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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., 80100 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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