Chronic obstructive pulmonary disease (COPD) is a disease of the air sacs and air passages of the lungs. Usually caused by smoking, damage to the lungs gradually worsens over time. People with COPD are short of breath and sometimes cough and wheeze. Symptoms slowly worsen over time, but most patients also have intermittent bouts of acute worsening of symptoms. Lung infections, cold weather, and exertion may bring on these bouts. With severe bouts, patients need close observation, treatment in the hospital, and, sometimes, a tube in the windpipe (trachea) to help them breathe (intubation). Rather than putting a tube down the trachea, doctors may try to force oxygen into the lungs with a mask over the mouth or nose that is connected to a breathing machine (ventilator) with a tube. This technique is called noninvasive positive-pressure ventilation (NPPV). Several recent studies have tested whether NPPV actually helps patients with severe bouts of COPD and whether it prevents the need for a tube in the trachea.