Experimental: Non-invasive ventilation (NIV) + standard medical treatment

The intervention will consist of application of NIV through a facemask. This will happen as soon as possible after the patient has been randomized by opening the opaque envelope. All patients will receive the same standardized ventilator settings at initiation. The IPAP will be set at 12 cm H2O and the PEEP at 5 cm H2O. These settings are preset on the ventilator used in the physician manned mobile emergency care unit (MECU) in the Central Denmark Region. Oxygen delivery will be adjusted to an arterial oxygen saturation of 88-92%. Further adjustment of ventilator settings and evaluation of treatment effect will be at the discretion of the prehospital physician.~NIV will be administered together with standard medical treatment described below.

No_intervention: Standard medical treatment alone

The standard treatment may include inhaled bronchodilators, intravenous corticosteroids, and titrated oxygen supplementation. Although this treatment is based on regional standard operating procedures (SOP), it may differ from patient to patient because it is based on the clinical judgement by the emergency physician in the MECU.~Inhaled bronchodilators can be given in the form of Fenoterol and Ipratropium as a combination drug and/or Salbutamol. Both as a nebulizer solution. One dose (4 ml) of the combination drug contains 1,25 mg Fenoterol and 0,5 mg Ipratropium. Salbutamol will be administered in a concentration of 1 mg/ml. Five mg will be given initially, which can be repeated if necessary.~Corticosteroids can be given in the form of Methylprednisolone 40-80 mg administered intravenously after establishing an intravenous access.~Oxygen therapy will be delivered through a nasal cannula if possible or a non-rebreather mask to maintain an arterial saturation of 88-92%.