Pneumothorax

By Donald R. Elton, MD, FCCP
Lexington Pulmonary and Critical Care



Definition
A pneumothorax is a collection of gas that is in the pleural space within the chest but outside of the lungs. The pleural space is a potential space bounced by the visceral and parietal pleura. This potential space exists to allow the chest wall to exert a suction to oppose the normal elastic recoil of the lungs. If this potential space is filled by air, this opposition is lost and a collapse of the involved lung occurs.

Classification
A pneumothorax is called spontaneous if it occurs in the absence of any trauma to the chest. It is considered to be primary when there is no obvious lung disease. A traumatic pneumothorax is normally caused by penetrating chest trauma but could be a result of barotrauma from a ventilator. A tension pneumothorax is one in which there is positive pressure in the pleural space sufficient to cause patient compromise.

Etiology
There are many causes of pneumothorax. Air enters the pleural space either from outside the chest (penetrating trauma), from lung parynchyma, or from the esophagus in cases of perforation. Obstructive lung diseases, either intrinsic, or resulting from foreign body airway obstruction, can lead to pneumothorax by allowing for inadequate exhalation of inspired gas. This can start out as "Auto-PEEP" which may damage lung tissues resulting in air leaks or can result from a "ball-valve" effect where an obstruction can be bypassed by enlarging airway diameter on inspiration but cannot be bypassed during exhalation when airway diameter is smaller. Such an obstruction could be caused by mucus plugging or by a foreign body in an airway. Pleural diseases can cause a pneumothorax by direct disruption of the pleura. Marfan syndrome patients are more susceptible to pneumothorax possibly due to a combination of abnormal connective tissue and by greater pleural pressure changes during breathing that result from the Marfan body habitus. This occurs mostly in young tall thin males.

Tension Pneumothorax
A tension pneumothorax results from situations where air can enter the pleural space but cannot leave. An example might be a stab wound to the chest that punctures the lung. While there is a hole in the chest, a knife wound tends to seal itself in the chest wall thus trapping any air leaked from the lung injury in the chest cavity. A gun shot wound is less likely to result in a tension pneumothorax as the leaked air can exit the chest via the entry wound in the chest wall. Most tension pneumothoraces are traumatic and many of these are iatrogenic and occur on patients who are receiving mechanical ventilation where positive airway pressures facilitate leakage of air into the pleural space. A tension pneumothorax is life threatening because there is more complete collapse of the affected lung and because the positive pressure exerts several adverse effects on the cardiovascular system including tamponade of the mediastinal structures and kinking of the great vessels at their point of entry to the thorax caused by mediastinal shift.

Diagnosis
A pneumothorax by have either an insidious or dramatic onset. Typical symptoms include pleuritic chest pain and dyspnea. Tachycardia and severe hypotension are the only noticeable findings in a mechanically ventilated patient or a patient who is otherwise unable to provide a history. There may be evidence of tracheal deviation and a deviated PMI if there is tension with mediastinal shift. Breath sounds will normally be absent or diminished but you may be able to hear breath sounds transmitted from the contralateral lung in some patients. In some cases, transillumination of the chest with a bright light source will light up the hemithorax of a patient with a pneumothorax though this technique is limited to pediatric patients or those with a thin chest wall. A chest x-ray, preferably with a lateral view or one taken during exhalation is normally the most definitive test to diagnose a pneumothorax but in some cases of tension pneumothorax, a patient may succumb before there is time to obtain one so it is necessary to consider treating an as-yet unconfirmed pneumothorax if the clinical situation dictates.

Radiographic diagnosis
The classic finding of pneumothorax on a chest x-ray is to see a peripheral lung margin with lucency between the lung margin and the chest wall. With a tension pneumothorax there will be mediastinal shift away from the affected side. It must be remembered, however, that the chest is a 3-dimensional structure and that the free air of a pneumothorax may be loculated anterior or posterior to lung or even interposed between the diaphragm and the lung. For this reason, presence of lung markings on a single view chest x-ray does not rule out a pneumothorax. One should be suspicious of any unusual lucencies in the chest and additional views should be obtained, perhaps even including CT scans of the chest in some cases.

