Four healthy firemen worked on the bicycle ergometer during four steps: (1) without a mask; (2) with a mean mask pressure equal to atmospheric pressure; (3) with a slight positive mean pressure of + 3.1 cm H2O inside the mask; (4) with a higher positive mean pressure of + 6.3 cm H2O inside the mask and a minimal mask pressure of + 2.2 cm H2O. Intraesophageal mean pressure increased only half as much as mask pressure. Only a slight tachycardia and a slight arterial hypotension was seen in step 4 compared to the control steps. Accordingly, a moderate increase of the preejection period (PEP) and a decrease of the left ventricular ejection time (LVET) was recorded, leading to a rise of the PEP/LVET ratio (left ventricular dysfunction). In contrast, no trace of arterial hypoxemia was seen. Opening of closed airways may explain this unexpected result.