Page 86 - Clinical Application of Mechanical Ventilation

52 Chapter 3

VENTILATOR CLASSIFICATION

Ventilator technology has evolved since the introduction of Engström 100, the first
volume-controlled mechanical ventilator in 1951. Since that time, a multitude of
manufacturers have produced and marketed ventilators of all sizes, descriptions,
and capabilities. Many manufacturers have coined new terms to describe their ven-
tilators and to accentuate how their product is different from the others. Several
different ventilator classification systems may be employed to describe mechanical
ventilators. The majority of these systems focus on the differences between ventila-
tors rather than the similarities.
Robert Chatburn (1992, 2007) has proposed a new way to classify mechanical
ventilators based on related features, physics, and engineering. Chatburn’s venti-
lator classification system has been featured in several articles and textbooks. It
allows flexibility as ventilator technology evolves in contrast to other systems that
employ more narrowly defined design principles or rely to a greater extent on
manufacturer’s terms.
With the evolution of ventilator technology over the next decade or more, the
flexibility of Chatburn’s classification system will be validated, as it is increasingly
adopted by practitioners. This author believes this system is important enough to
include in this text and in others that describe ventilator operational characteristics.
Students and practitioners learning about this classification system should refer to
the References section at the end of this chapter and read Chatburn’s original con-
tributions (Chatburn, 1991, 1992, 2001, 2007).

Ventilatory Work

Pulmonary physiologists have described the work ventilatory muscles perform
during inspiration, and how muscles can actively assist during exhalation. Dur-
ing inspiration, the primary ventilatory muscles cause the size (volume) of the
thoracic cage to increase, overcoming the elastic forces of the lungs and thorax
and the resistance of the airways. As the volume of the thoracic cage increases,
intrapleural pressure becomes more negative, resulting in lung expansion, as the
visceral pleura expands with the parietal pleura. Gas flows from the atmosphere
into the lungs as a result of the transairway pressure gradient. During expiration,
the muscles of inspiration relax. The elastic forces of the lung and thorax cause the
chest to decrease in volume. Exhalation occurs as a result of the greater pressure
at the alveolus when compared to atmospheric pressure. All of this muscle activity
to overcome the elastic and resistance properties of the lungs and thorax requires
energy and work.
The work that the muscles and/or the ventilator must perform is proportional to
the pressure required for inspiration times the tidal volume. The pressure required to
deliver the tidal volume is referred to as the load either the muscles or the ventilator
must work against. There is an elastic load (proportional to volume and inversely
proportional to compliance) and a resistance load (proportional to airway resistance

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