Atelektasis

01Pathophysiology

Atelectasis refers to the collapse of previously aerated lung parenchyma, resulting in loss of lung volume and impaired gas exchange. Understanding the mechanism behind each subtype is essential because the vignette will often hint at the underlying cause.

Obstructive (resorptive) atelectasis is the most commonly tested form. When an airway becomes completely blocked (by a mucus plug, aspirated foreign body, or endobronchial tumor), gas trapped distal to the obstruction is gradually absorbed into the pulmonary capillary blood. Since the alveoli cannot be replenished with fresh air, they collapse. This is particularly relevant in the post-operative setting, where general anesthesia suppresses the cough reflex, impairs mucociliary clearance, and reduces tidal volume, all of which promote mucus retention and subsequent airway obstruction.

Compressive atelectasis occurs when an external force pushes on the lung parenchyma from outside, preventing full expansion. Common causes include pleural effusion, pneumothorax, an elevated diaphragm (as in ascites or abdominal distension), and large mediastinal or thoracic masses. In this subtype, the airway itself is patent; the lung simply cannot expand against the compressive force.

Adhesive atelectasis results from surfactant deficiency or dysfunction. Surfactant normally reduces alveolar surface tension, preventing small alveoli from collapsing at end-expiration according to Laplace's law. When surfactant is absent or dysfunctional (as in neonatal respiratory distress syndrome or ARDS), alveoli collapse despite patent airways.

Cicatrization (contraction) atelectasis develops when pulmonary fibrosis or granulomatous disease causes scarring of the lung parenchyma. The fibrotic tissue contracts and pulls the surrounding alveoli inward, reducing their volume.

Relaxation (passive) atelectasis occurs when the normal negative intrapleural pressure is disrupted, as in pneumothorax. Without the negative pressure that keeps the lung expanded against the chest wall, the lung recoils inward.

The physiological consequence of all forms of atelectasis is intrapulmonary shunting: blood continues to perfuse the collapsed, non-ventilated alveoli but cannot participate in gas exchange. This produces ventilation-perfusion (V/Q) mismatch and, in extensive collapse, true right-to-left shunt. The result is hypoxemia with an elevated $A\text{-}a\text{ gradient}A\backslash text\{-\}a\backslash \; \backslash text\{gradient\}$. A key distinction: mild V/Q mismatch from small areas of atelectasis improves with supplemental oxygen, whereas true shunt from large areas of collapse is refractory to supplemental oxygen.

02Classification and Clinical Manifestation

By Mechanism

By Extent

By Severity

Atelectasis is the classic cause of fever on post-operative day 1 to 2 ("Wind"), making it the first consideration when a recently operated patient develops an early, low-grade fever.

The other causes of post-operative fever in sequence are urinary tract infection (day 3), wound infection (day 5 to 7), deep vein thrombosis/pulmonary embolism (day 7 to 10), and drug fever (any day).

03Diagnostic Workup

The chest X-ray is the best initial test for evaluating atelectasis. The hallmark finding is opacification of the affected lung region combined with signs of volume loss. This distinction is critical on the exam: both atelectasis and pneumonia can produce opacification on CXR, but only atelectasis demonstrates volume loss. The signs of volume loss include elevation of the ipsilateral hemidiaphragm, shift of the mediastinum and trachea toward the collapsed side, crowding of the ribs on the affected side, and compensatory hyperinflation of the remaining lung. In lobar collapse, each lobe produces a characteristic radiographic pattern:

The right upper lobe collapses superiorly and medially, producing a density along the right mediastinal border with elevation of the right hilum. The right middle lobe collapses anteriorly and medially, causing blurring of the right heart border on the PA view (silhouette sign) and a wedge-shaped density on the lateral view. The right lower lobe collapses posteriorly and medially, producing a triangular opacity behind the heart. The left upper lobe collapses anteriorly, creating a veil-like opacity over the left hemithorax without significant mediastinal shift. The left lower lobe collapses posteriorly and medially, producing a triangular retrocardiac density similar to the right lower lobe.

Arterial blood gas analysis reveals hypoxemia with an elevated $A\text{-}a\text{ gradient}A\backslash text\{-\}a\backslash \; \backslash text\{gradient\}$, confirming intrapulmonary shunting. Early in the course, the patient may demonstrate respiratory alkalosis due to compensatory tachypnea.

