Page 102 - Clinical Application of Mechanical Ventilation

68 Chapter 3

gases and to prevent atelectasis. The patient demand and pathology (resistance and
compliance) determines the volume delivered and the spontaneous frequency.

Dual Control within a Breath

Dual control within a breath implies that two variables become control variables
during inspiration within the same breath. During dual control within-a-breath
modes, the ventilator switches from pressure-controlled to volume-controlled. The
clinician sets a desired tidal volume, which becomes a volume target during the
breath. The ventilator begins the breath as a pressure controller, delivering a con-
stant pressure initially during the breath. During breath delivery, tidal volume is
measured and the pressure is adjusted automatically by the ventilator to maintain
the guaranteed tidal volume (volume control). Dual control within-a-breath modes
establish a high initial inspiratory flow (pressure-controlled breath), and a taper or
plateau in flow as the volume target is met. Examples of this mode include pressure
augmentation and volume-assured pressure support.

Dual Control Breath-to-Breath

Dual control breath-to-breath modes allow the clinician to set a volume target,
and the ventilator delivers pressure-controlled breaths attempting to achieve the
desired target tidal volume. The ventilator may operate in either pressure support
or pressure-controlled mode, with the pressure limit increasing or decreasing to
achieve the desired volume target (Branson et al., 2004).

Pressure-Limited Time-Cycled Breaths

Pressure-limited time-cycled breaths begin inspiration as pressure-limited
breaths (pressure increases to a set value or target), and they are time-cycled
(inspiration ends at a specified time interval). The clinician sets a target tidal
volume and maximum pressure (pressure limit). The ventilator delivers a test
breath and calculates the patient’s airway resistance and lung compliance. Once
resistance and compliance have been determined, pressure increases or decreases
automatically to reach the desired volume target. Pressure is adjusted in incre-
ments of 1 to 3 cm H O at a time between breaths, until the maximum pressure
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is reached or a set level below the upper pressure limit. If the desired volume is
not met, an alarm alerts the clinician to the fact and the upper pressure limit
is never exceeded. Examples of dual control breath-to-breath pressure-limited,
time-cycled modes include volume control plus (VC1) and pressure-regulated
volume control (PRVC).

Pressure-Limited Flow-Cycled Breaths

Pressure-limited flow-cycled breaths start as a pressure-support breath with a tar-
get tidal volume. Inspiration is flow-cycled (inspiration ends when inspiratory flow

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