Page 52 - Clinical Application of Mechanical Ventilation

18 Chapter 1

failure may develop when severe hypoxemia (PaO , 40 mm Hg) does not respond
2
to moderate to high levels (50% to 100%) of supplemental oxygen.
The important clinical signs of oxygenation failure and hypoxia include hypox-
emia, dyspnea, tachypnea, tachycardia, and cyanosis (Rochester, 1993). In addition,
The important clinical
signs of oxygenation failure patients often appear to have shortness of breath and may become disoriented. These
and hypoxia include hypox- signs are usually readily available in the medical records or at the bedside. They
emia, dyspnea, tachypnea,
tachycardia, and cyanosis. should be used in conjunction with laboratory results during “routine” ventilator
rounds to assess the patient so that appropriate action may be taken.

CLINICAL CONDITIONS LEADING TO
MECHANCIAL VENTILATION

Mechanical ventilation is often used to support ventilatory or oxygenation failure.
Mechanical ventilation is Failure to ventilate or oxygenate adequately may be caused by pulmonary or non-
often used to support ventila-
tory or oxygenation failure. pulmonary conditions. For example, adult respiratory distress syndrome is a pul-
monary condition commonly associated with mechanical ventilation and mortality.
Many nonpulmonary conditions (e.g., neuromuscular disease, acute brain injury)
also contribute to the need for mechanical ventilation (Pierson, 2002; Kelly et al.,
1993).
These pulmonary and nonpulmonary conditions often lead to a combination of
deadspace ventilation, V/Q mismatch, shunt, diffusion defect, ventilatory failure,
and oxygenation failure. For logical discussion and ease of patient management,
they are separated into three distinct groups: (1) depressed respiratory drive (e.g.,
drug overdose), (2) excessive ventilatory workload (e.g., airflow obstruction), and
(3) failure of ventilatory pump (e.g., chest trauma).

Depressed Respiratory Drive

Depressed or insufficient respiratory drive may lead to a decrease in tidal volume,
Depressed or insufficient frequency, or both. These patients may have normal pulmonary function but the
respiratory drive may lead to
ventilatory and oxygenation respiratory muscles do not have adequate neuromuscular impulses to function
failure. properly. Mechanical ventilation is used to support these patients until the cause of
insufficient respiratory drive has been reversed.
Table 1-9 lists the clinical conditions that may lead to a depressed respiratory
drive. They are drug overdose (Parsons, 1994), acute spinal cord injury (Bach, 1991),
acute brain injury (Pierson, 2002), neurologic dysfunction (Kelly et al., 1993), sleep
disorders, and compensation for metabolic alkalosis (Greene et al., 1994).

Excessive ventilatory Excessive Ventilatory Workload
workload may lead to muscle
fatigue and ventilatory/
oxygenation failure. Ventilatory workload is influenced by many clinical conditions (Table 1-10).
When it exceeds the patient’s ability to carry out the workload, ventilatory and

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