A 74-year-old woman is brought by ambulance to the emergency department and presents with a complaint of excruciating chest pain that started about 45 minutes ago. The patient was sitting in the garden when she 1st noticed the pain in the upper abdomen. The pain has persisted and now localizes underneath of the sternum and the left shoulder. Milk of magnesia and aspirin were tried with no relief. The patient had previous episodes of chest pain that were of lesser intensity and rarely lasted more than 10 minutes. She is diabetic and has been managed for hypertension and rheumatoid arthritis in the past. On examination, the patient is breathless and sweating profusely. The vital signs include blood pressure 140/90 mm Hg and heart rate 118/min. The electrocardiogram (ECG) shows Q waves in leads V2 and V3 and raised ST segments in leads V2, V3, V4, and V5. Laboratory studies (including cardiac enzymes at 6 hours after admission show: Hematocrit 45% Troponin T 1.5 ng/mL Troponin I 0.28 ng/mL Creatine kinase (CK)-MB 0.25 ng/mL The patient is admitted and started on analgesia and reperfusion therapy. She shows initial signs of recovery until the 6th day of hospitalization when she starts vomiting and complaining of dizziness. Physical examination findings at this time included heart rate 110/min, temperature 37.7°C (99.9°F), blood pressure 90/60 mm Hg. Jugular venous pressure is 8 cm. A harsh pansystolic murmur is present at the left lower sternal border. ECG shows sinus tachycardia and ST-segment elevation with terminal negative T waves. Laboratory studies show: Hematocrit 38% Troponin T 1.15ng/mL Troponin I 0.18 ng/mL CK-MB 0.10 ng/mL Which of the following best explains the patient's current clinical condition?

Acute ventricular septal rupture complicating myocardial infarction

Aortic dissection complicating myocardial infarction

A new myocardial infarction (re-infarction)

Acute pericarditis complicating myocardial infarction

Cardiac tamponade complicating myocardial infarction

A 27-year-old man is brought to the emergency department after a motorcycle accident 30 minutes ago. He was found at the scene of the accident with a major injury to the anterior chest by a metallic object that was not removed during transport to the hospital. The medical history could not be obtained. His blood pressure is 80/50 mm Hg, pulse is 130/min, and respiratory rate is 40/min. Evaluation upon arrival to the emergency department reveals a sharp metal object penetrating through the anterior chest to the right of the sternum at the 4th intercostal space. The patient is taken to the operating room immediately, where it is shown the heart has sustained a major injury. Which of the following arteries supplies the part of the heart most likely injured in this patient?

Left anterior descending artery

Left circumflex coronary artery

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

A 42-year-old Caucasian woman is enrolled in a randomized controlled trial to study cardiac function in the setting of several different drugs. She is started on verapamil and instructed to exercise at 50% of her VO2 max while several cardiac parameters are being measured. During this experiment, which of the following represents the relative conduction speed through the heart from fastest to slowest?

Purkinje fibers > atria > ventricles > AV node

Purkinje fibers > ventricles > atria > AV node

Atria > Purkinje fibers > ventricles > AV node

AV node > ventricles > atria > Purkinje fibers

Purkinje fibers > AV node > ventricles > atria

Mediastinum and heart for USMLE Step 3: High-Yield Anatomy Lessons

Mediastinum - The Body's Core Condo

Mediastinum divisions: axial, sagittal, and coronal views

Central thoracic compartment, divided at the sternal angle (T4/T5).
Superior: Thymus, great vessels (aortic arch, SVC), trachea, esophagus.
Inferior: Subdivided into three parts.
- Anterior: Lymph nodes, fat, thymus remnants.
- Middle: Heart, pericardium, phrenic nerves, great vessel roots.
- Posterior: Thoracic aorta, esophagus, azygos system, thoracic duct.

⭐ Mediastinal Syndromes: Compression is key. SVC syndrome causes facial/arm swelling. Recurrent laryngeal nerve compression causes hoarseness.

