This study will evaluate whether prolonged oral levosimendan can preserve respiratory function more effectively than placebo, resulting in better patient functionality as measured by the ALSFRS-R scale. In this randomized, double-blind, placebo-controlled, parallel-group, multicenter study, subjects are allocated in a 2:1 ratio to receive either levosimendan (1 -2 mg daily) or placebo for 48 weeks. The primary endpoint is slow vital capacity (SVC) at 12 weeks, with the impact on patient function assessed through 48 weeks, adjusted for patient outcome, using ALSFRS-R (combined assessment of function and survival, CAFS). Other important efficacy measures include time to respiratory events, clinical global impression (CGI), assessment of dyspnea using the Borg scale and sleep scales (Pittsburgh sleep quality index and Epworth sleepiness scale). Patient safety is monitored using conventional methods including adverse events, safety laboratory tests, vital signs and 12-lead EKG. Following screening and baseline visits, patients attend the clinic at 2, 4, 8, 12, 24, 36 and 48 weeks, with telephone assessments conducted at weeks 18, 30 and 42. An end of study visit is performed 14-25 days after the last study treatment administration. The study will be monitored by an independent data and safety monitoring board. A long-term extension study will be available for patients completing the study.