Question

A $70$-year-old man is brought to the emergency department due to sudden-onset severe shortness of breath, generalized weakness, and confusion that developed over the past $4$ hours. His medical history is significant for an inferior ST-elevation myocardial infarction $5$ days ago (for which he presented late and was managed medically), severe chronic obstructive pulmonary disease (COPD), bipolar I disorder, and chronic kidney disease.

On presentation, the patient is lethargic and oriented only to person. Temperature is $35.8^{\circ}\text{C}$ ($96.4^{\circ}\text{F}$), blood pressure is $82/52\text{ mmHg}$, heart rate is $118\text{/min}$, and respiratory rate is $28\text{/min}$. Oxygen saturation is $85\%$ on room air. Physical examination reveals distended neck veins up to the angle of the mandible while sitting at a $45$-degree angle. Chest auscultation demonstrates bilateral crackles extending to the mid-lung fields and diffuse expiratory wheezes. Cardiac auscultation reveals tachycardia with a new, harsh, holosystolic murmur heard best at the apex with radiation to the axilla. His extremities are cool and clammy with $2+$ bilateral pitting edema.

Laboratory studies show:
Leukocyte count: $12,500\text{/mm}^3$
Serum sodium: $131\text{ mEq/L}$
Serum potassium: $4.9\text{ mEq/L}$
Serum chloride: $96\text{ mEq/L}$
Serum bicarbonate: $14\text{ mEq/L}$
Blood urea nitrogen: $55\text{ mg/dL}$
Serum creatinine: $2.6\text{ mg/dL}$ (baseline $1.3\text{ mg/dL}$)
Serum lactate: $6.2\text{ mmol/L}$

Arterial blood gas on room air:
pH: $7.26$
$\text{PaCO}_2$: $29\text{ mmHg}$
$\text{PaO}_2$: $54\text{ mmHg}$

A sputum culture collected three days ago at his primary care clinic for a mild chronic cough has grown *Streptococcus pneumoniae*, which is fully susceptible to amoxicillin. His family mentions he has been profoundly depressed over the past three weeks and urgently requests he be started on fluoxetine.

Which of the following is the most appropriate next step in the management of this patient?