A pneumothorax generally needs a chest tube when it measures more than 2 cm from the lung edge to the chest wall at the level of the hilum (the midpoint where blood vessels enter the lung), or more than 3 cm from the lung apex to the top of the chest cavity, depending on which guideline your doctor follows. But size is only part of the equation. Your symptoms, breathing stability, and whether you have underlying lung disease all influence whether a chest tube is placed or a less invasive approach is tried first.
The Two Size Cutoffs and Why They Differ
There is no single universal number. The two most widely used guidelines define “large” pneumothorax differently. The British Thoracic Society (BTS) measures the gap between the lung edge and the chest wall at the hilum, the middle of the chest on an upright X-ray. If that gap exceeds 2 cm, the pneumothorax is considered large. The American College of Chest Physicians (ACCP) instead measures from the top of the collapsed lung down to the top of the chest cavity. If that distance exceeds 3 cm, it’s classified as large.
These two methods don’t always agree on the same patient. A study comparing both approaches found that the BTS 2 cm hilar cutoff tends to classify fewer pneumothoraxes as large, which could lead to underintervention: roughly a third of patients managed conservatively under BTS criteria came back needing drainage. The ACCP 3 cm apical cutoff casts a slightly wider net, flagging more cases for treatment. Neither measurement is perfect, which is why clinicians weigh size alongside your clinical picture.
Primary vs. Secondary Pneumothorax
The threshold for placing a chest tube drops significantly if you have an underlying lung condition like COPD, cystic fibrosis, or interstitial lung disease. This is called a secondary spontaneous pneumothorax (SSP), and it’s treated more aggressively because damaged lungs tolerate even small air leaks poorly and are less likely to re-expand on their own.
Under BTS guidance, a secondary pneumothorax smaller than 1 cm at the hilum can be managed with close inpatient observation alone. Between 1 and 2 cm, needle aspiration is typically attempted first. But if the pneumothorax is larger than 2 cm, or if aspiration fails, a chest tube is the next step. By contrast, a healthy person with a primary spontaneous pneumothorax of the same size might first get a trial of needle aspiration before anyone considers a tube. The bar is simply lower when your lungs are already compromised.
Age and smoking history also matter. Guidelines treat patients over 50 with a significant smoking history the same as those with known lung disease, because the odds of undiagnosed underlying damage are high.
When Size Doesn’t Matter
Certain situations call for a chest tube regardless of how big the pneumothorax looks on imaging. A tension pneumothorax, where trapped air builds enough pressure to push the heart and major blood vessels to the opposite side of the chest, is a life-threatening emergency. The hallmarks are sudden severe shortness of breath, low blood pressure, absent breath sounds on one side, visibly distended neck veins, and a windpipe that has shifted away from the affected side. If these signs are present, needle decompression is performed immediately, followed by chest tube placement, without waiting for an X-ray or measuring anything.
Traumatic pneumothorax, from a rib fracture, stabbing, or blunt chest injury, also tends to get a chest tube earlier because the mechanism of injury makes ongoing air leaks more likely. The same applies when someone is about to go on a ventilator, since positive-pressure breathing can rapidly worsen a small pneumothorax into a tension pneumothorax.
Symptoms alone can override size. If your oxygen saturation drops below 90%, your heart rate climbs, or you’re visibly struggling to breathe, those signs of clinical instability push the decision toward a tube even if the pneumothorax measures under the usual cutoff.
What Happens Before a Chest Tube
For a first-time primary spontaneous pneumothorax that’s large but not causing hemodynamic problems, most guidelines recommend trying needle aspiration first. A doctor inserts a needle into the chest wall, draws out the trapped air with a syringe, and then checks whether the lung re-expands on a follow-up X-ray. If the lung stays inflated, you may avoid a chest tube entirely.
Aspiration works well for many people with a first episode and no underlying lung disease. When it fails, meaning the lung collapses again or the patient’s condition worsens, a chest tube is placed. For secondary pneumothorax, aspiration is still sometimes attempted in the 1 to 2 cm range, but the success rate is lower, which is why guidelines favor moving to a tube sooner.
Small-Bore vs. Large-Bore Tubes
If you do need a chest tube, the device itself varies. Traditional large-bore chest tubes range from 20 to 28 French (roughly 7 to 9 mm in diameter). Smaller pigtail catheters, typically 10 to 14 French, are thinner, less painful to insert, and increasingly used as a first-line option. A study comparing the two in secondary spontaneous pneumothorax found identical success rates of about 73% for both, with no difference in safety. Many hospitals now default to pigtail catheters for straightforward pneumothorax cases, reserving large-bore tubes for trauma, large air leaks, or situations where blood is also draining from the chest.
From a patient perspective, a pigtail catheter is inserted with local anesthesia through a small puncture rather than the larger incision a traditional tube requires. It’s generally less uncomfortable during placement and while it remains in place, which can be anywhere from a couple of days to over a week depending on how quickly the air leak seals.
How Size Is Estimated
Doctors measure pneumothorax size on an upright chest X-ray. The simplest approach is the single-distance method used by BTS or ACCP guidelines, but these rough measurements don’t translate neatly into a percentage of lung collapse. A 2 cm rim of air around the lung can represent a much larger volume loss than it appears, because volume increases as a cube of the radius.
More precise methods exist. The Collins method adds up the distance between the lung edge and the chest wall at three points (the apex, the upper midlung, and the lower midlung) and plugs those into a formula to estimate percentage collapse. The Rhea method averages those same three distances and compares the result to a reference table. In practice, these formulas are used inconsistently, and CT scans give the most accurate volume estimate when precision matters. For most clinical decisions, though, the simple hilar or apical measurement on a plain X-ray is what determines whether you get observation, aspiration, or a chest tube.