Pneumothorax Lecture Notes

Anatomy of the Pleura and the Pleural Cavity

The anatomy of the pleura is characterized by two distinct divisions: the visceral pleura and the parietal pleura. The visceral pleura serves as the innermost layer, providing a direct covering for the lungs and extending into the lung fissures. In contrast, the parietal pleura comprises three distinct regions based on the structures they line: the costal pleura, which lines the inner surface of the thoracic cage; the diaphragmatic pleura, which covers the superior surface of the diaphragm; and the mediastinal pleura, which covers the mediastinum. Between these two pleural layers (parietal and visceral) lies an extremely narrow region known as the pleural cavity. Under normal physiological conditions, there is no air present within the pleural cavity. However, a small volume of pleural fluid, typically ranging from $10-20\,ml$, is present. This fluid acts as a crucial lubricant that facilitates smooth lung movement during respiration. Ordinarily, the lungs expand to fill the chest cavity completely, meaning there is no evident or visible pleural space in a healthy state.

Dynamics of Intra Pleural Pressure (IPP)

Intra Pleural Pressure (IPP) refers to the pressure within the pleural sac, and it maintains specific values relative to atmospheric pressure depending on the phase of respiration. At the end of a normal expiration, the IPP is approximately $-3\,mmHg$. At the end of a normal inspiration, this pressure becomes more negative, reaching $-6\,mmHg$. During a forced inspiration, the pressure can drop significantly to $-30\,mmHg$. Conversely, during a forced expiration performed with a closed glottis (the Valsalva maneuver), the IPP can become positive, reaching values of $+40\,mmHg$. If air is introduced into the pleural sac, a condition known as pneumothorax, the normal negativity of the IPP is lost. This loss of negative pressure causes the lung to collapse due to its inherent elastic recoil, while the thoracic cage simultaneously expands.

Definition and Classification of Pneumothorax

Pneumothorax is defined as the presence of air within the pleural space, which leads to either partial or complete collapse of the lung on the affected side. This condition is broadly classified into two main categories: spontaneous pneumothorax and traumatic pneumothorax. Spontaneous pneumothorax is further subdivided into primary and secondary types. Primary spontaneous pneumothorax occurs in the absence of any significant underlying lung disease, whereas secondary spontaneous pneumothorax occurs in patients with pre-existing lung conditions. Traumatic pneumothorax results from external injury or medical intervention and is further categorized into open, closed, or tension varieties.

Primary and Secondary Spontaneous Pneumothorax

Primary spontaneous pneumothorax typically affects patients who are young, tall, and thin, with lungs that appear otherwise healthy. There is a strong correlation with smoking, and patients often have a history of recurrence. The male-to-female ratio for this condition is $6:1$, and there is an equal frequency of occurrence in both the left and right lungs. Pathophysiologically, it is often caused by the rupture of a subpleural bleb, which is usually located apically. This is thought to be rooted in a congenital defect in the connective tissue of the alveolar walls. Secondary spontaneous pneumothorax, on the other hand, is associated with a wide range of diseases. Airway diseases such as chronic obstructive pulmonary disease (COPD), bronchial asthma, and cystic fibrosis are common contributors. It can also stem from interstitial lung diseases like interstitial pulmonary fibrosis, sarcoidosis, and occupational lung diseases. Infectious causes include pneumonia, tuberculosis (TB), and various fungal infections. Neoplasms, including bronchogenic carcinoma and lung metastasis, may also be responsible. Other associated factors include connective tissue disorders like Marfan's syndrome and drug abuse, specifically among cocaine or heroin addicts.

Traumatic and Iatrogenic Pneumothorax

Traumatic pneumothorax can be divided into iatrogenic and non-iatrogenic causes. Iatrogenic pneumothorax is a result of medical procedures, which may be diagnostic or therapeutic. Diagnostic techniques that carry this risk include pleural aspiration, pleural biopsy, percutaneous lung aspiration, and the use of rigid or fiberoptic bronchoscopes. Therapeutic techniques that may cause it include mechanical ventilation and the placement of a Central Venous Pressure (CVP) line. General traumatic pneumothorax results from either blunt trauma or penetrating trauma. These injuries can lead to an open pneumothorax (where a hole in the chest wall allows atmospheric air direct access to the pleural space), a closed pneumothorax, or a tension pneumothorax. In an open pneumothorax, the movement of air can cause a phenomenon known as a mediastinal flutter.

Tension Pneumothorax as a Medical Emergency

A tension pneumothorax is a critical medical emergency requiring immediate intervention. It occurs when a pleural tear creates a one-way valve effect, allowing air to be sucked into the pleural space during inspiration but preventing its expulsion during expiration. This results in the intrapleural pressure remaining positive throughout the entire breathing cycle. As a consequence, the lung on the affected side deflates further, and the increasing pressure causes the mediastinum to shift toward the contralateral side. This shift leads to decreased venous return and severe respiratory and cardiac embarrassment. Rapid diagnosis and treatment are essential to prevent cardiovascular collapse.

