Cardiac Physiology — Pressure-Volume Loops

The Four Phases of the Pressure-Volume Loop

• Isovolumetric Contraction (Phase 1): Following mitral valve closure at end-diastole, the ventricle contracts with no change in volume (vertical upward segment on loop). Pressure rises rapidly as all myocardial fibers shorten isometrically. The aortic valve remains closed until LV pressure exceeds aortic diastolic pressure (~80 mmHg). This phase represents the steep portion of the left margin of the loop.

• Ejection Phase (Phase 2): Once aortic valve opens, blood is ejected from the ventricle. The loop traces an upward and leftward path as pressure rises to systolic peak (~120 mmHg) and volume decreases. Early ejection (rapid ejection) shows steep slope; late ejection (reduced ejection) shows decreased slope as the ventricle approaches minimum volume. This phase represents the top and right side of the loop.

• Isovolumetric Relaxation (Phase 3): Following aortic valve closure (dicrotic notch), the ventricle relaxes with no change in volume, creating a vertical downward segment. Pressure falls rapidly while volume remains constant at end-systolic volume (ESV). This continues until LV pressure falls below left atrial pressure and the mitral valve opens. This phase represents the steep right margin of the loop.

• Ventricular Filling (Phase 4): The mitral valve opens, allowing passive filling of the ventricle. The loop traces downward and rightward as pressure decreases and volume increases to end-diastolic volume (EDV). This includes the rapid filling phase and atrial contraction (the "atrial kick"). This phase represents the bottom and left side of the loop.

Key Determinants of Loop Geometry

• Preload (End-Diastolic Volume): The starting point of the loop at the beginning of systole. Increased preload shifts the loop rightward, increasing both EDV and stroke volume through the Frank-Starling mechanism. The relationship between EDV and stroke work (area within the loop) follows a biphasic curve—initially increasing with preload, then plateauing and eventually declining with excessive preload (overstretching of sarcomeres reduces overlap of thick and thin filaments).

• Afterload (End-Systolic Pressure and Arterial Compliance): The impedance the ventricle must overcome to eject blood, primarily determined by systemic vascular resistance and aortic pressure. Increased afterload increases end-systolic volume (ESV) and shifts the loop rightward, increasing myocardial workload without increasing stroke volume. The end-systolic pressure-volume relationship (ESPVR) is relatively independent of loading conditions and reflects contractility; it is defined by the slope and position of the line connecting end-systolic points.

• Contractility (Inotropic State): The intrinsic ability of the myocardium to generate force independent of loading conditions. Increased contractility shifts the ESPVR line upward and leftward, decreasing ESV for any given systolic pressure and increasing stroke volume. Beta-adrenergic stimulation, digitalis glycosides, and positive inotropes increase contractility. Conversely, negative inotropes (beta-blockers, calcium channel blockers, myocardial infarction) shift ESPVR downward and rightward.

Stroke Work and Mechanical Efficiency

Diastolic Function and the Diastolic Portion of the Loop

Conditions Increasing Preload (Rightward Loop Shift)

• Mitral regurgitation
• Atrial septal defect
• Ventricular septal defect
• Anemia and hyperthyroidism (high cardiac output states)
• Fluid overload and volume expansion
• Pregnancy and athletic heart

Conditions Increasing Afterload (Rightward and Upward Loop Shift)

• Systemic hypertension
• Aortic stenosis
• Increased systemic vascular resistance (septic shock, vasoconstriction)
• Aortic regurgitation (mixed load)

Conditions Decreasing Contractility (Rightward, Upward, and Expanded Loop)

• Myocardial infarction
• Dilated cardiomyopathy
• Myocarditis
• Septic cardiomyopathy
• Hypothyroidism
• End-stage renal disease

Conditions Increasing Contractility (Leftward and Downward Loop Shift)

• Catecholamine excess
• Hyperthyroidism
• Exercise
• Positive inotropic agents (dobutamine, milrinone, digitalis)

Conditions Altering Diastolic Function (Altered Loop Filling Phase)

• Left ventricular hypertrophy (impaired relaxation)
• Restrictive cardiomyopathy
• Constrictive pericarditis
• Atrial fibrillation (loss of atrial contribution)
• Ischemic heart disease

Presentation in Increased Preload States

• Dyspnea, orthopnea, and paroxysmal nocturnal dyspnea (pulmonary edema results from elevated left atrial pressure transmitted backward)
• Peripheral edema, hepatomegaly, and jugular venous distention
• Fatigue and reduced exercise tolerance
• Physical exam findings: S3 gallop (rapid filling phase), pulmonary crackles, displaced apical impulse

Presentation in Increased Afterload States

• Angina pectoris and dyspnea on exertion (increased myocardial oxygen consumption from increased wall stress)
• Syncope (in aortic stenosis due to fixed stroke volume with inability to increase cardiac output)
• Hypertensive urgency or emergency
• Physical exam findings: sustained systolic murmur (aortic stenosis), widened pulse pressure (aortic regurgitation), sustained and forceful apical impulse

