Rationale: The progress of COVID-19 from moderate to severe may be precipitous, while the heterogenous characteristics of the disease pose challenges to the management of these patients. Objectives: To characterize the clinical course and outcomes of critically ill patients with COVID-19 during two successive waves. Methods: We leveraged the multi-center SuPAR in Adult Patients With COVID-19 (SPARCOL) study and collected data from consecutive patients requiring admission to the intensive care unit from April 1st to November 30th, 2020. Measurements and Main Results: Of 252 patients, 81 (32%) required intubation and mechanical ventilation. Of them, 17 (20.9%) were intubated during the first wave, while 64 (79%) during the second wave. The most prominent difference between the two waves was the overall survival (first wave 58.9% vs. second wave 15.6%, adjusted p-value=0.006). This difference is reflected in the prolonged hospitalization during the first wave. The mean ICU length of stay (19.1 vs. 11.7 days, p=0.022), hospital length of stay (28.5 vs. 17.1 days, p=0.012), and days on ventilator (16.7 vs. 11.5, p=0.13) were higher during the first wave. A significant difference between the two waves was the development of bradycardia. In the first wave, 2 (11.7%) patients developed sinus bradycardia only after admission to the intensive care unit, while in the second wave, 63 (98.4%) patients developed sinus bradycardia during hospitalization. Conclusions: Survival of critically ill patients with COVID-19 was significantly lower during the second wave. The majority of these patients developed sinus bradycardia during hospitalization.