A 37-year-old truck driver is seen with a history of recent onset of shoness of breath aggravated by bending over, lying down, and exeion. On physical exam, vital signs are normal. Lung exam shows decreased air movement at the left base with an area of dullness posteriorly. Abdomen is soft without palpable organomegaly. CXR is shown.The above diagnosis can best be confirmed by

This x-ray shows the left hemidiaphragm to be elevated in relation to the right side. There is no mediastinal pathology and the lung fields are clear. The hyperlucent gas shadow below the diaphragm suggests that there is no subdiaphragmatic pathology Unilateral elevation of the hemidiaphragm can be seen as a result of an enlargement or displacement of an abdominal organ, a subpulmonic process such as effusion, loss of volume of the lung with lobar atelectasis or surgical resection, or hemidiaphragmatic paralysis. Diaphragmatic paralysis results from interruption of the phrenic nerve supply to the diaphragm. The most common cause is malignancy, such as bronchogenic carcinoma, or postsurgical trauma. Twenty percent of patients who undergo open hea surgery sustain injury to the phrenic nerve. Other causes of diaphragmatic paralysis include polio, herpes, infections, lead poisoning, pulmonary infarctions, pneumonia, mediastinitis, and pericarditis. The diagnosis of unilateral paralysis of the diaphragm is suggested by the finding of an elevated hemidiaphragm on the chest x-ray. With diaphragmatic paralysis, the negative pleural pressure tends to pull the paralyzed diaphragm upward. Normally the right diaphragm is about 3 cm higher than the left. Confirmation of diaphragmatic paralysis is established by the sniff test. In this test the diaphragm is observed fluoroscopically as the patient sniffs. The normal diaphragm is moved downward during the sniff maneuver as the diaphragmatic muscles contract. A paralyzed diaphragm moves paradoxically upward because of negative pleural pressure. Patients with paralyzed diaphragms may be asymptomatic or may complain of dyspnea on lying down or with exeion. With complete paralysis, vital capacity and total lung capacity may be reduced about 25% from the baseline, and the maximum inspiratory pressure is reduced to about 40%