Weakness & Racing Heart: Differential Diagnosis & Urgency

The combination of weakness (asthenia) and tachycardia (HR >100 bpm) is a nonspecific but potentially serious presentation requiring systematic evaluation.

Priority Differential (Life‑Threatening First)

Category	Conditions	Key Features
Cardiovascular	Arrhythmias (AFib, SVT, VT), acute MI, heart failure, PE, tamponade, aortic dissection	Chest pain, dyspnea, hypotension, diaphoresis, ECG changes
Metabolic / Endocrine	Hypoglycemia, thyroid storm, adrenal crisis, pheochromocytoma, diabetic ketoacidosis	Diaphoresis, tremor (hypo); fever, tremor, exophthalmos (thyroid); hypotension, hyperpigmentation (adrenal)
Hematologic	Severe anemia (Hb <7), acute blood loss	Pallor, dyspnea, tachycardia disproportionate to activity
Infectious / Sepsis	Sepsis, severe pneumonia, endocarditis	Fever, rigors, hypotension, altered mental status, elevated lactate
Electrolyte / Toxicologic	Hypokalemia, hypomagnesemia, stimulant overdose (cocaine, amphetamines), caffeine, withdrawal (alcohol, benzos)	History, tox screen, ECG changes (U waves, QT)
Neurologic / Autonomic	Autonomic dysfunction, POTS, neurogenic shock	Orthostatic intolerance, fixed HR
Psychiatric	Panic attack, anxiety disorder	Hyperventilation, paresthesias, resolves with reassurance; diagnosis of exclusion

Immediate Assessment (ABCDE)

1. Airway, Breathing, Circulation — vitals, O₂ sat, ECG monitoring.
2. Fingerstick glucose.
3. IV access, labs: CBC, BMP, Mg, troponin, BNP, lactate, TSH, cortisol, tox screen, coags.
4. Portable CXR, bedside echo if indicated.

When to Call 911 / Go to ER

• Chest pain, SOB, syncope, confusion, cold sweats.
• HR >130 sustained, or <50 with symptoms.
• BP <90/60 or >180/120.
• Known heart disease, anticoagulation, diabetes on insulin.

Outpatient Follow‑Up If Stable

• Primary care: Holter/event monitor, echo, thyroid panel, iron studies, medication review.
• Cardiology if structural/electrical disease suspected.
• Endocrinology for thyroid/adrenal.