A 58-year-old man comes to the emergency department for complaints of crushing chest pain for 4 hours. He was shoveling snow outside when the pain started. It is rated 7/10 and radiates to his left arm. An electrocardiogram (ECG) demonstrates ST-segment elevation in leads V2-4. He subsequently undergoes percutaneous coronary intervention (PCI) and is discharged with aspirin, clopidogrel, carvedilol, atorvastatin, and lisinopril. Five days later, the patient is brought to the emergency department by his wife with complaints of dizziness. He reports lightheadedness and palpitations for the past 2 hours but otherwise feels fine. His temperature is 99.7°F (37.6°C), blood pressure is 95/55 mmHg, pulse is 105/min, and respirations are 17/min. A pulmonary artery catheter is performed and demonstrates an increase in oxygen concentration at the pulmonary artery. What finding would you expect in this patient?

Harsh, loud, holosystolic murmur at the lower left sternal border

Widespread ST-segment elevations

Drop of systolic blood pressure by 20 mmHg during inspiration

A 53-year-old man with a past medical history significant for hyperlipidemia, hypertension, and hyperhomocysteinemia presents to the emergency department complaining of 10/10 crushing, left-sided chest pain radiating down his left arm and up his neck into the left side of his jaw. His ECG shows ST-segment elevation in leads V2-V4. He is taken to the cardiac catheterization laboratory for successful balloon angioplasty and stenting of a complete blockage in his left anterior descending coronary artery. Echocardiogram the following day shows decreased left ventricular function and regional wall motion abnormalities. A follow-up echocardiogram 14 days later shows a normal ejection fraction and no regional wall motion abnormalities. This post-infarct course illustrates which of the following concepts?

Coronary collateral circulation

A 66-year-old female with hypertension and a recent history of acute ST-elevation myocardial infarction (STEMI) 6 days previous, treated with percutaneous transluminal angioplasty (PTA), presents with sudden onset chest pain, shortness of breath, diaphoresis, and syncope. Vitals are temperature 37°C (98.6°F), blood pressure 80/50 mm Hg, pulse 125/min, respirations 12/min, and oxygen saturation 92% on room air. On physical examination, the patient is pale and unresponsive. Cardiac exam reveals tachycardia and a pronounced holosystolic murmur loudest at the apex and radiates to the back. Lungs are clear to auscultation. Chest X-ray shows cardiomegaly with clear lung fields. ECG is significant for ST elevations in the precordial leads (V2-V4) and low-voltage QRS complexes. Emergency transthoracic echocardiography shows a left ventricular wall motion abnormality along with a significant pericardial effusion. The patient is intubated, and aggressive fluid resuscitation is initiated. What is the next best step in management?

Immediate transfer to the operating room

Immediate cardiac catheterization

Intra-aortic balloon counterpulsation

Administer dobutamine 5-10 mcg/kg/min IV

A cardiologist is studying how a new virus that infects the heart affects the electrical conduction system of the cardiac myocytes. He decides to obtain electrocardiograms on patients with this disease in order to see how the wave patterns and durations change over time. While studying these records, he asks a medical student who is working with him to interpret the traces. Specifically, he asks her to identify the part that represents initial ventricular depolarization. Which of the following characteristics is most consistent with this feature of the electrocardiogram?

Normal duration defined as less than 120 milliseconds

Elevated in patients with full thickness ischemic injury of the heart

Becomes peaked in states of hyperkalemia

Becomes prominent in states of hypokalemia

Normal duration defined as less than 200 milliseconds

A 55-year-old man comes to the emergency department because of left-sided chest pain and difficulty breathing for the past 30 minutes. His pulse is 88/min. He is pale and anxious. Serum studies show increased cardiac enzymes. An ECG shows ST-elevations in leads I, aVL, and V5-V6. A percutaneous coronary intervention is performed. In order to localize the site of the lesion, the catheter must pass through which of the following structures?

Left coronary artery → left circumflex artery

Right coronary artery → posterior descending artery

Left coronary artery → left anterior descending artery

Right coronary artery → right marginal artery

Left coronary artery → posterior descending artery

An 80-year-old man presents to the emergency department because of gnawing substernal chest pain that started an hour ago and radiates to his neck and left jaw. A 12-lead ECG is obtained and shows ST-segment elevation with newly developing Q waves. He is admitted for treatment. 4 days after hospitalization he suddenly develops altered mental status, and his blood pressure falls from 115/75 mm Hg to 80/40 mm Hg. Physical examination shows jugular venous distention, pulsus paradoxus, and distant heart sounds. What is the most likely cause of this patient's condition?

