Am. J. Respir. Crit. Care Med.,
Volume 165, Number 7, April 2002, 972-977
Expiratory Asynchrony in Proportional
Assist Ventilation
Hong-Lin
Du,
Mikiya
Ohtsuji,
Masaki
Shigeta,
David C.
Chao,
Katsunori
Sasaki,
Yutaka
Usuda,
and
Yoshitsugu
Yamada
Clinical Research Department, Newport Medical Instruments Inc., Newport Beach, California; Department of Anesthesiology, The University of
Tokyo Hospital, Tokyo, Japan; Emergency Life-saving Technique Academy of Tokyo, Tokyo, Japan; Barlow Respiratory Research Center,
Los Angeles, California; and Department of Anesthesiology, Yokohama City University School of Medicine, Yokohama, Japan
One of the proposed advantages of proportional assist ventilation
(PAV) has been the automatic synchrony between the end of the
patient's inspiratory effort and the ventilator cycle (i.e., expiratory
synchrony). However, recent clinical studies have shown a prolonged
ventilator inspiratory time or even a "runaway" phenomenon with
the normal use of PAV. We hypothesize that control-system delay
may account for this, because in reality there is always some degree of delays between control-system's input and output in all
ventilators. Computer simulation study to date has not taken into
account the potential effect of control-system delay on expiratory
synchrony. We therefore created a computer model in which the
parameter of control-system delay time was introduced. We found
that significant expiratory asynchrony may occur with this more
realistic model of PAV. The ventilator flow termination may fall behind the completion of the patient inspiration by as long as 0.33 seconds under the selected simulation conditions. The inspiratory
termination delay time is in proportion to the control-system delay time, the respiratory time constant, and the assist gain settings.
In conclusion, this model indicates that due to the unavoidable
control-system delay in the ventilators, expiratory asynchrony may
be an inherent shortcoming associated with PAV.