Lung Mechanics and Pathophysiology of Collapse

The mechanics of the lung are governed by the negative pleural pressure. Once a communication is established between the pleural space and the atmosphere, the negative pressure is lost, and the inherent elastic recoil of the lung tissue leads to deflation. If the communication between the lung and the pleural space is eventually closed, the air within the space will begin to be reabsorbed by the body. The reabsorption rate is estimated to be $1.25\%$ of the total radiographic volume of the hemithorax per day. Consequently, if a patient has a $50\%$ collapse of the lung, it will take approximately $40$ days for the air to be completely reabsorbed once the air leak has been sealed. The fundamental reason the lung collapses during a puncture, such as a stabbing, is that the pleural pressure becomes equal to atmospheric pressure, leaving no force to counter the lung's elastic recoil.

Clinical Presentation and Physical Signs

The clinical picture of pneumothorax is often characterized by the sudden onset of unilateral pleuritic chest pain accompanied by progressively increasing breathlessness as the pneumothorax enlarges. A dry cough may also be present. General signs of a simple pneumothorax include tachypnea and tachycardia. However, a tension pneumothorax presents with more severe indicators, including anxiety, restlessness, struggling for breath, and signs of overt respiratory distress. Patients may exhibit tachycardia, hypotension, cyanosis, and congested neck veins. On physical examination, a small pneumothorax may be clinically undetectable. In larger cases, inspection may reveal a unilateral localized bulge with decreased respiratory movement. Palpation may show decreased chest expansion and, in tension cases, a contralateral mediastinal shift; subcutaneous (sc) emphysema may also be noted. Percussion produces a hyperresonance note on the affected side, and auscultation typically reveals absent or significantly diminished breath sounds.

Diagnostic Investigations and Sizing

Chest X-ray is a primary diagnostic tool, showing unilateral hypertranslucency with a distinct, convex visceral pleural line. Lung markings are notably absent outside of this line. In traumatic cases, the X-ray might also reveal rib fractures or pleural fluid collections (hemothorax). In tension pneumothorax, imaging shows downward displacement of the diaphragm and a contralateral mediastinal shift. The size of a pneumothorax is estimated by measuring the distance from the lateral edge of the lung to the inner wall of the ribs. A measurement of $\geq 2\,cm$ indicates a large pneumothorax, suggesting a collapse of approximately $50\%$ of the lung volume, which necessitates Intercostal Tube (ICT) drainage. Other investigations include Arterial Blood Gases (ABG), which may show hypoxemia and hypocapnia. A CT scan of the chest may be utilized to evaluate underlying injury, pulmonary disease, or the presence of emphysematous bullae.

Therapeutic Strategies and Management

Conservative treatment is appropriate for healthy adults who are not breathless and where the pneumothorax volume is less than $20\%$ of the hemithorax. This approach includes simple observation and high-flow oxygen therapy at $10\,L/min$ to accelerate air absorption. Patients are advised to stop smoking and avoid air travel for at least $2$ weeks after successful treatment to prevent recurrence. Simple aspiration is another option, involving the infiltration of $2\%$ lidocaine down to the pleura in the second intercostal space at the Midclavicular Line (MCL). A $3-4\,cm$, $16$ French gauge cannula is pushed through the pleura and connected to a three-way tap and a $50\,ml$ syringe. Up to $2.5\,L$ of air is aspirated unless resistance is felt or the patient coughs excessively, followed by an expiratory chest X-ray. Tension pneumothorax requires needle decompression using a large-bore needle in the second intercostal space at the MCL; air gushing out converts it to a simple pneumothorax, which must be followed immediately by a tube thoracostomy.

Intercostal Tube Drainage and Surgery

Active management via Intercostal Tube (ICT) drainage is indicated for tension pneumothorax, the presence of dyspnea, a large pneumothorax ($> 20\%$ of hemithoracic volume), a small but enlarging pneumothorax, patients on mechanical ventilation, or cases involving underlying pulmonary diseases like COPD. Other indications include a previous contralateral pneumothorax, bilateral pneumothorax, or the presence of hydropneumothorax. Surgical options such as pleurectomy or pleurodesis are considered when there is a failure of lung expansion, a persistent air leak, or a recurrent pneumothorax. Additionally, surgery is recommended for the first occurrence of pneumothorax in professionals at high risk, such as pilots and divers. Potential complications of pneumothorax include recurrence, re-expansion pulmonary edema, tension pneumothorax, bronchopleural fistula, pneumopericardium, and pneumomediastinum.

References

The information presented is supported by the following academic texts: Kumar & Clark Clinical Medicine, Textbook of Internal Medicine, 7th edition, page 886; Guyton AC, and Hall JE (2010), Textbook of Medical Physiology, 12th ed., W.B. Saunders Company; Laura Lee Sherwood (2012), Human Physiology, From Cell to System, 8th ed., Blackwell Press; and Ganong WF (2012), Review of Medical Physiology, 24th ed., Lange Basic Science.