We searched for a PFO ( transesophageal echocardiography ) in healthy HA dwellers ( n = 22 ) and patients with chronic mountain sickness ( n = 35 ) at 3,600 m above sea level and studied its effects ( transthoracic echocardiography ) on right ventricular ( RV ) function , pulmonary artery pressure , and vascular resistance at rest and during mild exercise ( 50 W ) , an intervention designed to further increase pulmonary artery pressure .