A Study of Ketamine Infusions to Treat Clinically-depressed ICU Patients

Inclusion Criteria: * Has been in the Mayo Clinic Florida ICU for the past 1 week. * PHQ-9 score of 10 or higher. * One of the following diagnoses: acute myocardial infarction; acute renal failure; chronic obstructive pulmonary disease (COPD); congestive heart failure; end-stage liver disease; patients requiring intra-aortic balloon pump (IABP) therapy or continuous renal replacement therapy (CRRT); post-bone marrow transplant; acute Hypoxemic Respiratory Failure; acute or chronic hypoxemic respiratory failure; pre- or post- lung, liver or heart transplant or surgical overflows (such as abdominal aortic aneurysm, thoracotomy or tracheostomy). Exclusion Criteria: * Poor vital sign stability hypoxia: O2 \< 95%, hypotension: SBP \< 90 hypertension: SBP \> 180. * Heart rate: \< 50 or \> 120, or Respiratory Rate: \< 10 or \> 30. * Altered mental status. * Patient is unwilling to participate or provide informed consent. * Any allergy to ketamine or diphenhydramine. * Patient is female of child-bearing age and unwilling to provide urine or blood for HCG analysis. * Pregnant or breastfeeding. * Presence of intracranial mass or vascular lesion. * Presence of a history of psychosis or hallucinations (as assessed by electronic chart review). * Weight greater than 115 kg or less than 45kg. * History of increased intracranial pressure/hypertensive hydrocephalus or increased intraocular pressure. * Patient is acutely psychotic. * Provider feels that patient currently or likely will require chemical and/or physical restraints. * History of prolonged QT-interval. * Current treatment includes any medication known to affect the N-methyl-D-aspartate receptor system (e.g., lamotrigine, acamprosate, memantine, riluzole, or lithium). * Patient in withdrawal from substance abuse or who have used hallucinogens (including cannabis) in the last month for any indication, as determined by the clinical interview and urine drug screening.