Page 116 - Biomedical Engineering and Design Handbook Volume 2, Applications

OVERVIEW OF CARDIOVASCULAR DEVICES 95

patient management issues with a significant impact on morbidity and mortality, such as infection
control and thromboembolism, as well as device issues affecting quality of life and durability.

3.9 ARTIFICIAL LUNG

3.9.1 Market Size
According to the National Hospital Discharge Survey, approximately 220,000 membrane oxygena-
tors were used in the United States in 2005 (DeFrances et al., 2007) for acute surgical cardiopul-
monary bypass, representing over a 25 percent drop from the 300,000 used in 1997 (Owings and
Lawrence, 1999). The number of oxygenators required for acute cardiopulmonary bypass use still
dwarfs the number used for extended support of the failing lung, a modality termed extracorporeal
membrane oxygenation (ECMO). Changing ECMO demographics suggest new developments in
alternative medical therapies might be causing a reduction in the number of neonatal patients sup-
ported via ECMO therapy, a standard of care for respiratory support for decades (Roy et al., 2000).

3.9.2 Indications
The indications for cardiopulmonary bypass are surgical in nature, and are based on whether the pro-
cedure requires the heart to be stopped. Currently, most cardiac surgical procedures fall into this cat-
egory (McGiffin and Kirklin, 1995). Cardiopulmonary bypass provides the surgeon with a stable,
blood-free field to perform intracardiac repairs and procedures such as coronary artery bypass graft-
ing. Recent trends in minimally invasive surgery have led to surgical systems that allow some pro-
cedures such as coronary artery bypass grafting to be performed in the absence of oxygenator
support (Svennevig, 2000).
In contrast to cardiopulmonary bypass, medical criteria are the primary indicators for ECMO sup-
port. Conventional treatment of acute respiratory failure calls for high-pressure mechanical ventila-
tion with an elevated percentage of oxygen in the ventilation gas. Unfortunately, the high oxygen
concentration can result in oxidative damage to lung tissue (oxygen toxicity) and, in the case of the
newborn, proliferation of blood vessels in the retina leading to visual damage (retinoproliferative dis-
order) (Anderson and Bartlett, 2000). The high pressures used to achieve maximum ventilation area
also cause lung damage through a process known as barotrauma. In essence, the lungs are being sub-
jected to further damage by the therapy employed, preventing the healing necessary to restore proper
lung function. The purpose of ECMO is to take over the burden of gas exchange and allow the native
lung tissue time to heal.
ECMO is considered a standard therapy for the treatment of respiratory failure in neonatal
patients (Anderson and Bartlett, 2000). In adult and pediatric patients, it is a treatment of last resort
for individuals who would otherwise die despite maximal therapy (Anderson and Bartlett, 2000;
Bartlett et al, 2000). Even in neonatal cases, ECMO is a therapy reserved for those patients with
severe respiratory compromise and a high risk of death who are failing traditional ventilator-based
interventions. Common causes of respiratory failure in the neonatal population that are treatable with
ECMO support include pneumonia or sepsis, meconium aspiration syndrome, respiratory distress
syndrome, persistent fetal circulation, and congenital diaphragmatic hernia (Anderson and Bartlett,
2000). Contraindications to ECMO support include root causes that are unresolvable, such as a major
birth defect or genetic abnormality, and comorbid conditions such as intracranial hemorrhage or fetal
underdevelopment that suggest a poor outcome (Anderson and Bartlett, 2000). Indications for
ECMO use in the pediatric and adult populations are not dissimilar from those of the neonate, but
the causes for respiratory or cardiopulmonary failure are different, and many individuals suffer from
comorbid conditions. Indications include pneumonia, aspiration pneumonitis, acute respiratory dis-
tress syndrome, and recoverable heart failure as caused by infection or postsurgical complications
(Anderson and Bartlett, 2000). Despite the current limited application of ECMO in adults, a