This is a randomized, double-blind, placebo-controlled phase 3 clinical trial evaluating the additive effect of intravenous ketamine in combination with electroconvulsive therapy (ECT) in patients with treatment-resistant major depressive disorder (MDD). The study aims to determine whether ketamine enhances the antidepressant efficacy of ECT and reduces associated cognitive side effects. Thirty hospitalized patients diagnosed with treatment-resistant MDD will be randomized to receive either ketamine or placebo (saline) during ECT sessions 2, 4, and 6. Primary outcome is the change in depressive symptoms, measured by the Montgomery-Åsberg Depression Rating Scale (MADRS) at 4 weeks.