lungs, which is a benefit over mechanical ventilation,6,7 the most commonly used therapy. Intravenous respiratory assistance is less complicated than extracorporeal respiratory support because an external blood circuit and pump are not required.

Our group is developing an intravenous respiratory assist catheter to support patients with acute lung failure by improving oxygen delivery and CO2 removal.8–12 In its application in patients with COPD, the catheter focuses upon CO2 removal because oxygenation can be adequately addressed through nasal O2.13 Our catheter, seen in Figure 1, uses microporous hollow fiber membranes wrapped into a bundle around a pulsating balloon. Pulsation of this centrally located balloon at increasing frequencies creates higher blood velocities past the fiber surfaces than would exist with a passive device. This generates an active mixing environment, in turn facilitating greater gas transfer, subsequently increasing the CO2 removal capacity of the device.14 Our previous bench, ex vivo, and acute animal testing of our pulsating device5,10,12 have confirmed that enhanced gas exchange is possible with balloon pulsation10 when compared with a nonpulsating device, the IVOX.3,4,15–17



Figure 1

Catheter schematic including cross‐sections of pathway tubing and balloon/hollow fiber section.



The balloon within our respiratory assist catheter does not collapse and inflate axisymmetrically. Rather, the balloon collapses asymmetrically into a crescent shape, and the fibers around the balloon move similarly with balloon pulsation in an asymmetric manner. The fibers adjacent to the convex region of the crescent shaped balloon collapse are more stationary during balloon pulsation than the other fiber regions. We hypothesized that asymmetric balloon collapse and inflation within the fiber bundle might cause nonuniform flow patterns and nonuniform gas exchange in different regions of the fiber bundle. We also hypothesized that gas exchange may differ from fibers on the outside of the fiber bundle compared with fibers on the inside of the bundle near the pulsating balloon. To test these hypotheses, we evaluated the local carbon dioxide gas exchange for individual fiber bundle quarters around the central balloon and for inner and outer fiber rings around the balloon (i.e., the inner layer of the fiber bundle closest to the balloon and the outer most layers away from the balloon).

Evaluation of Local Gas Exchange in a Pulsating Respiratory Support Catheter

We are developing an intravenous respiratory assist catheter, which uses hollow-fiber membranes wrapped around a pulsating balloon that increases oxygenation and CO 2 removal with increased balloon pulsation. Our current pulsation system operates with a constant rate of pulsation and delivered balloon volume. This study examined the hypothesis that random balloon pulsation would disrupt fluid entrainment within the fiber bundle and increase our overall gas exchange. We implemented two different modes for random (rates and delivered volume) versus constant pulsation. The impact on gas exchange was measured in a 3 1/min water flow loop at 37°C. CO 2 gas exchange for randomized beat rate mode was comparable to its corresponding average constant pulsation (e.g., constant 286 beats/min versus randomized 200-400 beats/min was 299.5 ± 0.9 and 302.2 ± 1.4 ml/ min/m 2 , respectively). Random volume mode CO 2 exchange was also comparable to constant delivered balloon volume (100% inflation and deflation) (e.g., 294.3 ± 0.6 and 301.1 ± 1.7 ml/min/m 2 , random 50-100% inflation and constant, respectively). Greater active mixing was seen with constant pulsation as compared with randomly changing the parameters of balloon pulsation.

Investigating the effects of random balloon pulsation on gas exchange in a respiratory assist catheter.

Carbon dioxide transport by blood is often overlooked when considering the design and clinical potential of cell-free O2 carriers. However, approximately the same amount of CO2is removed from respiring tissue as O2delivered to, and the implications for blood substitutes must be considered, especially in critically ill patients where tissue CO2 build-up could be very high. Approximately 23% of total CO2 is transported as carbamate (i.e. bound to hemoglobin) and is “oxylabile” (the affinity of deoxy hemoglobin for CO2 is higher than that of oxyhemoglobin). In addition to this important role of hemoglobin, red cells are also critical to overall CO2 transport because they contain carbonic anhydrase which permits the rapid hydration of CO2 to bicarbonate and hydrogen ion. Without this enzyme, the hydration reaction would be much slower than the circulation time. Except for αα-hemoglobin, which has reduced CO2 binding, little is known about the effects of crosslinking on CO2 binding. It is of interest to consider how CO2 transport is handled naturally by underwater (crocodiles) and high altitude animals (sheep and goats) who are faced with O2 shortages in nature.

Carbon Dioxide Transport by Hemoglobin-Based Blood Substitutes

This open clinical study was aimed at testing the hypothesis that an intravascular oxygenator (IVOX) may help to perform permissive hypoventilation in 10 patients with severe ARDS. After initial evaluation, we tried to reduce ventilator settings before and after IVOX implantation. Before IVOX, poor clinical tolerance and worsening oxygenation did not allow for a significant decrease in ventilator settings. With IVOX, peak inspiratory pressure (PIP) was reduced from 47 to 39 cm H2O (p = 0.005) and minute ventilation from 13 +/- 3.5 to 11 +/- 3 L/min. CO2 removal by IVOX allowed a significant decrease in PaCO2 from 66 +/- 15 to 59 +/- 13 mm Hg. Improvement of oxygenation with IVOX was not significant. Furthermore, interruption of oxygen flow through IVOX did not change oxygenation variables. Tolerance of the IVOX device was good, but insertion of the device was followed by a significant decrease in both cardiac index and pulmonary wedge pressure. In conclusion, IVOX improves tolerance of hypoventilation by limiting hypercapnia in ARDS patients. These preliminary results must be confirmed by a randomized controlled study.

Permissive Hypercapnia and Intravascular Oxygenator in the Treatment of Patients with ARDS

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Permissive Hypercapnia and Intravascular Oxygenator in the Treatment of Patients with ARDS