Cardiology USMLE Practice Questions: Hemodynamics & Case Bank

Right ventricular myocardial infarction is preload-dependent; venodilators (e.g., nitroglycerin) or diuretics cause profound hypotension, which must be immediately treated with rapid intravenous isotonic crystalloid boluses.

1. Clinical Scenario: The patient presents with an acute inferior ST-elevation myocardial infarction (leads II, III, aVF) complicated by right ventricular infarction (confirmed by ST elevation in right-sided lead V4R).
2. Pathophysiology: The infarcted, stiff right ventricle has impaired contractility and cannot pump blood effectively through the pulmonary vasculature unless driven by elevated right atrial filling pressures. It is exquisitely dependent on adequate venous return (preload).
3. Pharmacologic Precipitant & Solution: Nitroglycerin causes widespread venodilation, sharply decreasing systemic venous return to the right heart. This causes an abrupt drop in left ventricular filling and catastrophic systemic hypotension. Immediate aggressive volume loading with isotonic saline restores RV filling pressure, augmenting stroke volume and systemic perfusion.

• RV infarction accompanies up to 30–50% of inferior STEMIs (due to occlusion of the proximal right coronary artery).
• Triad of RV infarction: hypotension, clear lungs, and elevated jugular venous pressure.
• Nitrates and diuretics must be avoided; treatment of hypotension begins with isotonic volume expansion.

Intravenous beta-blockers (e.g., esmolol or labetalol) are the mandatory initial medical therapy in acute aortic dissection to reduce heart rate (target < 60/min) and shear stress (dP/dt) before or alongside vasodilators.

1. The patient presents with classic features of an acute Stanford type A aortic dissection: sudden, severe tearing chest pain radiating to the back, significant pulse/blood pressure differential between arms, and an early diastolic murmur of acute aortic regurgitation.
2. Acute aortic dissection management requires rapid reduction of aortic wall shear stress, which is proportional to the rate of left ventricular pressure rise (dP/dt). Intravenous beta-blockade reduces both dP/dt and heart rate.
3. The goal of medical therapy is to lower the heart rate to ≤ 60 beats/min and systolic blood pressure to 100–120 mm Hg. Beta-blockers must be administered prior to vasodilators (e.g., nitroprusside or nicardipine) to prevent reflex sympathetic activation and reflex tachycardia, which would paradoxically increase shear stress and propagate the dissection flap.

• Initial medical management for suspected acute aortic dissection is heart rate and blood pressure control.
• Short-acting IV beta-blockers (esmolol, labetalol) are first-line to lower heart rate to < 60/min and systolic BP to 100–120 mm Hg.
• Vasodilators should never be started without prior or concurrent beta-blockade due to reflex tachycardia.

Atrial fibrillation in a patient with Wolff-Parkinson-White (WPW) syndrome presents as an irregularly irregular wide-complex tachycardia; first-line therapy in hemodynamically stable patients is intravenous procainamide or ibutilide.

1. The ECG shows an irregularly irregular wide-complex tachycardia with beat-to-beat variation in QRS morphology (due to fusion beats from varying conduction via the AV node and accessory pathway). This is pathognomonic for pre-excited atrial fibrillation in Wolff-Parkinson-White syndrome.
2. In hemodynamically stable patients, the therapeutic goal is to prolong the refractory period of the accessory pathway and myocardium rather than selectively blocking the AV node.
3. Class Ia antiarrhythmics (e.g., intravenous procainamide) or Class III agents (e.g., ibutilide) slow accessory pathway conduction and convert atrial fibrillation to sinus rhythm. AV nodal blocking agents (adenosine, beta-blockers, calcium channel blockers, digoxin) are strictly contraindicated because they divert impulses down the bypass tract, which can precipitate ventricular fibrillation.

• Pre-excited atrial fibrillation (WPW + AF) is identified by an irregularly irregular, wide-complex tachycardia with polymorphic QRS complexes.
• Stable patients are treated with IV procainamide or ibutilide.
• AV nodal blocking drugs (adenosine, beta-blockers, diltiazem/verapamil, digoxin) are contraindicated.
• If the patient is unstable, prompt synchronized electrical cardioversion is required.

Left ventricular free wall rupture typically occurs 3 to 14 days after myocardial infarction, leading to hemopericardium, cardiac tamponade (Beck's triad, pulsus paradoxus), and rapid death unless emergent pericardiocentesis and surgical repair are performed.

1. The patient is 4 days post-anterior myocardial infarction and develops sudden hemodynamic collapse. Physical exam reveals hypotension, jugular venous distension, and muffled heart sounds (Beck's triad).
2. Examination also identifies an exaggerated drop in systolic blood pressure during inspiration (> 10 mm Hg), known as pulsus paradoxus (here, 18 mm Hg). Clear lungs distinguish this presentation from acute mitral regurgitation or ventricular septal rupture, both of which cause pulmonary edema and loud systolic murmurs.
3. These findings indicate acute cardiac tamponade, resulting from myocardial free wall rupture with hemopericardium. The immediate lifesaving measure in an acutely unstable patient with tamponade is emergent bedside pericardiocentesis to relieve intrapericardial pressure and restore venous return, followed by emergency cardiosurgical repair.

• Free wall rupture occurs 3–14 days post-MI and presents with sudden hemodynamic collapse, Beck's triad, and pulsus paradoxus (> 10 mm Hg drop in systolic blood pressure during inspiration).
• Clear lungs and lack of new murmurs help differentiate free wall rupture from papillary muscle rupture and interventricular septal rupture.
• Immediate management of tamponade with shock is emergent pericardiocentesis.