Page 117 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 83

Since a pressure gradient is needed to generate gas flow and volume, mechanical ven-
tilators achieve this condition by creating either a negative or positive pressure gradient.

Negative Pressure Ventilation

Negative pressure ventilation creates a transairway pressure gradient by decreas-
ing the alveolar pressures to a level below the airway opening pressure (i. e., below
the atmospheric pressure). Unless airway obstruction is present, negative pressure
ventilation does not require an artificial airway. Two classical devices that provide
negative pressure ventilation are the “iron lung” and the chest cuirass or chest shell.
Iron Lungs. An “iron lung” ventilator encloses the patient’s body except for the head
and neck in a tank, and the air in it is evacuated to produce a negative pressure
around the chest cage. This negative pressure surrounding the chest and underlying
alveoli results in chest wall and alveolar expansion. The tidal volume delivered to

The tidal volume deliv- the patient is directly related to the negative pressure gradient. For example, a more
ered by a negative pressure negative pressure applied to the chest wall will yield a larger tidal volume. Since
ventilator is directly related to
the negative pressure negative pressure ventilation does not require tracheal intubation, this noninvasive
gradient. method of ventilation has been used extensively and successfully to support chronic
ventilatory failure (Corrado et al., 1994; Frederick, 1994).
Disadvantages and complications associated with the iron lung type of negative
pressure ventilator are (1) poor patient access and (2) potential for a decreased car-
diac output known as “tank shock” (Frederick, 1994).
Since the iron lung encloses the patient, it restricts access to the patient for rou-
tine health care. Tank shock may result from a decreased venous blood return to the
right atrium. Normally, the heart and vena cava are surrounded by negative pleural
pressure, while the remainder of the vascular system outside the thorax is subjected
to atmospheric pressure. This creates a vascular pressure gradient between the vena
cava and the venous drainage that enhances venous blood return to the right atrium.
However, if a patient is placed in an iron lung, this vascular pressure gradient is lost
because the peripheral vasculature is subjected to negative pressures that closely ap-
proximate the pleural pressure. This results in a potential decrease in venous return
that could lead to a decreased cardiac output.
Chest Cuirass. The chest cuirass or chest shell (see Figure 18-1) is a form of negative
pressure ventilation that was intended to alleviate the problems of patient access
and tank shock associated with iron lungs. This shell device covers only the pa-
tient’s chest and leaves the arms and lower body exposed. Although the chest shell
improves patient access and decreases the potential for tank shock, ventilation with
this device may be limited by the difficulties in maintaining an airtight seal be-
tween the shell and the patient’s chest wall (Newman et al., 1988).
To overcome the problem of air leakage, individually designed cuirass “respira-
tors” minimize air leaks, and they have been used successfully to ventilate patients
with chest wall diseases such as scoliosis (Kinnear et al., Hockley, 1988; Kinnear
et al., Petch, 1988). Because of the availability of positive pressure ventilators, chest
cuirass ventilators are seldom used in an acute care facility. However, they are rather

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