1. patients who need mechanical ventilation (invasive or non-invasive), high flow nasal cannula or admission to icu at the moment of randomization. 2. current diagnosis of acute bronchial asthma attack. 3. history or clinical suspicion of pulmonary fibrosis. 4. current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. 5. patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. 6. patients with pneumonectomy or lobectomy. 7. renal failure with glomerular filtration \<30 ml/min/1.73m2 8. patients with contraindication for anticoagulant treatment. 9. congenital bleeding disorders. 10. hypersensitivity to tinzaparin or ufh or some of its excipients. 11. history of heparin-induced thrombocytopenia. 12. active bleeding or situation that predispose to bleeding. 13. moderate or severe anaemia (hb\<10 g/dl) 14. low platelet count \< 80000/µl 15. patients with life expectancy less than 3 months due to primary disease evaluated by the physician. 16. patients currently intubated or intubated between the screening and the randomization. 17. pregnancy.

1. patients who need mechanical ventilation (invasive or non-invasive), high flow nasal cannula or admission to icu at the moment of randomization. 2. current diagnosis of acute bronchial asthma attack. 3. history or clinical suspicion of pulmonary fibrosis. 4. current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. 5. patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. 6. patients with pneumonectomy or lobectomy. 7. renal failure with glomerular filtration \<30 ml/min/1.73m2 8. patients with contraindication for anticoagulant treatment. 9. congenital bleeding disorders. 10. hypersensitivity to tinzaparin or ufh or some of its excipients. 11. history of heparin-induced thrombocytopenia. 12. active bleeding or situation that predispose to bleeding. 13. moderate or severe anaemia (hb\<10 g/dl) 14. low platelet count \< 80000/µl 15. patients with life expectancy less than 3 months due to primary disease evaluated by the physician. 16. patients currently intubated or intubated between the screening and the randomization. 17. pregnancy.

patients who need mechanical ventilation (invasive or non-invasive), high flow nasal cannula or admission to icu at the moment of randomization. current diagnosis of acute bronchial asthma attack. history or clinical suspicion of pulmonary fibrosis. current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. patients with pneumonectomy or lobectomy. renal failure with glomerular filtration <30 ml/min/1.73m2 patients with contraindication for anticoagulant treatment. congenital bleeding disorders. hypersensitivity to tinzaparin or ufh or some of its excipients. history of heparin-induced thrombocytopenia. active bleeding or situation that predispose to bleeding. moderate or severe anaemia (hb<10 g/dl) low platelet count < 80000/µl patients with life expectancy less than 3 months due to primary disease evaluated by the physician. patients currently intubated or intubated between the screening and the randomization. pregnancy.

patients who need mechanical ventilation (invasive or non-invasive), high flow nasal cannula or admission to icu at the moment of randomization. current diagnosis of acute bronchial asthma attack. history or clinical suspicion of pulmonary fibrosis. current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. patients with pneumonectomy or lobectomy. renal failure with glomerular filtration <30 ml/min/1.73m2 patients with contraindication for anticoagulant treatment. congenital bleeding disorders. hypersensitivity to tinzaparin or ufh or some of its excipients. history of heparin-induced thrombocytopenia. active bleeding or situation that predispose to bleeding. moderate or severe anaemia (hb<10 g/dl) low platelet count < 80000/µl patients with life expectancy less than 3 months due to primary disease evaluated by the physician. patients currently intubated or intubated between the screening and the randomization. pregnancy.

1. patients who need mechanical ventilation (invasive or non-invasive), high flow nasal cannula or admission to icu at the moment of randomization. 2. current diagnosis of acute bronchial asthma attack. 3. history or clinical suspicion of pulmonary fibrosis. 4. current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. 5. patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. 6. patients with pneumonectomy or lobectomy. 7. renal failure with glomerular filtration <30 ml/min/1.73m2 8. patients with contraindication for anticoagulant treatment. 9. congenital bleeding disorders. 10. hypersensitivity to tinzaparin or ufh or some of its excipients. 11. history of heparin-induced thrombocytopenia. 12. active bleeding or situation that predispose to bleeding. 13. moderate or severe anaemia (hb<10 g/dl) 14. low platelet count < 80000/µl 15. patients with life expectancy less than 3 months due to primary disease evaluated by the physician. 16. patients currently intubated or intubated between the screening and the randomization. 17. pregnancy.

1. patients who need mechanical ventilation (invasive or non-invasive), high flow nasal cannula or admission to icu at the moment of randomization. 2. current diagnosis of acute bronchial asthma attack. 3. history or clinical suspicion of pulmonary fibrosis. 4. current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. 5. patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. 6. patients with pneumonectomy or lobectomy. 7. renal failure with glomerular filtration <30 ml/min/1.73m2 8. patients with contraindication for anticoagulant treatment. 9. congenital bleeding disorders. 10. hypersensitivity to tinzaparin or ufh or some of its excipients. 11. history of heparin-induced thrombocytopenia. 12. active bleeding or situation that predispose to bleeding. 13. moderate or severe anaemia (hb<10 g/dl) 14. low platelet count < 80000/µl 15. patients with life expectancy less than 3 months due to primary disease evaluated by the physician. 16. patients currently intubated or intubated between the screening and the randomization. 17. pregnancy.

1. patients who need mechanical ventilation or admission to icu at the moment of randomization. 2. current diagnosis of acute bronchial asthma attack. 3. history or clinical suspicion of pulmonary fibrosis. 4. current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. 5. patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. 6. patients with pneumonectomy or lobectomy. 7. renal failure with mdrd<30 ml/min 8. patients with contraindication for anticoagulant treatment. 9. congenital bleeding disorders. 10. hypersensitivity to tinzaparin or ufh or some of its excipients. 11. history of heparin-induced thrombocytopenia. 12. active bleeding or situation that predispose to bleeding. 13. moderate or severe anaemia (hb<10 g/dl) 14. low platelet count < 80000/µl 15. patients with life expectancy less than 3 months due to primary disease evaluated by the physician. 16. patients currently intubated or intubated between the screening and the randomization. 17. pregnancy.

1. patients who need mechanical ventilation or admission to icu at the moment of randomization. 2. current diagnosis of acute bronchial asthma attack. 3. history or clinical suspicion of pulmonary fibrosis. 4. current diagnosis or suspicion of pulmonary thromboembolism or deep vein thrombosis. 5. patients who need anticoagulant treatment due to previous venous or arterial thrombotic disease, or due to atrial fibrillation. 6. patients with pneumonectomy or lobectomy. 7. renal failure with mdrd<30 ml/min 8. patients with contraindication for anticoagulant treatment. 9. congenital bleeding disorders. 10. hypersensitivity to tinzaparin or ufh or some of its excipients. 11. history of heparin-induced thrombocytopenia. 12. active bleeding or situation that predispose to bleeding. 13. moderate or severe anaemia (hb<10 g/dl) 14. low platelet count < 80000/µl 15. patients with life expectancy less than 3 months due to primary disease evaluated by the physician. 16. patients currently intubated or intubated between the screening and the randomization. 17. pregnancy.