Page 426 - Clinical Application of Mechanical Ventilation

392 Chapter 12

distress possibly due to conditions such as hypoxia, pain, anxiety, inadequate inspiratory
flow or pressure support. Tachypnia shortens the expiratory time, increases the mean
airway pressure, and alters the ventilation/perfusion relationship. For these reasons, per-
sistent tachypnia must be investigated and corrected. Triggering of the high frequency
alarm may be corrected by simple steps such as endotracheal suctioning, increasing the
F O , peak flow or pressure support setting. The high frequency alarm limit must not be
2
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increased without clear justification (e.g., reversal of sedation or anesthesia).
Another cause of the high frequency alarm is an inappropriate sensitivity set-
ting. When this control is set excessively sensitive to the patient’s inspiratory effort,
minimum inspiratory efforts or movements will cause the ventilator to initiate auto-
triggering and increase in total frequency.

Apnea/Low Frequency Alarm

The apnea/low frequency limit is set to ensure that a minimum number of breaths
is delivered to the patient.
The apnea or low frequency alarm is triggered when the total frequency drops
Disconnection of the below the low frequency limit. Disconnection of the ventilator circuit from the
ventilator circuit from the
patient’s endotracheal tube is patient’s endotracheal tube is the most frequent trigger of the apnea alarm, since the
the most frequent trigger of ventilator cannot sense any air movement (respiratory effort) from a disconnected
the apnea alarm.
circuit. Other triggers of the apnea/low frequency alarm include a patient under
respiratory depressants or muscle-paralyzing agents, conditions of respiratory center
dysfunction, and respiratory muscle fatigue.
Some ventilators merely alert the practitioner that the patient is having periods
of apnea; the practitioner must increase ventilation to alleviate the situation.
Most ventilators switch to a backup ventilation mode until the problem is
corrected.

High PEEP Alarm

The high PEEP limit is set to prevent excessive PEEP imposed on the patient. The
alarm is triggered when the actual PEEP exceeds the preset PEEP limit. Auto-PEEP
may occur in conditions of air trapping, insufficient inspiratory flow (long I-time),
or insufficient expiratory time (short E-time).
Air trapping may be reduced by decreasing the ventilator tidal volume and
frequency, and by using bronchodilators in patients with reversible airway obstruc-
tion. Increasing the inspiratory peak flow provides a shorter I-time and a longer
E-time. More time for exhalation helps to reduce air trapping.

Low PEEP Alarm

The low PEEP limit is set to ensure that the preselected PEEP is delivered to the
patient. The alarm is triggered when the actual PEEP drops below the preset low
PEEP limit. Failure of the ventilator circuit to hold the PEEP is usually due to
leakage in the circuit or ET tube cuff.

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