Am. J. Respir. Crit. Care Med., Vol 149, No. 5, 05 1994, 1354-1358.
Should inverse ratio ventilation be used in adult respiratory distress syndrome?
C Shanholtz and R Brower
Department of Medicine, Pulmonary and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland.
IRV-induced increases in MAP are clearly associated with shunt reduction,
but we find no studies that show shunt reduction without increased
end-expiratory alveolar pressure. On the other hand, various studies in
humans with ARDS and hyaline membrane disease and animal models of acute
lung injury indicate that shunt reduction does not occur with IRV if there
is no increase in end-expiratory alveolar pressure (21), that shunt
reduction is the same with IRV as with conventional ventilation with PEEP
when there are comparable levels of end-expiratory volume or alveolar
pressure (16, 32), and that shunt reduction is greater when MAP is raised
with PEEP than with IRV (27). Improved ventilation-perfusion matching with
IRV is theoretically unlikely and, given the high FIO2 used in ARDS,
improvements in oxygenation from more even ventilation would not be great.
Deadspace reduction by extended inspiratory phase ventilation may allow
only minor improvements in gas exchange (14-16, 21, 25, 26, 34). Thus,
there is little evidence to indicate that oxygenation can be maintained or
improved with IRV while volotrauma risk is reduced. Some have suggested
that IRV may promote gradual shunt reduction over hours or days, and that
slower inspiratory airflow may reduce injurious parenchymal shear forces.
However, these potentially salutary effects of IRV are unproven. On the
other hand, there are potential deleterious effects of IRV, including
increased risk of volotrauma and the requirements for heavy sedation and
neuromuscular blockage. IRV remains of unproven value in the management of
ARDS.
