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The past three decades have seen
immense advances in the technologies used to replace the functions
of failing organs or systems. During this time, mechanical ventilation
has evolved from being used only as a last rite, when there was
little hope of survival, to being a widely used and versatile technique
that allows thousands of patients to recover from respiratory failure
every year. Extracorporeal membrane oxygenation (ECMO), reintroduced
into neonatal and pediatric intensive care medicine in the 1980s,
is rapidly reaching similar status, as its indications and, perhaps more
importantly, its limitations are better defined. More recently,
external and implantable ventricular assist devices have undergone
sufficient development to become viable alternatives for the continued support
of patients with severe circulatory dysfunction, often as a bridge
toward cardiac transplantation. While these technologies offer many
possibilities, intensive care specialists must use them wisely by
carefully selecting the patients who can benefit, by providing patients
and families with realistic assessments of their potential, and
by carefully evaluating the results to define better indications and
to improve efficacy.
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Today, any modern intensive care unit has at its disposal a wide
array of ventilator models, many of them capable of effectively
ventilating the lungs of adults as well as those of premature infants.
Although these devices certainly make the task easier, their application
to the treatment of infants and children requires more than a passing
knowledge of respiratory physiology—it requires careful
attention to monitoring the patient’s responses.
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All commonly used methods of mechanical ventilation are based
on the same principle: A mechanical device forces a gas mixture
into the lungs during inspiration and then allows the passive recoil
of lungs and chest wall to force gas out of the lungs during expiration.
In most cases, the device uses a bellows or other type of pneumatic
mechanism to inject a gas mixture into the airway lumen, either
through a cannula inserted in the trachea or through a mask applied
around the mouth and nose (positive-pressure ventilation). More
rarely, the device lowers pressure around the patient’s
chest and abdomen (negative-pressure ventilation), decreasing pleural
and alveolar pressures relative to atmospheric pressure and drawing
gas into the alveoli. Usually, a cannula must be placed in the trachea
to prevent subatmospheric airway pressures from collapsing the pharynx
and larynx during inspiration. Contrary to common misconception,
positive- and negative-pressure ventilation apply similar strains
to the lung tissue for a given level of lung inflation.3 Thus,
it seems there would be no difference in the degree of lung injury
caused by both methods, provided that tidal volume and functional
residual capacity are kept equal. This consideration is, of course,
limited by the practicality of its assumptions. Without entering
a several-decade-long controversy on which method of ventilation
is better,4 it seems clear that, by reducing rather
than increasing pleural pressure relative to the atmosphere, negative-pressure
ventilation may have a more beneficial effect on venous return to
the heart than positive-pressure ventilation.5...