A hemothorax is a collection of blood in the pleural space, the thin gap between the lung and the chest wall. Under normal conditions, this space contains only a small amount of fluid that helps the lungs expand and contract smoothly. When blood fills this space, it compresses the lung and can reduce the heart’s ability to pump effectively. The condition ranges from minor (under 400 mL of blood) to massive (over 1,000 mL), and the severity determines how urgently it needs to be treated.
How Blood in the Chest Affects Breathing and Circulation
The body’s response to a hemothorax has two parts: a breathing problem and a blood loss problem. As blood pools in the pleural space, it physically prevents the lung on that side from fully expanding, reducing the amount of air you can take in with each breath. At the same time, losing blood from the circulatory system drops blood volume, which forces the heart to work harder with less to pump.
In severe cases, the pressure from a large volume of blood pushes against the major veins returning blood to the heart and compresses lung tissue. This creates a dangerous feedback loop: the heart receives less blood, pumps less effectively, and blood pressure drops. This is called tension hemothorax physiology, and without treatment, it leads to cardiovascular collapse.
Common Causes
Trauma is the most frequent cause. Blunt chest injuries from falls, car accidents, and similar impacts account for the majority of cases. Rib fractures are a particularly common trigger. Roughly 33% of people who fracture ribs from blunt trauma go on to develop a hemothorax, because the sharp edges of broken bone can tear blood vessels or puncture lung tissue. Among blunt trauma patients in one large study, simple falls caused 36% of cases, falls from height caused about 20%, and vehicle accidents caused 11%.
Penetrating injuries, like stab wounds or gunshot wounds, can directly damage arteries in the chest wall, the lung itself, or major vessels like the aorta.
Non-traumatic hemothorax, sometimes called spontaneous hemothorax, is less common but still important to recognize. It can result from a ruptured aortic aneurysm, abnormal blood vessel formations in the lungs, or blood clotting disorders. Certain medical procedures carry a small risk as well, including central line placement, lung biopsies, and fluid drainage from the chest. People on blood-thinning medications face a higher risk because their blood doesn’t clot as readily to seal off small vascular injuries.
What It Feels Like
Symptoms depend on how much blood has accumulated and how quickly. A small hemothorax may cause mild chest discomfort and slight shortness of breath that develops gradually. A large or rapidly growing hemothorax feels much more dramatic: sharp chest pain, difficulty breathing, a racing heart, lightheadedness, and pale or clammy skin. These are signs the body is losing significant blood volume.
On a physical exam, a doctor will typically notice decreased breath sounds on the affected side (because the lung is compressed) and a dull sound when tapping the chest, rather than the normal hollow resonance of an air-filled lung. If the hemothorax is large enough, the affected side of the chest may visibly move less during breathing. People with underlying conditions like anemia or heart disease may develop severe symptoms even from a moderate amount of blood, because they have less reserve to compensate.
How It’s Diagnosed
Chest X-ray is the traditional first step, but it has meaningful limitations. It detects hemothorax with about 63% sensitivity, meaning it misses roughly one in three cases, particularly smaller collections. Bedside ultrasound performs better, catching about 79% of cases while maintaining near-perfect specificity (97.9%), which means false alarms are rare with either method.
When compared head to head using CT scan as the gold standard, the gap widens further. Ultrasound detected hemothorax in about 66% of confirmed cases, while X-ray caught only 34%. This is why emergency departments increasingly rely on ultrasound (often as part of a rapid trauma scan called eFAST) for initial assessment, especially when speed matters. CT scanning remains the most accurate tool and is used when the diagnosis is uncertain or doctors need detailed information about the source of bleeding.
Formally, hemothorax is defined as pleural fluid with a blood cell concentration greater than 50% of the patient’s circulating blood, distinguishing it from other types of fluid buildup in the chest.
Treatment
The primary treatment is draining the blood through a chest tube, a procedure called tube thoracostomy. For hemothorax specifically, a large-bore tube (36 French or larger) is needed because blood is thicker than air or clear fluid and would clog a smaller tube. The tube is inserted through the chest wall, typically at the level of the nipple line along the side of the chest, into the pleural space. A chest X-ray afterward confirms the tube is in the right position.
Most hemothoraces resolve with chest tube drainage alone. The tube stays in place until the bleeding stops and the fluid output slows, which may take a few days. During that time, the medical team monitors how much blood drains per hour.
Surgery becomes necessary when the bleeding is too heavy for a chest tube to manage. Current guidelines call for surgical intervention if more than 1,500 mL of blood drains immediately after the tube is placed, or if bleeding continues at more than 200 mL per hour over the first two to four hours. Surgery is also required for injuries involving the heart, major blood vessels, or the airway. The operation (thoracotomy) allows surgeons to directly find and repair the source of bleeding.
Complications of Incomplete Drainage
Getting all the blood out matters. When blood remains in the pleural space after initial treatment, it creates a breeding ground for infection. Empyema, a bacterial infection of the pleural space, complicates chest tube drainage in up to 10% of trauma cases overall. But the risk is dramatically higher when blood is left behind: 33% of patients with residual hemothorax develop empyema, compared to just 2% of those whose blood is fully drained.
There’s also a concern about fibrothorax, a condition where the leftover blood organizes into thick scar tissue that traps the lung and prevents it from expanding normally. This can lead to chronic breathing problems and, in severe cases, respiratory failure. However, clinically significant fibrothorax is usually a consequence of infection rather than retained blood alone, which is one reason preventing empyema through complete drainage is so important.
Severity Classification and Outlook
Hemothorax is classified by volume. Less than 400 mL is considered minimal, 400 to 1,000 mL is medium, and anything above 1,000 mL is massive. Minimal hemothoraces sometimes resolve on their own without a chest tube, though they still need monitoring. Massive hemothorax is a life-threatening emergency.
Outcomes depend heavily on the cause and speed of treatment. An isolated, promptly treated traumatic hemothorax generally has a good prognosis. The picture gets more complicated when major arteries like the aorta, intercostal arteries, or internal mammary arteries are involved, because these can bleed fast enough to cause fatal blood loss before surgical control is achieved. Non-traumatic cases tied to aortic aneurysm or vascular malformations also carry higher risk because the underlying condition itself is dangerous.