Compression of heart chambers by blood in the pericardial space

Arrhythmia caused by ventricular fibrillation

Acute pulmonary edema from left heart failure

A 71-year-old woman with a past medical history of type 2 diabetes, hypercholesterolemia, and hypertension was admitted to the hospital 8 hours ago with substernal chest pain for management of acute non-ST-elevated myocardial infarction (NSTEMI). The ECG findings noted by ST-depressions and T-wave inversions on anterolateral leads, which is also accompanied by elevated cardiac enzymes. Upon diagnosis, management with inhaled oxygen therapy, beta-blockers and aspirin, and low-molecular-weight heparin therapy were initiated, and she was placed on bed rest with continuous electrocardiographic monitoring. Since admission, she required 2 doses of sublingual nitroglycerin for recurrent angina, and the repeat troponin levels continued to rise. Given her risk factors, plans were made for early coronary angiography. The telemetry nurse calls the on-call physician because of her concern with the patient's mild confusion and increasing need for supplemental oxygen. At bedside evaluation, The vital signs include: heart rate 122/min, blood pressure 89/40 mm Hg, and the pulse oximetry is 91% on 6L of oxygen by nasal cannula. The telemetry and a repeat ECG show sinus tachycardia. She is breathing rapidly, appears confused, and complains of shortness of breath. On physical exam, the skin is cool and clammy and appears pale and dull. She has diffuse bilateral pulmonary crackles, and an S3 gallop is noted on chest auscultation with no new murmurs. She has jugular venous distention to the jaw-line, rapid and faint radial pulses, and 1+ dependent edema. She is immediately transferred to the intensive care unit for respiratory support and precautions for airway security. The bedside sonography shows abnormal hypodynamic anterior wall movement and an ejection fraction of 20%, but no evidence of mitral regurgitation or ventricular shunt. The chest X-ray demonstrates cephalization of pulmonary veins and pulmonary edema. What is the most appropriate next step in the stabilization of this patient?

Initiate dopamine therapy and diuresis

Obtain blood cultures and start preliminary broad-spectrum antibiotics

Start intravenous fluids and epinephrine therapy

Intubate the patient and perform an emergency cardiocentesis

Insert two large-bore intravenous catheters and start rapid fluid resuscitation

A 50-year-old man presents the emergency department for intense chest pain, profuse sweating, and shortness of breath. The onset of these symptoms was 3 hours ago. The chest pain began after a heated discussion with a colleague at the community college where he is employed. Upon arrival, he is found conscious and responsive; the vital signs include a blood pressure of 130/80 mm Hg, a heart rate at 90/min, a respiratory rate at 20/min, and a body temperature of 36.4°C (97.5°F). His medical history is significant for hypertension diagnosed 7 years ago, which is well-controlled with a calcium channel blocker. The initial electrocardiogram (ECG) shows ST-segment depression in multiple consecutive leads, an elevated cardiac troponin T level, and normal kidney function. Which of the following would you expect to find in this patient?

Incomplete occlusion of a coronary artery

A 49-year-old man was brought to the emergency department by ambulance with complaints of sudden-onset chest pain that radiates into his neck and down his left arm. This substernal pain started 2 hours ago while he was having dinner. His past medical history is remarkable for hypercholesterolemia that is responsive to therapy with statins and coronary artery disease. His temperature is 37.0°C (98.6°F), blood pressure is 155/90 mm Hg, pulse is 112/min, and respiratory rate is 25/min. Troponin I levels are elevated. A 12-lead ECG was performed (see image). What is the most likely etiology of this patient’s presentation?

Right coronary artery occlusion

Left circumflex artery occlusion

Left anterior descending artery occlusion

Left main coronary artery occlusion

A 57-year-old man is brought to the emergency department for crushing substernal chest pain at rest for the past 2 hours. The pain began gradually while he was having an argument with his wife and is now severe. He does not take any medications. He has smoked 1 pack of cigarettes daily for 35 years. He is diaphoretic. His temperature is 37.1°C (98.8°F), pulse is 110/min, respirations are 21/min, and blood pressure is 115/65 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 97%. Cardiac examination shows an S4 gallop. The lungs are clear to auscultation. An ECG is shown. Which of the following is the most likely underlying cause of this patient's condition?

Occlusion of the left anterior descending artery

Occlusion of the left circumflex artery

Tear in the intimal lining of the aorta

Thromboembolism to the right interlobar pulmonary artery

Ischemia and infarction patterns for USMLE Step 1: High-Yield Physiology Lessons

ECG Ischemia Basics - The Silent Scream

Myocardial ischemia from ↓ coronary blood flow creates a spectrum of ECG changes reflecting cellular distress. The location and duration of ischemia determine the pattern.

Subendocardial (NSTEMI): Affects inner myocardial layer. Manifests as ST-segment depression or T-wave inversion.
Transmural (STEMI): Involves the entire myocardial wall. Causes ST-segment elevation, the hallmark of acute injury.

⭐ De Winter's T-waves (tall, prominent, symmetric T-waves in precordial leads) are a STEMI equivalent for acute LAD occlusion, even without classic ST elevation.

STEMI vs. NSTEMI - The Great Divide

Pathophysiology: The core difference lies in the degree of coronary artery occlusion.

