Biomedical Engineering Reference
In-Depth Information
earlier polio epidemics, as well as other intensive medical cases, still used of the iron lung.
Lifecare Services, Inc., reported that as of January 1985 approximately 300 iron lungs
were still being used in the United States (Drinker and McKhann, 1986).
In the last few decades, the United States has been largely free of polio and has been
able to manage the rare occurrences with existing machines. As a result, J. H. Emerson
Co. ceased production of its respirator in 1970, and Respironics Colorado discontinued
repair, service, and parts on iron lungs as of March 2004.
According to a report from September 2004, there are an estimated 40 survivors of
polio still living in the iron lung (Nelson, 2004). On November 1, 2009, June Middleton
died in a Melbourne nursing home. She was 83 and had been confined to an iron lung for
60 years after being struck down by polio.
Despite wide-scale vaccination efforts by the World Health Organization, recent out-
breaks of poliomyelitis in Africa and South America have led health workers back to
negative-pressure ventilators as a possible life-saving technology for emergency cases of
paralytic poliomyelitis. In addition, several studies have reported benefits of intermittent
negative-pressure ventilation in patients with chronic respiratory failure due to chest wall
deformity, neuromuscular diseases, and central hypoventilation.
However, in recent years, negative-pressure ventilation has been used infrequently
for the management of patients with acute respiratory failure. In a review of the literature
on noninvasive ventilation, Hillberg and Johnson (1997) note that the role of negative-
pressure ventilation in the management of acute respiratory failure is unclear. In some
studies of the use of the body ventilator or poncho wrap for patients with acute respiratory
failure and chronic obstructive pulmonary disease (COPD), neuromuscular disease, or
chest wall deformity, some benefit from these devices has been found. However another
large randomized controlled trial found that negative-pressure ventilation had no benefit
(Shapiro, Ernst et al., 1992).
Negative-pressure ventilation has not been widely used because of poor acceptance
by patients, inadequate effectiveness for many patients, the awkward size of the devices,
and the development of upper airway obstruction in some. Patients with stable or slowly
progressive neuromuscular diseases, central hypoventilation, or chest wall deformities are
the best candidates for noninvasive negative-pressure ventilation.
Noninvasive ventilation is not effective in diseases that affect the upper airways, such
as amyotrophic lateral sclerosis. Additionally, patients with rapidly progressive neuro-
muscular processes like Guillian-Barre syndrome are also poor candidates. In general,
noninvasive ventilation should not be used in patients who are unable to cooperate or
who have impaired consciousness, problems with retained secretions, or hemodynamic
instability.
9.7
THE PHYSICS OF EXTERNAL
NEGATIVE-PRESSURE VENTILATION
External ventilation systems consist of two main components: the chamber and the pump
mechanism. The function of the chamber is to enclose either the whole body or the chest
cavity effectively so that negative pressure will result in an expansion of the chest and draw
air into the lungs. The pump must be capable of providing the correct negative pressure
level in a cyclical fashion to drive the forced ventilation function. Various parameters of
the pumps should be controllable to suit the differing requirements of a range of patients.
