Paracorporeal Lung Devices: Thinking Outside the Box

Abstract

Extracorporeal Membrane Oxygenation (ECMO) is a resource intensive, life-preserving support system that has seen ever-expanding clinical indications as technology and collective experience has matured. Clinicians caring for patients who develop pulmonary failure secondary to cardiac failure can find themselves in unique situations where traditional ECMO may not be the ideal clinical solution. Existing paracorporeal ventricular assist device (VAD) technology or unique patient physiologies offer the opportunity for thinking "outside the box." Hybrid ECMO approaches include splicing oxygenators into paracorporeal VAD systems and alternative cannulation strategies to provide a staged approach to transition a patient from ECMO to a VAD. Alternative technologies include the adaptation of ECMO and extracorporeal CO₂ removal systems for specific physiologies and pediatric aged patients. This chapter will focus on: (1) hybrid and alternative approaches to extracorporeal support for pulmonary failure, (2) patient selection and, (3) technical considerations of these therapies. By examining the successes and challenges of the relatively select patients treated with these approaches, we hope to spur appropriate research and development to expand the clinical armamentarium of extracorporeal technology.

Keywords: VAD; hybrid; lung assist; oxygenator; paracorporeal.

Figure 1

Examples of a short path (A) and long path (B) oxygenator design. Streamlines denote the potential path of blood cells through the oxygenator. The longer the path line, the higher the pressure drop (and thus higher resistance) of the oxygenator.

Figure 2

General flow path of blood through the Quadrox iD and NovaLung iLA devices (left). These devices employ an orthogonal flow path for the other medium (right). Blood flow is around the fibers while gas flow is through polymethylpentene fibers (Quadrox and NovaLung) and water flows through a separate set of polypropylene fibers (Quadrox only) in the right half of the oxygenator. The fibers are separated by a plastic divider (red vertical line in the left image).

Figure 3

VADs used in VAD+Oxy configurations. (A) In a centrifugal pump, blood flow is directed inward from the inlet, accelerated circumferentially by the impeller, and then expelled along the axial line of the outlet. (B) The pressure change across a typical centrifugal pump is fixed by the impeller speed. The resultant flow is therefore a function of the inlet and outlet resistance to flow. (C) A pneumatically driven paracorporeal VAD has an internal blood-filled sac compressed externally by air forced between it and the housing. One-way valves create unidirectional flow from the pump. Adapted with permission from ASME (35).

Figure 4

Configurations for paracorporeal oxygenators. (A) Centrifugal pump with oxygenator in the shunt line [modified with permission from Annals of Thoracic Surgery (20)]. (B) Pulsatile pump with oxygenator and shunt line [reproduced with permission from the Journal of Extracorporeal Technology (21)]. (C) Pumpless oxygenator configuration with return through ASD.

Figure 5

Graphical representation of an ECCO₂R device with renal support (top). The oxygenator is typically placed in series with the hemofilter to increase the resistance through the oxygenator and reduce the chance for bubble formation. The HemoLung (bottom) is an out-of-the-box ECCO₂R device with a unique active mixing feature to improve mass transfer at the fiber surface.