Treatment
Not all pneumothoraces require specific therapy. In asymptomatic or mildly symptomatic patients, serial chest x-rays can be obtained to allow for spontaneous resorption of a small pneumothorax. Resorption can be accelerated by placing a patient on 100% oxygen to wash nitrogen from the pleural space with a resultant decrease in the size of the collected free gas. There has been some success reported with needle thoracentesis of spontaneous pneumothoraces though recurrences of pneumothorax are not unusual with this technique and a repeat procedure or chest tube may be required. If a patient is being mechanically ventilated there is a higher risk of progression to a tension pneumothorax and in these cases a chest tube should be used to evacuate any free air and allow time for the original air leak to heal.

Chest Drainage Systems
In an emergency, the initial treatment of a pneumothorax, tension or otherwise, is to place a large bore IV needle into the pleural space to decompress any tension and/or to remove free air to allow the lung to re-expand. Free air will normally collect anteriorly in a supine patient so the needle would be placed into the chest, over a rib, normally in the 2-4th rib interspace between the anterior axillary and mid-clavicular lines. In many cases, a patient will improve as soon as tension is released with a needle procedure though some patients will not improve until their lung has been re-expanded. Once a chest tube has been inserted in the pleural space (anteriorly if possible) a system is needed that will allow air and fluids out of the chest without allowing air to leak back into the chest. In some cases, it may be desirable to allow for suction to be applied to the pleural space to facilitate removal of air or fluids. A Heimlich valve can be used to satisfy the first requirement and consists essentially of a one-way valve that is attached directly to a chest tube. A make shift Heimlich valve can be produced by attaching the finger of a surgical glove over the end of a chest tube and placing a small hole in the end of the glove finger. The glove finder would tend to allow gasses to escape but would collapse in response to negative pressure in the pleural space. 
A 3-bottle or Atrium or Pleurovac drainage system can provide valuable information about the patient's pleural pressure and the presence of absence of an active air leak (bronchopleural fistula for example). The first chamber (White on a Pleurovac) is a simple trap to catch any fluids drained from the chest tube. The second chamber (Pink on a Pleurovac) is the underwater seal or water seal chamber. This chamber exists to provide the one-way valve that will allow air to exit the chest but not to enter it. Note that the tube from the drainage chamber is immersed under water about 2 cm deep. Any pressure in the pleural space greater than 2 cm H2O will result in bubbles forming in this chamber. If you see bubbles in this chamber then there is air coming from the chest tube or connecting tubing. Note that a hole in the chest tube that is outside of the chest will result in heavy bubbling in this chamber so not all leaks are from the pleural space. The third chamber (Blue on a Pleurovac) is the suction control chamber. If the "S" connection is attached to wall suction and there is bubbling from the vent tube, then the level of negative pressure applied to the pleural space is equal to the depth that the vent tube is placed under water. 20 cm H2O would be a typical level of suction. Some chest drainage systems substitute a needle valve suction regulator for the suction control chamber. To measure the pleural pressure, one can pinch off the suction tubing "S" and read the water level in the underwater seal chamber (second chamber).

Prevention
Iatrogenic pneumothoraces can be prevented in some but not all cases.  If a thoracentesis is to be performed, use of ultrasound guidance can help in avoiding lung injury and avoidance of pleural adhesions which may be likely to result in a pneumothorax if punctured. The biggest danger in an iatrogenic pneumothorax is in not recognizing that one has occurred. Listening to breath sounds before an invasive procedure starts will make it much easier to tell if a pneumothorax exists afterwards. In high risk patients (i.e. high airway pressures and/or unstable hemodynamics) plan ahead by having large bore needles, stop cocks, and syringes available before the procedure is started so there will be minimal delay in draining a life-threatening pneumothorax.

References
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