Growing evidence has supported rapid and robust antidepressant effects with subanesthetic doses of intravenous (IV) ketamine for treatment resistant depression (TRD). However, no completed or ongoing RCTs have evaluated the effects of repeated doses of IV ketamine for a homogenous sample of patients with treatment-resistant bipolar disorder depression (TRBD). The primary research goal is to determine the acute antidepressant efficacy, safety and tolerability of repeated sub-anesthetic maintenance doses of IV ketamine in, over a period of twelve weeks. Open-label ketamine infusions will be provided on a flexible schedule (every 2-4 weeks) with flexible dosing (0.5-1.0mg/kg over 40 minutes) titrated to optimize benefits, while minimizing the dosage and frequency over a 12-week extension period. All patients participating in this open-label study will have completed an acute course of infusions in a parent two-site, phase II, double-blinded midazolam-controlled RCT trial. In addition to this acute course of four infusions, a maximum of six infusions will be provided over the 12-week period. Secondary aims include evaluating effects of IV ketamine on suicidal ideations, quality of life, function and duration of effects. Herein, a two-site (University Health Network and Canadian Rapid Treatment Centre of Excellence), single-arm, open label, 12-week extension trial evaluating the effects of flexibly-dosed adjunctive ketamine infusions for TRBD to maintain antidepressant effects in participants who achieved an antidepressant response (MADRS decrease by >50%) or remission (MADRS < 12) following an acute course of four ketamine infusions is proposed. The primary outcome will be Montgomery-Åsberg Depression Rating Scale (MADRS) scores, determining by a linear mixed model from baseline to week 12. Secondary outcomes include evaluating response and remission rates, safety, tolerability (including treatment-emergent mania), and effects on suicidality, anxiety, quality of life, function and the duration of effects.