Page 128 - Clinical Application of Mechanical Ventilation

94 Chapter 4

TABLE 4-2 Characteristics of the Control Mode

Characteristic Description

Type of breath Each breath delivers a mechanical tidal
volume.
Triggering mechanism Every breath in the control mode is
time-triggered.

Cycling mechanism Inspiration is terminated by the delivery
of a preset tidal volume (volume-cycled).
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Complications of Control Mode

Since the patient’s spontaneous respiratory drive will have been blunted with seda-
In a sedated or apneic tion and neuromuscular block in the control mode, the patient is totally dependent
patient, the primary hazard
of the control mode is the on the ventilator for ventilation and oxygenation. As a result, the primary hazard
potential for apnea and associated with the control mode is the potential for apnea and hypoxia if the pa-
hypoxia if the patient should
become disconnected from tient should become accidentally disconnected from the ventilator or the ventilator
the ventilator or the ventilator should stop to operate due to mechanical or electrical failure.
should fail to operate.
Another physiologic problem with the control mode is the rapid disuse atrophy of
diaphragm fibers. One study shows that a combination of 18 to 69 hours of complete
diaphragmatic inactivity during mechanical ventilation results in marked atrophy of
human diaphragm myofibers (Levine et al., 2008). Furthermore, prolonged mechani-
cal ventilation leads to diaphragmatic oxidative injury, elevated proteolysis, and reduced
function of the diaphragm (Betters et al., 2004). Because of the patient’s dependence on
the ventilator, the most important alarms in the control mode become those that alert
the caregiver of any interruption in the patient’s ventilation. The important ventilator
alarms include the low exhaled volume alarm and the low inspiratory pressure alarm.
Table 4-2 summarizes the major characteristics of the control mode.

ASSIST/CONTROL (AC)

With the assist/control (AC) mode, the patient may increase the ventilator frequency
(assist) in addition to the preset mechanical frequency (control). Each control breath
provides the patient with a preset, ventilator-delivered tidal volume. Each assist
breath also results in a preset, ventilator-delivered tidal volume. The assist control
mode does not allow the patient to take spontaneous breaths (Figure 4-5).

Assist Control Triggering Mechanism

The mandatory mechanical breaths may be either patient-triggered by the patient’s
spontaneous inspiratory efforts (assist) or time-triggered by a preset frequency

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