Feature	STEMI (ST-Elevation MI)	NSTEMI (Non-ST-Elevation MI)
Vessel Blockage	100% occlusive thrombus	Subtotal occlusion; severe stenosis
Infarction	Transmural (full thickness)	Subendocardial (partial thickness)
ECG	ST-segment elevation (new)	ST depression, T-wave inversion
Biomarkers	↑ Troponins	↑ Troponins
Immediate Goal	Emergent Reperfusion	Anti-ischemic therapy, risk stratify
%%{init: {'flowchart': {'htmlLabels': true}}}%%
flowchart TD
START["🩺 Chest Pain • ACS symptoms• Acute presentation"]

ECG["🔬 12-Lead ECG
• Perform stat• Evaluate rhythm"]

DECISION["📋 ST Elevation?
• Assess segments• Check leads"]

STEMI["⚠️ STEMI
• Activate Cath Lab• Emergent reperfusion"]

NSTEMI["🩺 NSTEMI / UA
• Unstable Angina• Check Troponins"]

START --> ECG ECG --> DECISION DECISION -->|Yes| STEMI DECISION -->|No| NSTEMI

style START fill:#F7F5FD,stroke:#F0EDFA,stroke-width:1.5px,rx:12,ry:12,color:#6B21A8 style ECG fill:#FFF7ED,stroke:#FFEED5,stroke-width:1.5px,rx:12,ry:12,color:#C2410C style DECISION fill:#FEF8EC,stroke:#FBECCA,stroke-width:1.5px,rx:12,ry:12,color:#854D0E style STEMI fill:#FDF4F3,stroke:#FCE6E4,stroke-width:1.5px,rx:12,ry:12,color:#B91C1C style NSTEMI fill:#F7F5FD,stroke:#F0EDFA,stroke-width:1.5px,rx:12,ry:12,color:#6B21A8


> ⭐ **Exam Favorite:** In STEMI, "time is muscle." The primary goal is immediate reperfusion, typically via percutaneous coronary intervention (PCI) within **90 minutes** of first medical contact. This is the most critical management step.

![ECG and heart changes in STEMI vs NSTEMI heart attack](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Physiology_ECG_interpretation_Ischemia_and_infarction_patterns/a7a7b855-6751-446a-aea8-263ccb2602e6.jpg)

## Infarct Location - Pinpointing the Problem

![Coronary arteries and ECG leads](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Physiology_ECG_interpretation_Ischemia_and_infarction_patterns/2e5080b2-9a92-42f7-8549-0ff9f0729d73.jpg)

*   **ST elevation** in specific lead groups localizes the myocardial injury.
*   Look for **reciprocal ST depression** in opposing leads.

| Infarct Location | Leads with ST Elevation | Artery Involved |
| :--- | :--- | :--- |
| **Anteroseptal** | V1-V4 | Left Anterior Descending (LAD) |
| **Lateral** | I, aVL, V5-V6 | Left Circumflex (LCX) |
| **Inferior** | II, III, aVF | Right Coronary Artery (RCA) |
| **Posterior** | ST depression V1-V3, Tall R | Posterior Descending Artery (PDA) |📌 **Mnemonic:**
- **L**ateral: **I**, a**VL**, V5, V6 (Use the "L" in aVL)
- **I**nferior: **II**, **III**, aV**F** (Use the "F" in aVF for "Feet")

> ⭐ **Reciprocal Changes:** An inferior wall MI (↑ST in II, III, aVF) often shows reciprocal ST depression in lateral leads (I, aVL). This pattern strongly confirms the diagnosis of an acute infarction.

## MI Timeline - A Scar is Born

*   **Acute (0-12 hrs):** Begins with hyperacute, peaked T-waves, followed by marked ST-segment elevation, indicating transmural injury. Reciprocal ST-depression may be present.
*   **Evolving (Hours-Days):** ST-elevation diminishes. Pathological Q-waves (>**0.04s**, >**25%** of R-wave height) develop, and T-waves invert, signifying necrosis and ischemia.
*   **Chronic (Weeks-Years):** ST segments and T-waves often normalize. The pathological Q-wave persists indefinitely as electrical evidence of the fibrous scar.

![Ischemic vs. Non-Ischemic ST-Segment Elevation & Examples](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Physiology_ECG_interpretation_Ischemia_and_infarction_patterns/e0253b43-4166-45bb-8fe7-89b1ffe0ff5e.webp)

> ⭐ Persistent ST-elevation for several weeks post-MI strongly suggests the formation of a ventricular aneurysm.

> *   **ST-segment elevation (STE)** in contiguous leads signifies an **acute transmural injury (STEMI)**.
> *   **ST depression** and **T-wave inversion** are classic signs of **subendocardial ischemia**.
> *   The presence of **pathological Q waves** indicates a **prior or old infarction**.
> *   Always look for **reciprocal changes** (i.e., ST depression) in anatomically opposite leads.
> *   **Anterior MI**: STE in **V1-V4**; **Inferior MI**: STE in **II, III, aVF**; **Lateral MI**: STE in **I, aVL, V5-V6**.

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