Page 129 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 95

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Figure 4-5 Assist/control mode pressure tracing. Each assisted or controlled breath triggers
a mechanical tidal volume. (A) An assisted breath; note the negative deflection at the beginning
of inspiration. (B) Another assisted breath that is initiated by the patient sooner than (A). (C) A
controlled breath; note the absence of a negative deflection at the beginning of inspiration.

(control). If a breath is patient-triggered, it is referred to as an assisted breath; if a
breath is time-triggered, the breath is referred to as a control breath.
For example, if the patient has a stable assist frequency of 12/min, then the pa-
tient is triggering breaths every 5 sec. If the control frequency is preset at 10/min,
the ventilator would deliver time-triggered breaths every 6 sec. However, since the
interval between the assisted breaths is shorter than 6 sec, no time-triggered breaths
will be delivered. If however, the patient’s spontaneous breaths were to decrease less
than the preset control frequency, then the ventilator would begin delivering time-
triggered breaths.

Assist Control Cycling Mechanism

Inspiration in the AC mode is terminated by volume cycling. When the preset tidal
volume is delivered, the ventilator is cycled to expiration.

Indications for AC Mode

The AC mode is most often used to provide full ventilatory support for patients
when they are first placed on mechanical ventilation. Full ventilatory support is
defined as any ventilator mode that provides all of the work of breathing.
The AC mode is typically used for patients who have a stable respiratory drive
(a stable spontaneous frequency of at least 10 to 12/min) and can therefore trigger
the ventilator into inspiration. Essentially, the time-triggering control frequency is
generally considered as a safety net to provide adequate ventilation in the event that
the patient stops triggering the ventilator at an acceptable frequency (Sassoon et al.,
1990). The generally accepted minimum control frequency in the AC mode is 2 to
4/min less than the patient’s assist frequency, or a minimum control frequency of
from 8 to 10/min.

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