Page 423 - Clinical Application of Mechanical Ventilation

Management of Mechanical Ventilation 389

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report would return to the patient’s normal pH 7.39, PaCO 55 mm Hg HCO
3
2
32 mEq/L (compensated respiratory acidosis).
Alveolar Hypoventilation Due to Sedation
or Patient Fatigue

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The blood gas report pH 7.30, PaCO 50 mm Hg HCO 24 mEq/L is interpreted
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2
as acute respiratory acidosis. The associated corrective action would be increasing
the ventilator frequency, tidal volume, or pressure support. However, this type of
report can occur if the mechanically ventilated patient hypoventilates because of
excessive sedation or respiratory muscle fatigue (e.g., premature weaning attempt).
Again, the underlying causes must be found and corrected (e.g., delay weaning).
Increasing the ventilator frequency would not be the proper action to correct this
“respiratory acidosis.”

Metabolic Acid-Base Abnormalities
Ventilatory interven-
tions should not be done
to compensate or correct Metabolic acid-base abnormalities should be corrected by treating their respec-
primary metabolic acid-base
problems. tive causes. Three major causes of metabolic acidosis are renal failure, diabetic
ketoacidosis, and lactic acidosis. One of the major causes of metabolic alkalosis is
hypokalemia (Shapiro et al., 1994). Ventilatory (respiratory) interventions should
not be done to compensate or correct primary metabolic acid-base problems. The
Blood gas interpreta-
tion must correlate with the reader should refer to a blood gas textbook for further information on the diagnosis
clinical signs of the patient. and treatment of metabolic acid-base abnormalities.
Incorrect interpretation can
lead to inappropriate changes Blood gas interpretation must correlate with the clinical signs of the patient. In-
of ventilator settings or harm- correct interpretation can lead to inappropriate changes of ventilator settings or
ful clinical decisions.
harmful clinical decisions.

TROUBLESHOOTING OF COMMON VENTILATOR
ALARMS AND EVENTS

The type of ventilator alarm is easy to spot since most ventilators provide an indica-
alarm: An absolute value of
a parameter on the ventilator tor (light or sound) for each event that triggers the alarm. Once the type of alarm
beyond which an alert is invoked is identified, steps can be taken to alleviate the problem by process of elimination.
to warn that the safety limit has
been breached. This section provides the common causes for each alarm.

Low Pressure Alarm

The low pressure limit is set to ensure that a minimum pressure is present in the
ventilator circuit during each inspiratory cycle.
Low pressure alarms are triggered when the circuit pressure drops below the preset
low pressure limit. If the preset low pressure limit is set at 40 cm H O and the
2
circuit pressure drops below 40 cm H O, the low pressure alarm will be triggered. In
2

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