Mariska Hargitay, the 44-year-old actress who plays a tough but empathetic detective on Law & Order SVU, was diagnosed this week with a partially collapsed lung, also known as pneumothorax. The treatable condition can be caused by either lung disease or an injury that allows air to fill up the chest cavity, preventing the lung from properly inflating.

Hollywood gossip site TMZ says this is the second time that Hargitay's lung collapsed in the past month. It notes that she was spotted sledding in Long Island, New York’s Hamptons on New Year's Eve, and SVU's Richard Belzer confirmed reports on "The View" that his co-star had a sledding accident. Hargitay's reps, however, have denied that.

The actress is expected to appear in all episodes of the series set to air this TV season, according to People magazine.  However, the magazine also quoted an unnamed production source as saying, "We don't know when she'll be back in production . . . We're just going to wait and see."

For more information on the condition, we spoke with Shahriar Zehtabchi, a physician and researcher in the Department of Emergency Medicine at the Kings County Hospital in New York and the State University of New York Downstate Medical Center

An edited transcript of our conversation follows:

What is pneumothorax?

Pneumothorax is when air gets between the lung and the chest wall.  When that happens, it compresses the lung and does not allow the lung to expand enough for respiration.

Primary spontaneous pneumothorax is when a bulla or air bubble in the lung tissue ruptures and the air makes its way out of the lung and accumulates between the lung and chest wall. Usually, we see this in smokers, but it can be in anyone.  We don't know for sure why it's most common in smokers, but, theoretically, it could be because when they smoke they hold more air in the lungs, increasing pressure inside the chest and making the bulla more likely to rupture. In primary spontaneous pneumothorax, 99 percent of the time we really don't know why it happens. 

Secondary spontaneous pneumothorax can be caused by lung diseases that produce the bulla, such as emphysema or asthma. Emphysema is mostly found in smokers and causes lung tissue to die. Air can get trapped in this dead tissue and any of these trapped bubbles can break and cause pneumothoax. There are also some organisms that infect the lung, such as Pneumocystis carinii, which is associated with HIV and can cause bullae.

If someone gets stabbed, shot in the chest, or otherwise injured, that would be secondary pneumothorax, but you can't call it spontaneous.

What type of pneumothorax do you most commonly see in the emergency room?

We are one of the biggest trauma centers in the country, and we see a lot of secondary pneumothorax from stab wounds, gunshot wounds, and accidents. But we also see a good number of cases of spontaneous pneumothorax, and we have a lot of patients with emphysema and HIV.  I have seen at least two cases of primary spontaneous pneumothorax this year. 

I had a very young, fit athlete -- a basketball player in his 20s, very healthy and active.  He came in for shortness of breath.  He couldn't breathe and had chest tightness for two to three weeks. I listened to his lungs and there was decreased air movement on one side. We got a chest X-ray and one of the lungs was almost completely collapsed.

How do you treat it?

We generally put a tube into the chest wall. We connect the tube to a vacuum and the vacuum sucks the air out.  We leave the tube in for four to five days and then take it out.  Usually that takes care of it.

If it happens again, we refer them for a pleurodesis. Using video guidance, they usually insert talc or other chemicals between the lung and the chest wall.  That produces irritation, inflammation, tissue fibrosis, and adhesion which permanently closes the gap between the lung and chest wall. That's a procedure that is done by a thoracic surgeon.

In 2007, I published a paper on using needle aspiration for primary pneumothroax. If the patient doesn't have any lung disease, instead of jamming a big tube in the lung and admitting her or him to the hospital, we put a large bore catheter inside the lung and simply aspirate the air, and send the patient home. In Europe, it’s a very popular procedure, but in the U.S. people are hesitating, because everyone is afraid of sending patients with pneumothorax home. A lot of young physicians are open to this procedure and patients are very happy.  I've done it twice in the past year. It's just a needle like any other injection as opposed to a big tube in your chest for five days.

This is Mariska Hargitay's second pneumothorax in the last month.  How common is that and what does it mean for her?

She's going to need the pleurodesis. For a single incident, usually we can treat it with a chest tube or needle aspiration. When it's recurrent, most physicians refer the patient to a thoracic surgeon to do the adhesion.  The prognosis is pretty good.  It's not a debilitating problem when it is treated.  It's not something that will prevent her from acting, exercising and doing daily activities.