Page 135 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 101

MMV is an additional function of the SIMV mode and is intended to prevent
hypercapnia by “automatically” ensuring that the patient receives a minimum preset
minute volume. It is especially useful in preventing hypoventilation and respiratory
acidosis in the final stages of weaning with SIMV when the patient’s spontaneous
breathing is assuming a significant portion of the total minute volume.
For example, a patient may have been weaned down to a mandatory SIMV fre-
quency of 4/min with a mandatory tidal volume of 800 mL; the patient’s ventilator-
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delivered minute volume would then be 3.2 L/min (V = f * V ). If this patient’s
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spontaneous minute volume is 6 L/min, then the total minute volume is the sum
of the ventilator-delivered minute volume and the spontaneous breathing minute
volume (9.2 L/min in this example). If the patient’s spontaneous minute volume
suddenly decreases by a significant amount, or if the patient becomes apneic, then
without MMV the reduced minute volume would cause hypercapnia and respira-
tory acidosis. However, on MMV-equipped ventilators, a decrease in the patient’s
spontaneous minute volume would trigger an automatic increase in the ventilator’s
mandatory frequency.
The way that MMV functions on the majority of ventilators is that a desired
minimum minute volume is preset on the ventilator—usually only slightly less than
the minute volume required to “normalize” the PaCO . The ventilator then mea-
2
sures the total minute volume and compares it with the preset minimum minute
volume. As long as the patient’s total minute volume equals or exceeds the preset
minimum minute volume, the MMV function is not activated. However, if the
patient’s spontaneous minute volume decreases to the point that the total minute
volume becomes less than the preset mandatory minute volume, then the ventila-
tor will automatically increase the SIMV mandatory frequency until it reaches the
preset mandatory minute volume.
In the MMV mode, it is important to monitor not only the patient’s spontaneous
minute volume, but also the patient’s estimated spontaneous alveolar minute vol-
ume. The reason for this is that if the patient becomes distressed, the tendency is to
increase the spontaneous frequency at the expense of a decreased tidal volume (i.e.,
the patient will typically adopt the spontaneous breathing pattern that minimizes
the work of breathing). A minute volume supported by a rapid frequency and low
A minute volume sup- tidal volume may avert the MMV function but at the same time provides a signifi-
ported by rapid frequency
and low tidal volume (e.g., cant amount of deadspace ventilation. This results in a decreased alveolar minute
distressed patient) may avert volume.
the MMV function but at the
same time provides a sig- Perhaps the most efficient method of ensuring that this condition does not occur
nificant amount of deadspace is to set the high frequency alarm at approximately 10/min greater than the patient’s
ventilation. This results in a
decreased alveolar minute “baseline” spontaneous frequency.
volume. Although MMV operates in the manner previously described on most ventila-
tors, one exception is seen in the Hamilton Veolar ventilator. Selecting the MMV
mode on this ventilator automatically places the patient in a “pure” pressure support
mode (i.e., every breath is a spontaneous pressure-supported breath and no manda-
tory breaths are given). A minimum desired mandatory minute volume is selected
and the ventilator automatically compares the patient’s total minute volume with
the preset minimum minute volume. On the Veolar, if the patient’s total minute

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