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Few studies exist on the effects, in terms of work capacity and safety, of exposure to moderately high altitudes in patients with stable ischemic left ventricular dysfunction. Moreover no data are currently available on the cardiorespiratory response to walks in the mountains.The objective of this study is to evaluate the effects of altitude on effort tolerance during walks in the mountains and to determine whether exposure to altitude may be harmful to patients with ischemic left ventricular dysfunction.Forty-five patients with stable chronic ischemic left ventricular dysfunction (ejection fraction=35±4%, and peak VO2≥18/ml/kg per min in a preliminary effort test) were compared to 24 normal subjects. All subjects underwent a series of 6-min walking tests at three different altitudes: 500, 2000 and 2970 m above sea level. Cardiorespiratory response was assessed by a validated portable instrument. The resting arterial PO2 was measured at the three altitudes.No complications were observed during any tests in either the patients or the healthy controls. Overall, healthy subjects had higher values of 6-min walking test VO2 and walked longer distances in the test than did the patients with left ventricular dysfunction. The mean distances walked in the 6-min walking test were similar at 500 and at 2000 m in both the healthy controls and the patients; at 2970 m, however, the distances decreased in both groups, and more so in the patients (−11±3%) than in the controls (−5±2%) (P<0.01). VO2 during the 6-min walking test remained stable when the test was carried out at 500 and 2000 m (20.4±3.6 versus 19.9±4.1 ml/kg per min in patients, and 30.2±3.4 versus 29.8±4.2 ml/kg per min in the controls; P, NS), but decreased at 2970 m by 13.9±3% in patients (P<0.01) and by 6.6±2.1% in controls (P<0.01) (patients versus controls, P<0.01). Finally, a similar, significant decrease in arterial PO2 was observed in both groups only at 2970 m (−29%, P<0.01).Patients with stable ischemic left ventricular dysfunction had good tolerance while walking at high altitudes, but showed a moderate decrease in work capacity at 2970 m, which was greater than in normal subjects.