Hamilton G5 - Introduction; C.1 Introduction

C-2 624074/07
CASV
®
(Adaptive Support Ventilation)
C.1 Introduction
WARNING
This appendix describes ASV as it is implemented
in the HAMILTON-G5 device. It does not replace the
clinical judgment of a physician and is not to be
used for clinical decision making.
In 1977, Hewlett et al. introduced mandatory minute volume
(MMV). “The basic concept is that the system is supplied with
a metered, preselected minute volume of fresh gas, from
which the patient breathes as much as he is able, the remain-
der being delivered to him via a ventilator. Thus the patient is
obliged to breathe, one way or the other, a Mandatory Minute
Volume MMV” (Hewlett 1977).
Since then, many ventilators have included versions of MMV
under different names. However, all commercially available
MMV algorithms have clear limitations, which lead to certain
risks for the patient (Quan 1990). These include rapid shallow
breathing, inadvertent PEEP creation, excessive dead space
ventilation, and inadvertent wrong operator settings due to
very complicated use.
Adaptive Support Ventilation (ASV
®
) was designed to minimize
those risks and limitations. ASV maintains an operator-preset,
minimum minute ventilation independent of the patient‘s
activity. The target breathing pattern (tidal volume and rate) is
calculated using Otis’ equation, based on the assumption that
if the optimal breath pattern results in the least work of breath-
ing, it also results in the least amount of ventilator-applied
inspiratory pressure when the patient is passive. Inspiratory
pressure and machine rate are then adjusted to meet the
targets. A lung protection strategy ensures ASV’s safety. In
contrast to MMV, ASV attempts to guide the patient using a
favorable breathing pattern and avoids potentially detrimental
patterns like rapid shallow breathing, excessive dead space
ventilation, breath stacking (inadvertent PEEP), and excessively
large breaths.