Midodrine

Mechanism of Action

Midodrine is a directly acting α₁-adrenergic receptor agonist (prodrug converted to its active metabolite desglymidodrine). Crucially, it does not cross the blood-brain barrier, so its vasopressor effects are purely peripheral — it increases arterial and venous tone, raising blood pressure without significant CNS effects (Harrison's, p. 12488).

Primary Indications

Indication	Notes
Neurogenic orthostatic hypotension (OH)	First-line pharmacologic agent; used in autonomic neuropathy, MSA, PAF, PD
Vasovagal/reflex syncope	Adjunctive in refractory cases
POTS (postural orthostatic tachycardia syndrome)	Off-label; improves venous return
Dialysis-associated hypotension	Off-label; used intradialytically
Hepatorenal syndrome	Off-label adjunct with albumin and octreotide

Dosing

Standard: 5–10 mg orally three times daily (TID)
Decremental dosing (preferred by some patients):
- 15 mg on awakening
- 10 mg at midday
- 5 mg in the afternoon
Last dose: Must be taken no later than 6:00 PM to avoid nocturnal supine hypertension (Harrison's, p. 12488)
Onset: ~1 hour; duration of action: 2–4 hours

Side Effects

Side Effect	Details
Supine hypertension	Most clinically significant; dose-dependent
Pruritus / piloerection	Due to peripheral α₁ stimulation of piloerector muscles
Urinary retention	α₁ stimulation of bladder neck/sphincter
Bradycardia	Reflex response to increased BP
Paresthesias, scalp tingling	Common

Contraindications

Severe organic heart disease
Acute renal failure / urinary retention
Pheochromocytoma
Thyrotoxicosis
Persistent/excessive supine hypertension
Use with caution in patients with diabetes or visual problems (may exacerbate)

Comparison with Other Agents for Neurogenic OH

Drug	Mechanism	Key Advantage	Key Drawback
Midodrine	Direct α₁ agonist	Proven, titratable	Supine HTN, frequent dosing
Droxidopa	NE prodrug	Effective in PAF, PD, MSA	Costly
Fludrocortisone	Mineralocorticoid	Volume expansion	Worsens supine HTN, fluid retention
Pyridostigmine	AChE inhibitor	Minimal supine HTN effect	Modest BP benefit

(Harrison's, p. 12488)

Key Clinical Pearls

Patients should be upright or seated when taking midodrine — do not take while supine or recumbent
Monitor supine blood pressure regularly; a reasonable threshold is to hold if supine BP >180 mmHg systolic
Elevating the head of bed (10–20°) at night reduces supine hypertension risk
In hepatic failure/HRS, used in the classic triplet: midodrine + octreotide + albumin infusion
FDA approved for symptomatic orthostatic hypotension; many other uses are off-label

octreotide

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octreotide mechanism of action indications dosing

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octreotide side effects contraindications pharmacology somatostatin analog

GradeResponse

Octreotide

Mechanism of Action

Octreotide is a synthetic octapeptide analog of somatostatin (natural somatostatin has a half-life of ~2 minutes; octreotide's is ~90–120 minutes). It binds primarily to somatostatin receptors 2 and 5 (SSTR2, SSTR5), mediating:

Inhibition of GH, IGF-1, glucagon, insulin, gastrin, secretin, VIP, and other gut hormones
Splanchnic vasoconstriction (reduces portal venous pressure)
Reduced GI secretion and motility
Antiproliferative effects on neuroendocrine tumor cells

Formulations

Formulation	Route	Half-life / Duration	Use
Octreotide acetate (short-acting)	SC or IV	~90–120 min	Acute conditions, titration
Octreotide LAR (long-acting release)	IM (monthly)	~23–30 days	Chronic maintenance

Indications & Dosing

1. Acromegaly

Short-acting: 50–100 mcg SC TID (titrated up to 500 mcg TID)
LAR: 20 mg IM q4 weeks (titrate to 10–40 mg)

2. Carcinoid Syndrome / Neuroendocrine Tumors (NETs)

Short-acting: 100–600 mcg/day SC in 2–4 divided doses
LAR: 20–30 mg IM q4 weeks
Short-acting formulation still used for breakthrough symptoms and perioperatively around surgery or PRRT (Neuroendocrine Neoplasms, p. 67)

3. Variceal Hemorrhage (acute upper GI bleeding)

50 mcg IV bolus, then 25–50 mcg/hour continuous IV infusion for 3–5 days
Mechanism: splanchnic vasoconstriction → reduced portal pressure

4. VIPoma / Zollinger-Ellison / Other Secretory Tumors

200–300 mcg/day SC in divided doses; titrate to symptom control

5. Hepatorenal Syndrome (HRS)

Used as part of the midodrine + octreotide + albumin triplet
100–200 mcg SC TID (or IV infusion in ICU settings)

6. Other (off-label)

Refractory diarrhea (e.g., dumping syndrome, short bowel syndrome)
Chylothorax / chylous ascites
Insulinoma, glucagonoma
Thyroid storm (adjunctive)

Side Effects

Category	Effects
GI (early, usually transient)	Nausea, abdominal cramps, bloating, loose stools — up to 1/3 of patients in first 1–2 months, typically subsides (Harrison's, p. 2582)
Pancreatic insufficiency	Fat malabsorption with prolonged use; treat with pancreatic enzyme supplements
Glucose dysregulation	Mild hyperglycemia (inhibits insulin) OR hypoglycemia (inhibits glucagon counter-regulation)
Cholelithiasis	Significant long-term risk — impaired gallbladder contractility → delayed emptying → gallstones; consider prophylactic cholecystectomy if surgery planned (Harrison's, p. 2582)
Bradycardia / conduction abnormalities	QT prolongation rare; sinus bradycardia more common
Hypothyroidism	With prolonged use
Injection site reactions	Pain, induration (SC route)

Contraindications / Cautions

No absolute contraindications in acute settings
Use with caution in:
- Diabetes — unpredictable glucose effects
- Cholelithiasis — risk of worsening
- Renal/hepatic impairment — dose reduction may be needed (LAR especially)
- Patients on cyclosporine (reduced absorption)

Key Clinical Pearls

In variceal bleeding, combine with endoscopic therapy (band ligation or sclerotherapy) — octreotide alone is insufficient
For carcinoid crisis (life-threatening flushing/hypotension perioperatively), IV octreotide is the treatment of choice
Somatostatin receptor scintigraphy (Octreoscan) exploits the same receptor binding for imaging NETs — high SSTR2 expression predicts better response to octreotide therapy
LAR requires 4–6 weeks to reach steady state; always overlap with short-acting SC doses initially when transitioning
Gallbladder ultrasound at baseline and periodically during long-term therapy is recommended

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