Pericardium - The Heart's Private Sac

Pericardial Sac Layers and Heart Anatomy

Structure: A double-walled fibroserous sac covering the heart and great vessel roots.
**Layers (superficial to deep):
- Fibrous Pericardium: Tough, inelastic connective tissue.
- Serous Pericardium:
  - Parietal layer: Lines the inner surface of the fibrous pericardium.
  - Visceral layer (Epicardium): Adheres to the heart.
Pericardial Cavity: Potential space between serous layers, contains ~15-50 mL of fluid.
Innervation: Phrenic nerve (C3-C5) → referred pain to the shoulder.

⭐ Pericarditis: Inflammation of the pericardium causing sharp, pleuritic chest pain, often relieved by sitting up and leaning forward. A pericardial friction rub may be audible.

Heart Chambers & Valves - The Four-Room Pump

Four Chambers: Two atria (receiving) & two ventricles (pumping).
- Right Atrium (RA): Receives deoxygenated blood from SVC, IVC, & coronary sinus.
- Right Ventricle (RV): Pumps blood to the pulmonary artery; thinner wall.
- Left Atrium (LA): Receives oxygenated blood from pulmonary veins.
- Left Ventricle (LV): Pumps blood to the aorta; thickest myocardium.
Four Valves: Prevent backflow (regurgitation).
- Atrioventricular: Tricuspid (RA→RV), Mitral/Bicuspid (LA→LV).
  - 📌 Mnemonic: LAB RAT (Left Atrium: Bicuspid; Right Atrium: Tricuspid).
- Semilunar: Pulmonary (RV→Pulmonary Artery), Aortic (LV→Aorta).

⭐ The posterior left atrium lies anterior to the esophagus. LA enlargement (e.g., from mitral stenosis) can cause dysphagia.

Blood flow through the heart

Coronary Circulation - The Heart's Own Fuel Line

Coronary arteries and supply territories (anterior view)

Arterial Supply: Originates from the root of the aorta.
- Left Coronary Artery (LCA): Arises from the left aortic sinus.
  - Left Anterior Descending (LAD): Supplies anterior 2/3 of interventricular septum, anterior LV wall. 📌 Nicknamed the "widow-maker."
  - Left Circumflex (LCX): Supplies lateral and posterior LV walls.
- Right Coronary Artery (RCA): Arises from the right aortic sinus.
  - Supplies SA/AV nodes, RV free wall.
  - Posterior Descending Artery (PDA): Supplies posterior 1/3 of septum, posterior walls.
Coronary Dominance: Determined by the artery supplying the PDA.
- Right Dominant (~85%): PDA arises from RCA.
- Left Dominant (~15%): PDA arises from LCX.

⭐ The LAD is the most commonly occluded coronary artery in myocardial infarction.

Cardiac Conduction - The Heart's Electric Beat

Pacemaker: Sinoatrial (SA) node, the heart's natural pacemaker, fires at 60-100 bpm.
Pathway: SA node → AV node (introduces a delay for ventricular filling) → Bundle of His → Right/Left Bundle Branches → Purkinje fibers.
Pacing Rates: SA node (60-100) > AV node (40-60) > Ventricles (20-40).

ECG waveform with labeled P, QRS, T waves and segments

⭐ The PR interval on an EKG, normally 0.12-0.20s, represents the AV nodal delay. A prolonged PR interval signifies an AV block.

High‑Yield Points - ⚡ Biggest Takeaways

The pericardium has two layers: an outer fibrous layer and an inner serous layer (parietal and visceral).

Cardiac tamponade classically presents with Beck's triad: hypotension, jugular venous distention (JVD), and muffled heart sounds.

The left anterior descending (LAD) is the most frequently occluded coronary artery.

Right-dominant circulation is most common, where the PDA arises from the right coronary artery.

Left atrial enlargement can compress the esophagus, causing dysphagia.

The phrenic nerve (C3-C5) passes anterior to the lung root to innervate the diaphragm.

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