CT chest is ordered when the CXR findings are equivocal, when there is concern for an underlying endobronchial lesion or mass causing the obstruction, or when rounded atelectasis needs to be distinguished from a lung malignancy. Rounded atelectasis, which is associated with prior asbestos exposure and pleural thickening, appears as a rounded peripheral opacity with the characteristic "comet tail sign" (curvilinear bronchovascular structures sweeping into the mass).

Bronchoscopy serves as both a diagnostic and therapeutic tool. It is indicated when obstructive atelectasis does not resolve with conservative measures, or when an endobronchial lesion is suspected. It allows direct visualization, mucus plug removal, foreign body extraction, and tissue biopsy if a tumor is identified.

04Management and Treatment

Prevention is the most commonly tested management concept for atelectasis. In the post-operative setting, the single most important intervention is incentive spirometry, which encourages sustained maximal inspiration to recruit collapsed alveoli. The patient should perform 10 breaths every 1 to 2 hours while awake, aiming for the highest inspiratory volume possible. This is combined with early ambulation and effective pain control, as inadequate analgesia limits the patient's ability to take deep breaths, perpetuating the cycle of hypoventilation and collapse.

For established atelectasis, chest physiotherapy (percussion, vibration, and postural drainage) helps mobilize secretions and facilitate their clearance. In mechanically ventilated patients, suctioning is used to remove accumulated secretions from the airways.

Positive airway pressure with CPAP or BiPAP provides a pneumatic splint that holds alveoli open and recruits collapsed lung units. This is useful in patients with diffuse atelectasis or those who cannot perform incentive spirometry effectively. Starting pressures are typically 5 to 10 cmH${}_2\_2$O for CPAP.

When atelectasis is refractory to conservative measures (persistent lobar collapse despite 24 to 48 hours of respiratory therapy), therapeutic bronchoscopy is the next best step. This allows direct suction of mucus plugs and lavage of the obstructed airway. Bronchoscopy is also indicated urgently when foreign body aspiration is suspected.

For compressive atelectasis, treatment targets the compressing lesion: thoracentesis or chest tube placement for pleural effusion or pneumothorax, paracentesis for tense ascites causing diaphragmatic elevation, or surgical intervention for mediastinal masses.

For adhesive atelectasis in neonates (RDS), the treatment is exogenous surfactant replacement (beractant or poractant alfa) administered via the endotracheal tube, combined with CPAP or mechanical ventilation.

An important point: antibiotics are not indicated for atelectasis itself. Although post-operative atelectasis causes fever, this is not an infectious process. Antibiotics should only be started if there is clinical evidence of superimposed pneumonia (purulent sputum, leukocytosis with left shift, new or worsening infiltrate without volume loss signs).

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common trap with atelectasis questions centers on the direction of tracheal and mediastinal shift. Students frequently confuse atelectasis with pleural effusion or tension pneumothorax, all of which produce opacification or abnormality on the same side. The rule is straightforward: atelectasis pulls structures toward the collapsed lung (volume loss), while pleural effusion and tension pneumothorax push structures away (volume gain). If the vignette describes a post-operative patient with tracheal deviation toward the side of the opacity, atelectasis is the answer. If the trachea deviates away, think effusion or tension pneumothorax.

A second common error is treating post-operative day 1 fever with antibiotics. Test-writers present an early post-operative fever with a CXR showing basilar opacities and expect you to recognize atelectasis and choose incentive spirometry, not empiric antibiotics. The presence of fever alone does not equal infection.

Another tested concept is distinguishing atelectasis from pneumonia on imaging. Both cause opacification, but the presence of air bronchograms without volume loss points toward pneumonia (the airways remain patent and visible against the consolidated, fluid-filled alveoli). Atelectasis, by contrast, demonstrates volume loss signs and may lack air bronchograms in the obstructive subtype (because the airway is blocked).

Finally, be aware of the right middle lobe syndrome scenario: recurrent or chronic right middle lobe atelectasis due to the anatomy of the right middle lobe bronchus, which is long, narrow, and surrounded by lymph nodes. When a vignette describes persistent right middle lobe collapse, the test is assessing whether you will order bronchoscopy to evaluate for an underlying obstructing lesion.

The core competency being tested in atelectasis questions is the ability to recognize volume loss on imaging, correctly identify the direction of mediastinal shift, choose appropriate non-pharmacologic interventions (incentive spirometry and early ambulation), and avoid premature antibiotic use for non-infectious post-operative fever.