This poable chest x-ray taken in the ICU shows an intubated patient with hyperlucent lines in the soft tissue with striations along the fibromuscular bundles of the neck and chest musculature. There is a faint paracardiac hyperlucent line representing air around the pericardium. The diaphragm leaflets are seen clearly across the midline due to the contrasting air shadow representing the "continuous diaphragm sign". All the signs mentioned are representative of a pneumomediastinum. The lower edge of the diaphragm is also visible due to a pneumoperitoneum. Alveolar rupture with increased alveolar-interstitial space gradient can cause pneumomediastinum and subcutaneous emphysema. Subcutaneous emphysema may occur after trauma such as esophageal rupture with direct introduction of air in the mediastinum. It can also occur where there is abdominal and thoracic muscular contraction against a closed glottis. Infection with a gas-forming organism can cause subcutaneous gas formation. Inflammatory bronchiolitis or overinflated alveoli due to mechanical ventilation can cause alveolar rupture, especially if there is airway obstruction with air moving along the bronchovascular sheaths. Pneumomediastinum refers to abnormal air collection within the mediastinum. Air can dissect into the mediastinum from areas of the neck and thorax or from the GI tract or lungs. Pathologically there is continuity between the periaerial and the peribronchial interstitium when an alveolar rupture occurs, creating an air collection within the interstitial connective tissues. Patient-related factors that are found to predispose to volutrauma include lung disease that weakens alveolar walls, such as COPD and necrotizing pneumonia. Mediastinal air accumulates and then decompresses into the subcutaneous tissues and the retroperiteoneal areas. Later, mediastinal pleura may rupture, resulting in a pneumothorax. The Hamman sign, a crunching sound synchronous with the cardiac cycle, is seen in 40% to 50% of patients with pneumomediastinum. When the pneumomediastinum extends caudally, it shows a so-called "continuous diaphragm sign." Treatment is usually conservative, with attempts to reduce airway resistance with bronchodilator therapy and minimize tidal volume and plateau pressure.