Page 141 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 107

time-triggered. Once a breath is triggered, the ventilator tries to reach the pressure
support level as soon as possible. The delivered volume is then compared with the
preset tidal volume for further action by the ventilator.
If the delivered volume equals the preset volume, the breath is considered a pres-
sure support breath. Since pressure support breaths are dependent on the patient
effort, the delivered volume may be larger than the preset volume. It is essential to
set the pressure support level that provides a volume that is lower than the preset
volume. On the other hand, if the delivered volume falls short of the preset volume,
the ventilator switches from a pressure-limited breath to a volume-limited breath.
This results in a longer inspiratory time (at a constant flow) until the preset volume
VAPS may prolong the is delivered.
inspiratory time. Patients with
airflow obstruction should be Since VAPS may prolong the inspiratory time automatically, patients with air-
monitored closely in order to flow obstruction should be monitored closely in order to prevent air trapping
prevent air trapping and other
related side effects. and other undesirable cardiovascular effects associated with prolonged inspira-
tory time.

PRESSURE-REGULATED VOLUME CONTROL (PRVC)

pressure-regulated volume Pressure-regulated volume control (PRVC) is first available in the Siemens 300.
control (PRVC): A mode of Similar modes to PRVC in subsequent ventilators are known as: adaptive pressure
ventilation that provides volume-
controlled breaths with the lowest control (Servo-I, Maquet), AutoFlow (Evita XL, Drager), adaptive pressure ven-
pressure possible by altering the tilation (Galileo, Hamilton), volume control 1 (Puritan Bennett, Tyco), volume
flow and inspiratory time.
targeted pressure control and pressure controlled volume guaranteed (Engstrom,
General Electric) (Mireles-Cabodevila et al., 2009). In the Siemens 300 ventilator,
PRVC is only active during CMV whereas in other ventilators, this type of dual
control mode is also active in SIMV. Table 4-8 outlines the characteristics of the
PRVC mode.
PRVC is used primarily to achieve volume support while keeping the peak inspira-
PRVC provides volume
support while keeping the tory pressure (PIP) at a lowest level possible. This is achieved by altering the peak flow
PIP at a lowest level possible
by altering the peak flow and and inspiratory time in response to changing airway or compliance characteristics.
inspiratory time in response For example, Equation (1) shows that at constant flow, the PIP is increased due to
to changing airway or compli-
ance characteristics. increasing airflow resistance. Note that the increasing airflow resistance may be due

TABLE 4-8 Characteristics of the Pressure-Regulated Volume Control Mode

Characteristic Description
Type of breath CMV in Siemens 300; SIMV in other ventilators

Triggering mechanism Time-triggered or patient-triggered
Cycling mechanism Volume-cycled under normal operating
conditions
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