The portable chest X-ray is the initial screening tool and may identify lung parenchymal causes of hypercarbia but given the supine acquisition may not be a sensitive test. Since pulmonary embolism is on the differential diagnosis contrast enhanced CT is the gold standard imaging technique permitting comprehensive assessment of mediastinum, pleural space, lung parenchyma and pulmonary vasculature.

Central causes of reduced respiratory drive resulting in hypercarbia may be drug induced, which will often be clinically apparent. Neuropathy or myopathy is usually diagnosed on clinical grounds. Imaging using chest radiograph or CT may identify features of phrenic nerve injury and malignant or compressive causes.

Following neurological evaluation brain imaging may be required to exclude a central cause for hypoventilation. While MRI is the reference standard for detecting brainstem disease, brain CT may be the initial test in view of access and safety. This does not preclude subsequent MRI if clinical suspicion is high.

Barotrauma is a common complication of mechanical ventilation. There is evidence that it may occur in all mechanically ventilated patients but only presents as a clinical problem in approximately 10–20 %. Barotrauma is a consequence of alveolar distension in the context of prolonged ventilation with high airway pressures. Although most commonly occurring in patients with ARDS, barotrauma can become a problem in any mechanically ventilated patient (Fig. 21.1). Manifestations of barotrauma are mostly subtle. If patients become increasingly difficult to ventilate clinicians should look out for radiological signs such as subcutaneous emphysema, pneumopericardium, pneumomediastinum, and uncommonly pneumoperitoneum.