Page 134 - Clinical Application of Mechanical Ventilation

100 Chapter 4

TABLE 4-4 Characteristics of the Synchronized IMV Mode

Characteristic Description

Type of breath The ventilator delivers mechanical tidal volume at a preset frequency.
The patient may breathe spontaneously between mandatory breaths.
Triggering mechanism Mandatory breaths may be either time-triggered or patient-triggered.
Spontaneous breaths are patient-triggered (i.e., the demand flow
valve opens in response to the patient’s spontaneous inspiratory
effort).

Cycling mechanism The mandatory breaths are volume-cycled. The patient controls
spontaneous frequency and volume.
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The mean airway pressure is an important consideration because the greater the
mean airway pressure, the greater the potential for a reduced venous return, cardiac
output, and arterial perfusion pressure. Reduction of the mean airway pressure during
SIMV indirectly enhances the patient’s cardiovascular functions (Wilkins et al., 2003).
Facilitates Weaning. SIMV facilitates weaning due to its ability to decrease the man-
datory frequency in small increments. This may offer some advantage to those
“hard-to-wean” patients who cannot tolerate an abrupt decrease of the mechanical
frequency or spontaneous breathing trial (Downs et al., 1973).

Complications of SIMV Mode

Prior to PSV (pressure support ventilation), the approach of SIMV weaning is to
provide a spontaneous breathing workload that gradually increases a patient’s mus-
cle strength and endurance. The primary disadvantage associated with SIMV is the
desire to wean the patient too rapidly, leading first to a high work of spontaneous
breathing and ultimately to muscle fatigue and weaning failure. Without PSV, the
best practice is to decrease the SIMV mandatory frequency slowly and monitor the
patient closely for signs of fatigue (Wilkins et al., 2003).
Table 4-4 summarizes the major characteristics of the synchronized IMV mode.

MANDATORY MINUTE VENTILATION (MMV)

Mandatory minute ventilation (MMV), also called minimum minute ventila-
mandatory minute ventilation
(MMV): MMV is a feature of some tion, is a feature of some ventilators that provides a predetermined minute venti-
ventilators that causes an increase lation when the patient’s spontaneous breathing effort becomes inadequate. For
of the mandatory frequency (Note:
In Hamilton Veolar, the pressure example, an apnea episode (lack of spontaneous breathing) may cause the actual
support level), when the patient’s minute ventilation to drop below the preset level. When this occurs, the mandatory
spontaneous breathing level
becomes inadequate. This compen- frequency is increased automatically to compensate for the decrease in minute ven-
sation by the ventilator ensures a tilation caused by the apnea. This compensation by the ventilator ensures a desired
safe minimal minute ventilation.
minute ventilation.

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