Page 130 - Clinical Application of Mechanical Ventilation

96 Chapter 4

TABLE 4-3 Characteristics of the Assist/Control Mode

Characteristic Description

Type of breath Each breath, assist or control, delivers a preset mechanical tidal volume.

Triggering mechanism Mechanical breaths may be either patient-triggered (assist) or
time-triggered (control).

Cycling mechanism Inspiration is terminated either by the delivery of a preset tidal volume
(volume-cycled) or by the high pressure limit (pressure-cycled).
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Advantages of AC Mode

There are two primary advantages with the AC mode. First, the patient’s work of
breathing requirement in the AC is very small when the triggering sensitivity (pres-
sure or flow) is set appropriately and the ventilator supplies an inspiratory flow that
The AC mode allows
the patient to control the meets or exceeds the patient’s inspiratory flow demand. The second advantage of
frequency and therefore the AC is that, if the patient has an appropriate ventilatory drive, this mode allows the
minute volume required to
normalize the patient’s PaCO 2 . patient to control the frequency and therefore the minute volume required to nor-
malize the patient’s PaCO (Kirby et al., 1988).
2
Complications of AC Mode

The potential hazard associated with AC is alveolar hyperventilation (respiratory alka-
losis). In two separate studies, the pH was found to be higher and the PaCO was lower
2
intermittent mandatory in the AC mode than the results obtained in the intermittent mandatory ventilation
ventilation (IMV): IMV is a mode (IMV) mode (Culpepper et al., 1985; Hopper et al., 1985). If the patient’s respiratory
in which the ventilator delivers
control (mandatory) breaths and center is either injured or diseased, the patient may have an inappropriately high respi-
allows the patient to breathe
2
spontaneously to any tidal volume ratory drive leading to an excessive assist frequency despite a low PaCO . If the patient
the patient is capable of between is assisting at a high frequency (i.e., .20 to 25/min) and the tidal volume is preset at 10
the mandatory breaths.
to 15 mL/kg, this will usually result in hypocapnia and respiratory alkalosis.
Mechanical deadspace may be used in this situation, but it is generally considered
safer to switch the patient to another mode of ventilation (e.g., SIMV) that limits
the patient’s ability to generate excessive minute volumes.
Table 4-3 summarizes the major characteristics of the assist/control mode.

INTERMITTENT MANDATORY VENTILATION (IMV)

Intermittent mandatory ventilation (IMV) is a mode in which the ventilator delivers
Since IMV breaths are control (mandatory) breaths and allows the patient to breathe spontaneously at any
delivered at a frequency
independent of the patient’s tidal volume the patient is capable of in between the mandatory breaths (Figure 4-6).
spontaneous frequency, Historically, IMV was a separate circuit adapted to ventilators that were designed
breath stacking may occur.
to provide either assist/control or control mode ventilation. As such, it was the first
widely used mode that allowed partial ventilatory support (i.e., a mode that allowed

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