Experimental: Ketamine (HIKER)

Patients in the HIKER arm will receive a single dose of ketamine 0.50 mg/kg, which is enough to achieve a full anaesthetic effect (i.e., unconsciousness mimicking the GA regimen above), on 8 successive weekdays.

Active_comparator: Ketamine-ECT (EAST)

Patients in the EAST arm will initially receive intravenous ketamine 0.75 mg/kg, remifentanil 1 mcg/kg (to reduce discomfort), and succinylcholine 0.75 mg/kg (for safety). Based on patients' anaesthetic response, the attending anaesthesiologist is given the freedom to vary the dose of remifentanil and succinylcholine as well as administer propofol to achieve safe and acceptable anaesthetic conditions. As per the Saskatoon Health Region's care standard, patients in the EAST arm will receive eight ECT sessions (on a bi/triweekly schedule) delivered by the attending psychiatrist with either unilateral or bilateral electrode placement and monitoring of seizure threshold by the half-age method.