Page 103 - Clinical Application of Mechanical Ventilation

Classification of Mechanical Ventilators 69

falls to predetermined value). The clinician sets a volume target, PEEP and pres-
sure limit. The breath begins as a pressure-controlled (pressure-support) breath and
the ventilator measures the tidal volume delivered. If the tidal volume falls below
the target level, inspiratory pressure is increased on the next breath to attempt to
achieve the target. As in pressure support, the breath ends when inspiratory flow
decays to a percentage of the peak flow. If the patient becomes apneic, the ventilator
switches to a volume-controlled backup ventilation mode. This mode is available on
the Maquet Servo-i as Volume Support Ventilation (VSV).

Automode

Automode combines PRVC and volume support into a single mode (Branson
et al., 2004). In the absence of spontaneous patient effort, the ventilator de-
livers mandatory breaths using a time-triggered, pressure-limited, time-cycled
mode, adjusting the pressure limit to maintain the clinician-set tidal volume.
If the patient breathes spontaneously for two consecutive breaths, the ventilator
will switch to volume-support ventilation (VSV) where the breaths are patient-
triggered (pressure or flow), pressure-limited, and flow-cycled. If the patient be-
comes apneic (12 seconds for adults, 8 seconds for pediatrics), the ventilator will
switch back to PRVC mode.

Proportional Assist Ventilation

Proportional assist ventilation (PAV) is a mode in which the ventilator will
proportionally assist the patient’s spontaneous ventilation. The ventilator does
so by proportionally amplifying the delivered pressure (pressure support) in
proportion to the measured inspiratory flow and volume (Branson et al., 2004).
The amount of support provided by the ventilator is tailored or adjusted to the
patient’s spontaneous effort, increasing or decreasing pressure support relative
to the patient’s work of breathing. PAV may be pressure- or flow-triggered and
is cycled when the patient’s volume or flow demands are met. PAV is available
on the Puritan-Bennett 840 ventilator.

Automatic Tube Compensation

Automatic tube compensation is a mode of ventilation that automatically com-
pensates for the resistance of the endotracheal tube. The pressure applied is
based upon the size and type of artificial airway (endotracheal tube or trache-
ostomy tube) and how much support is desired by the clinician. Automatic
tube compensation can eliminate the resistance imposed by the artificial air-
way. The ventilator adjusts the pressure to compensate for airway size or flow
demands (Branson et al., 2004). Automatic tube compensation is active both
during inspiration and expiration, and may reduce air trapping and intrin-
sic PEEP. Automatic tube compensation is available on the Puritan-Bennett
840 ventilator.

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