Hypoxemia: Chest x-ray

Hypoxemia is defined as a low level of oxygen in the blood. This is not to be confused with hypoxia, which is a condition where the oxygen delivered to an organ or the entire body is insufficient. Hypoxemia can occur in patients with hypoventilation, V/Q mismatch, right-to-left shunts, diffusion limitations, and a low inspired fraction of oxygen. Out of these five categories, conditions that produce either a V/Q mismatch or a diffusion limitation will likely show findings on chest x-ray related to the underlying pathology. As there are many diseases and pathologies that can result in either a V/Q mismatch and/or a diffusion limitation, several of the most common ones and their findings on chest x-ray will be listed below. It should be noted that not all findings presented below are specific for the certain disease and many findings may be seen in more than one disease. Therefore, as always, it is important to consider the clinical context when interpreting radiographs.

Emphysema (COPD):

• Increased radiolucency and enlarged lung fields, a flat diaphragm, a long and narrow small heart, and possible bullous changes. On lateral chest x-ray a “barrel chest” with a widened anterior-posterior diameter may be seen. In advanced disease, pulmonary hypertension and cor pulmonale may develop resulting in enlargement of the main pulmonary arteries and eventual enlargement of the heart due to right heart failure.

Pneumonia:

• Lobar pneumonia: Pattern of consolidation characterized by a solitary peripheral focus of dense opacity that spares the larger airways producing air bronchograms.
• Bronchopneumonia: Multiple areas of patchy consolidation that are often bilateral and may coalesce with time. Lack of air bronchograms.
• Interstitial pneumonia: Diffuse interstitial (diffuse reticular opacities) infiltrates typically involving both lungs and more than one lobe.
• Aspiration pneumonia: Typically shows an infiltrate in the dependent sections of the lungs (superior or posterior basal segments of a lower lobe or the posterior segment of an upper lobe). Can result in an aspiration-related lung abscess which would show a cavitary lesion.

Acute respiratory distress syndrome:

• Radiograph findings vary depending on stage of disease.
• Acute phase (typically first week) is characterized by either normal appearing lungs or diffuse bilateral patchy alveolar infiltrates that often obscure the pulmonary vascular markings. Air bronchograms may be visible and lung volumes are usually reduced. Pleural effusions may also be seen, however this is a more common finding in patients with coronary heart failure. In very severe cases, a complete “white out” of the lungs is possible.
• Intermediate or proliferative phase (second week) is characterized by stabilization of alveolar opacities.
• Late or fibrotic phase (over 2 weeks) the pulmonary opacities begin to clear resulting in normal looking lungs or lungs with reduced lung volumes and coarse reticulations.

Pulmonary edema:

• Chest x-ray findings can vary depending on the underlying cause of the pulmonary edema (i.e., cardiogenic vs noncardiogenic).
• Cardiogenic pulmonary edema can show cephalization of the pulmonary vessels, peribronchial cuffing, septal lines (Kerley B lines), and a “bat wing” pattern in the lungs. If the pulmonary edema is bad enough, you may not be able to see the costophrenic angles. Additionally, the heart is typically enlarged, however not all cardiogenic pulmonary edema will show an enlarged heart.
• Noncardiogenic pulmonary edema (e.g., capillary permeability edema) can show patchy, peripheral, and nongravitational distribution of edema with an absence of septal lines, peribronchial cuffing, and usually a normal-sized heart.

Pleural effusion:

• Pleural effusions typically accumulate in the most gravity dependent portion of the thoracic cavity because the lung is less dense than the fluid.
• Typical features seen on PA and AP erect films include blunting of the costophrenic and cardiophrenic angles, and fluid within the horizontal or oblique fissures. Pulmonary vessels are typically visible and air bronchograms are absent. Large-volume effusions can cause mediastinal shift away from the effusion.
• Subpulmonic effusions, also known as infrapulmonary effusions, are a type of pulmonary effusion that elevates the lung base and is typically seen on the right. Subpulmonic effusions can shift the apex of the curvature of the lung laterally, causing its slope to slant towards the lateral costophrenic sulcus. This produces a visual finding called “Rock of Gibraltar sign” and is best seen on the lateral views. If they occur on the left, you may see a marked separation (>2 cm) of the lung from the stomach bubble.
• Loculated pleural effusions are a type of pleural effusion that typically occurs as a result of adhesions. They can appear as a homogeneous mass with tapered borders that drops on upright imaging due to the effects of gravity on its liquid contents.

Pneumothorax

• In a simple pneumothorax, which makes up most pneumothoraxes, you will typically see the presence of a white visceral pleural line with no visible bronchovascular markings beyond the visceral pleural edge.
• A tension pneumothorax, which is more likely seen in a trauma patient or a patient on mechanical ventilation, may show mediastinal and tracheal shift toward the contralateral lung. Splaying of the ribs and flattening of the ipsilateral diaphragm may also be present.
• Hydropneumothorax will show a liquid-gas level when the patient is upright.

Atelectasis

• Atelectasis can occur as lobar, multilobar, segmental, and atelectasis of the entire lung.
• Increased opacification of the airless lobe and displacement of fissures may be seen. Additional findings may include: displacement of the cardiomediastinal and hilar structures toward the side of collapse, elevation of the ipsilateral hemidiaphragm, compensatory hyperinflation and hyperlucency of the remaining aerated lung, and obscuration of adjacent structures.
• If the atelectasis effects the entire lung, opacification of the hemithorax and ipsilateral cardiomediastinal shift may be seen.