Page 133 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 99

breathing in the SIMV mode is typically supplied by a demand valve. The
Spontaneous frequency demand valve is always patient-triggered, either by pressure or flow depending
and tidal volume taken by
the patient in the SIMV mode on the ventilator.
are totally dependent on the It is important to understand that the spontaneous breaths taken by the patient in
patient’s breathing effort.
the SIMV mode are truly spontaneous. The ventilator provides the humidified gas
at the selected F O , but the spontaneous frequency and spontaneous tidal volume
2
I
are totally dependent on the patient’s breathing effort.
Indications for SIMV Mode

The primary indication for SIMV is to provide partial ventilatory support, i.e., a
The primary indication desire to have the patient actively involved in providing part of the minute vol-
for SIMV is to provide partial
ventilatory support to the ume. In a practical sense, when a patient is first placed on ventilatory support, full
patient. ventilatory support is appropriate to provide a period of rest, typically for the first
24 hours. After this initial period of full ventilatory support, it is a typical practice
to place the patient on a trial run of partial ventilatory support with SIMV. It is
customary to ease the patient from full support to partial support by gradually de-
creasing the mandatory frequency as tolerated by the patient. This depends on, of
course, reversal of the clinical conditions that committed the patient to the ventila-
tor in the first place.

Advantages of SIMV Mode

Since SIMV promotes spontaneous breathing and use of respiratory muscles, SIMV
(1) maintains respiratory muscle strength/avoids muscle atrophy, (2) reduces venti-
lation to perfusion mismatch, (3) decreases mean airway pressure, and (4) facilitates
weaning.
Maintains Respiratory Muscle Strength/Avoids Muscle Atrophy. SIMV helps to maintain
respiratory muscle activity and strength. Patients maintained in full ventilatory
support for extended periods tend to experience partial loss of ventilatory muscle
strength. This can be minimized by using the respiratory muscles during spontane-
ous breathing (Zelt et al., 1972).

Reduces Ventilation to Perfusion Mismatch. Deadspace ventilation (i.e., high ventila-
tion to low perfusion) is typical in the upper zone of the lungs because pulmo-
nary perfusion is gravity-dependent and favors the lower lung zone. This problem
is intensified during positive pressure ventilation, as the lung units in the upper
zone are hyperinflated. Spontaneous breathing during SIMV tends to distribute the
spontaneous tidal volume more evenly thus reducing alveolar deadspace ventilation
(Weisman et al., 1983).

Decreases Mean Airway Pressure. The mean airway pressure is directly related to
the peak inspiratory pressure and inspiratory time (Note: Two other factors are
frequency and positive end-expiratory pressure). Since spontaneous breaths during
SIMV have a lower peak inspiratory pressure and inspiratory time, SIMV tends to
have a lower mean airway pressure.

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