Gary D. Reeder

Development of a low flow resistance intravenous oxygenator.

A potentially attractive support device for patients with acute respiratory failure is an intravenous membrane oxygenator. One problem, however, is that the membrane surface area required for sufficient gas exchange can unduly increase vena cavai pressure drop and impede venous return. The purpose of this study was to design and develop an intravenous oxygenator that would offer minimal venous flow resistance in situ. The device uses a constrained fiber bundle of smaller cross sectional size than the vena cava so as to effect an intentional shunt flow of venous blood around the fiber bundle and reduce the venous pressure drop caused by the device. A pulsating balloon within the fiber bundle redirects part of this shunt flow into reciprocating flow in and out of the fiber bundle. This offers dual advantages: 1) Blood flow through the fiber bundle is mainly perpendicular to the fibers; and 2) the requisite energy for driving flow comes largely

Recent progress in engineering the Pittsburgh intravenous membrane oxygenator.

The University of Pittsburgh intravenous membrane oxygenator (IMO) is undergoing additional engineering development and characterization. The focus of these efforts is an IMO device that can supply as much as one-half basal O2 consumption and CO2 elimination rates while residing within the inferior and superior vena cavae after peripheral venous insertion. The current IMO design consists of a bundle of hollow fiber membranes potted to manifolds at each end, with an intra-aortic type balloon integrally situated within the fiber bundle. Pulsation of the balloon using helium gas and a balloon pump console promotes fluid and fiber motion and enhances gas exchange. During the past year, more than 15 IMO prototypes have been fabricated and extensively bench tested to characterize O2 gas exchange capacity, balloon inflation/deflation over relevant frequency ranges, and the pneumatics of the sweep gas pathway through the device. The testing has led to several engineering changes, including redesign of the helium and sweep gas pathways within the IMO device. As a result, the maximum rate of balloon pulsation has increased substantially above the previous 70 bpm to 160 bpm, and the vacuum pressure required for sufficient sweep gas flow has been reduced. The recent IMO prototypes have demonstrated an O2 exchange capacity of as much as 90 ml/min/m2 in water, which appears within 70% of our design goal when extrapolated to scaled up devices in blood.

Respiratory dialysis. A new concept in pulmonary support.

Use of a new intravenous oxygenator made of hollow fiber membranes arranged around a centrally positioned balloon is reported. In vitro studies using fluorescent image tracking velocimetry and gas exchange analysis demonstrated enhanced convective mixing with balloon pulsations and augmented gas flux (100% increase in pO2) compared with the device in its static configuration. In vivo observations confirmed a greater than 50% increase in O2 flux with balloon activation. Those parameters that produce radial flow and convective mixing in vitro enhance gas flux in vivo, thus confirming the efforts to exceed the fluid limit translate into improved gas exchange.

Acute in vivo studies of the Pittsburgh intravenous membrane oxygenator.

The efficacy of an innovative intravenous membrane oxygenator (IMO) was tested acutely (6-8 hrs) in seven calves. The IMO prototypes consisted of a central polyurethane balloon within a bundle of hollow fibers with a membrane surface area of 0.14 m2. The IMO devices were inserted through the external jugular vein into the inferior vena cava of anesthetized calves (68.9 +/- 2.3 kg). Rhythmic balloon pulsation (60-120 bpm) was controlled with an intra-aortic balloon pump console. Oxygen sweep gas was delivered through the device at 3.0 L/min. Gas concentrations were monitored continuously by mass spectroscopy. The principal results were as follows: 1) oxygen and carbon dioxide exchange ranged from 125 to 150 ml/min/m2 and 150 to 200 ml/min/m2, respectively; 2) there was at least a 30-50% augmentation of gas exchange with balloon pulsation; 3) maximum exchange occurred with 60-90 bpm balloon pulsations; and 4) hemodynamic parameters remained unchanged. There were no device related complications, and the feasibility of insertion of the device by a cervical cut-down was established. These acute in vivo experiments show that the Pittsburgh IMO device can exchange oxygen and carbon dioxide gases in vivo at levels consistent with this current prototype design, and that intravenous balloon pulsation significantly enhances gas exchange without causing any end-organ damage.

Temporary Support of the Lungs - the Artificial Lung

The adult respiratory distress syndrome (ARDS), described first by Ashbaugh and his colleagues[1] almost 30 years ago, continues to present a dilemma to the clinician, for the mortality associated with the syndrome has remained exceedingly high in spite of a better understanding of the mechanisms that contribute to this pulmonary disease process. For the patient with ARDS, maintaining gas exchange at a level consistent with survival is associated frequently with progressively increasing levels of respiratory support. The final outcome in these patients is contributed to not only by the degree of pulmonary injury, but also by the level of dysfunction in multiple other organs=. Recognizing these facts, and that any therapy for ARDS involves more than just providing a setting where the lungs have the potential for recovery, we will concentrate this discussion on new therapies with intracorporeal devices under development for the treatment of ARDS.