Page 263 - Clinical Application of Mechanical Ventilation

Initiation of Mechanical Ventilation 229

wave flow pattern is considered more physiologic because it is similar to the flow
pattern during spontaneous breathing. The sine wave may also improve the distri-
bution of ventilation and therefore improve gas exchange.
For ventilators that do not permit a preset inspiratory time, the inspiratory
time may increase if the patient’s peak inspiratory pressure increases. This is be-
cause as the PIP increases, the pressure gradient between the ventilator and the
patient’s airway opening increases, resulting in an increased inspiratory time.
However, on ventilators in which the inspiratory time is preset, such as the
Hamilton Veolar, the inspiratory time is held constant for any flow pattern
selected.
In performing calculations that involve the inspiratory flow as a variable (e.g.,
Resistance 5 Pressure/Flow), the mean inspiratory flow should be used. Since the
only flow pattern in which the peak flow equals the mean inspiratory flow is the
square wave pattern, the ventilator should be switched to a constant flow pattern
prior to measurement.

VENTILATOR ALARM SETTINGS

Although different ventilators have different alarm systems, the following alarms
should be basic to any ventilator: low exhaled volume alarm, low inspiratory pres-
sure alarm, high inspiratory pressure alarm, apnea alarm, high frequency alarm,
and F O alarm. These alarms should be backed up by a battery source to prevent
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I
malfunction in the event of electrical failure.
Low Exhaled Volume Alarm
The low exhaled volume
alarm (low volume alarm)
should be set at about 100 mL The low exhaled volume alarm (low volume alarm) should be set at about 100 mL
lower than the expired
mechanical tidal volume. lower than the expired mechanical tidal volume. This alarm is triggered if the
patient does not exhale an adequate tidal volume. This alarm is typically used to
detect a system leak or circuit disconnection.

Low Inspiratory Pressure Alarm
The low inspiratory
pressure alarm (low pressure
alarm) should be set at The low inspiratory pressure alarm (low pressure alarm) should be set at 10 to 15 cm
10 to 15 cm H 2 O below the
observed peak inspiratory H O below the observed peak inspiratory pressure. This alarm is triggered if the
2
pressure. peak inspiratory pressure is less than the alarm setting. The low inspiratory pressure
alarm complements the low exhaled volume alarm and is also used to detect system
leaks or circuit disconnection.

The high inspiratory High Inspiratory Pressure Alarm
pressure alarm (high pressure
limit alarm) should be set at
10 to 15 cm H 2 O above the The high inspiratory pressure alarm (high pressure limit alarm) should be set at
observed peak inspiratory
pressure. 10 to 15 cm H O above the observed peak inspiratory pressure. This alarm is
2
triggered when the peak inspiratory pressure is equal to or higher than the high

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