Unilateral or Markedly Asymmetric Lung Disease
Patients who have lobar pneumonia, lobar or whole-lung atelectasis, and
other markedly asymmetric pulmonary involvement present a special problem
in mechanical ventilation, particularly in the presence of severe
hypoxemia. Such patients illustrate why PEEP should not automatically be
applied as treatment for hypoxemic respiratory failure. Respiratory system
compliance is much higher in relatively normal areas of lung than in areas
of consolidation or collapse. As a result, application of PEEP may
preferentially expand these more normal areas and not produce the desired
effect in the involved lung. Distention of normal lung tissue stretches
and narrows pulmonary vessels, which can raise pulmonary vascular
resistance sufficiently to divert blood to the abnormal areas.
Accordingly, applying PEEP can worsen rather than improve arterial
oxygenation in such instances. Some patients who have hypoxemic
respiratory failure and apparently asymmetric lung involvement respond
favorably to PEEP, however, which emphasizes the need to perform PEEP
trials as routine.
Neuromuscular disease
Patients who have acute neuromuscular disease or cervical spinal cord
injury and whose lung function is relatively normal may benefit from
ventilator management at higher than usual Vts and flows. Such patients
may experience dyspnea at Vts of 10-12 mL/kg, which improves when larger
volumes (12-16 mL/kg) are used, although this is controversial. Similarly,
these patients typically prefer faster inspiratory flows (e.g., 80–100
L/min). Such settings often result in a mild-to-moderate respiratory
alkalosis, which is usually well tolerated and soon accompanied by a
compensatory metabolic acidosis. Alternatively, low levels of PEEP may
accomplish the same relief of dyspnea as the attendant hyperventilation.
Many patients who have traumatic quadriplegia or other acute
neuromuscular disorder experience recurrent atelectasis, which can cause
more severe hypoxemia than is usually seen with lobar collapse in other
clinical settings. Ventilation with larger than usual Vts, with or without
the addition of low-level PEEP, is important in such patients to prevent
recurrence. Frequent changes in posture may also be beneficial.
Acute brain injury
Patients who have closed head injury or other acute brain insult may
lose the normal autoregulation of cerebral perfusion pressure (CPP). In
such patients anything that decreases mean arterial pressure or raises
central venous pressure must be avoided. Thus, PEEP is used cautiously, if
at all, in patients who have acute brain injury, as the raised
intrathoracic pressure is transmitted via the vertebral veins to the
central nervous system. Maneuvers that induce coughing and may raise
intracranial pressure, such as tracheal suctioning, are avoided whenever
possible in these patients.
For many years, deliberate hyperventilation to arterial PaCO2
levels of 25-30 mmHg was an integral part of ventilator management in
patients who had acute brain injury. Results of recent studies call this
practice into question, however, and hyperventilation is no longer used in
many institutions, except as a temporary emergency measure while other
treatments for intracranial hypertension are initiated.
Flail chest
Several studies demonstrate that the clinical course and outcome of
flail chest injury are determined mainly by the underlying pulmonary
injury rather than the flail segment per se. Patients who sustain multiple
rib fractures without associated lung contusion or pneumonia generally
recover uneventfully, while flail chest in the setting of acute lung
injury typically follows the course of that illness, with little separate
contribution from the chest wall instability.
Ventilator management of patients who have a flail chest injury is thus
essentially that used to manage the underlying pulmonary condition.
Attention must be given to pain control, however, particularly when
ventilator modes providing only partial ventilatory support are used, as
these force the patient to use the intercostal muscles associated with the
flail. Many clinicians prefer to use full ventilatory support until the
patient is ready for weaning. Intercostal nerve blocks or administration
of epidural narcotics can greatly aid in pain control and ventilator
weaning in such patients.
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