The aim of this barchelor thesis is to investigate the frequency of invasive and non-invasive mechanical ventilation and outcomes of patients who were diagnosed with acute respiratory insufficiency caused by coronavirus after initial treatment with high-flow oxygen therapy in the Covid respiratory center of the Intensive Care Unit of the General Hospital Karlovac. This research is retrospective analysis of patients data from the hospital information system of the General Hospital Karlovac in the period from October 1, 2020. to December 31, 2021. The patients were divided into two groups. Group A are the patients that received only HFNC therapy and group B that received NIMV and/or IMV after initial HFNC therapy. In this resarch it was percieved that NIMV/IMV treatment in group B was applied more often (72.36%) compared to HFNC therapy in group A (27.64%). Also the difference in age and gender was percieved. The patients of group A were younger than those from group B and in both groups there was a higher proportion of males than females. The average number of days of treatment with HFNC was higher in group A (6.82 days) than in group B ( 2.70 days). Admission parameters, as indicators of low risk for mechanical ventilation (ROX index), were in favor of group A. The number of hospitalization days in CRC ICU was shorter in group A (7.56 days) compared to group B (12.63 days). 71 patients (79.78%) had death as an treatment outcome in group B, while not one death was noted in group A. Based on the obtained results we conclused that COVID-19 positive patients who were successfully treated with HFNC had fef wer days of hospitalization in the CRC ICU, as well as lower mortality rate compared to patients who were treated with NIMV and/or IMV after the application of HFNC.