Presentation in Decreased Contractility States

• Progressive dyspnea and orthopnea (pulmonary edema from elevated ventricular filling pressures)
• Fatigue, weakness, and exercise intolerance
• Syncope or presyncope (cardiogenic shock with inadequate cerebral perfusion)
• Cool extremities and decreased urine output (peripheral hypoperfusion)
• Physical exam findings: displaced apical impulse, S3 gallop, pulmonary crackles, hepatomegaly, peripheral edema, narrow pulse pressure, tachycardia

Presentation in Altered Diastolic Function

• Dyspnea disproportionate to degree of systolic dysfunction
• Signs of pulmonary edema despite preserved ejection fraction
• Atrial fibrillation with rapid ventricular response
• Physical exam findings: S4 gallop (atrial contraction against stiffened ventricle), prominent jugular venous waveform with rapid descent

Invasive Hemodynamic Assessment

• High-fidelity pressure transducers measure instantaneous LV pressure throughout the cardiac cycle with accuracy ±1–2 mmHg. The pressure waveform includes systolic peak, diastolic baseline, and dicrotic notch (aortic valve closure).
• Ventriculography (contrast or radioisotopic) determines ventricular volume at multiple time points throughout the cardiac cycle. End-diastolic volume (EDV) is measured at end-diastole (typically 120–130 mL/m² body surface area); end-systolic volume (ESV) is measured at end-systole (typically 40–50 mL/m² body surface area).
• Stroke volume = EDV − ESV; ejection fraction = Stroke volume / EDV (normal >50%)
• Cardiac output = Stroke volume × Heart rate
• Pressure-volume relationship: End-systolic pressure-volume relationship (ESPVR) slope reflects contractility (steeper slope = better contractility); normal ESPVR slope is 2–3 mmHg/mL. End-diastolic pressure-volume relationship (EDPVR) slope reflects diastolic stiffness (steeper slope = stiffer ventricle).

Echocardiographic Assessment

• 2D echocardiography measures LV dimensions in systole and diastole using M-mode or 2D-guided measurements. Increased end-diastolic dimension (>55 mm in men, >50 mm in women) indicates ventricular dilatation.
• 3D echocardiography provides more accurate volume measurements compared to 2D geometry-based calculations.
• Tissue Doppler imaging (TDI) and strain imaging assess relaxation velocity and myocardial deformation, providing indirect assessment of contractility and diastolic function.
• Diastolic function parameters: E/A ratio (early-to-atrial filling velocity), E/e' ratio (mitral inflow to tissue velocity ratio), left atrial volume index, and pulmonary venous flow patterns.

Laboratory Assessment

• B-type natriuretic peptide (BNP) or N-terminal pro-BNP (NT-proBNP) elevation indicates elevated ventricular filling pressures and ventricular wall stress. Normal BNP <100 pg/mL; elevated with BNP >400 pg/mL or NT-proBNP >900 pg/mL.
• Troponins (high-sensitivity assays) detect myocardial necrosis and are elevated in acute myocardial infarction.
• Electrolytes, renal function, and liver function assess end-organ dysfunction from cardiogenic shock.

Cardiac Magnetic Resonance (CMR)

• Provides precise volumetric assessment with highest reproducibility
• Tissue characterization (late gadolinium enhancement) identifies fibrosis, ischemia, or infiltration
• Assessment of myocardial strain and tagging techniques
• Gold standard for ejection fraction and volume measurement in clinical research

Cardiac Catheterization Findings in Specific Pathology

• Dilated cardiomyopathy: Rightward-shifted loop with increased EDV and ESV, decreased ejection fraction, and depressed ESPVR slope
• Hypertrophic cardiomyopathy: Normal to small ESV with increased wall thickness, abnormal ESPVR, and mid-cavity obstruction with dynamic pressure gradient
• Restrictive cardiomyopathy: Small ventricular volumes with steeply ascending EDPVR, elevated filling pressures, and characteristic "dip and plateau" pressure tracing
• Aortic stenosis: Increased systolic pressure with leftward loop shift, increased relative wall thickness, and decreased stroke volume
• Aortic regurgitation: Large stroke volume with increased EDV and decreased ESV, wide pulse pressure, and hyperdynamic loop
• Mitral regurgitation: Large stroke volume with increased EDV, prominent systolic bulge into left atrium

Management of Increased Preload (Dilated Loop)

• First-line: Diuretics — Loop diuretics (furosemide) reduce preload by increasing renal sodium and water excretion. Typical dosing: furosemide 40–80 mg PO daily or IV for acute decompensation. Achieve euvolemia (absence of orthopnea, peripheral edema, and elevated jugular venous pressure).
• Second-line: Aldosterone antagonists — Spironolactone or eplerenone provide neurohormonal modulation; typical dosing: spironolactone 12.5–50 mg daily. Improves outcomes in systolic heart failure through RAAS blockade.
• Non-pharmacological: Sodium restriction (<2 g/day) reduces fluid retention; fluid restriction (<1.5 L/day) in cases of severe volume overload or hyponatremia.
• Mechanical: Ultrafiltration or hemodialysis for refractory volume overload in acute decompensated heart failure.

Management of Increased Afterload (Upward and Rightward Loop Shift)

• First-line: ACE inhibitors or ARBs — Lisinopril, enalapril, or losartan reduce systemic vascular resistance and improve forward flow. Typical dosing: lisinopril 5–40 mg daily; losartan 50–100 mg daily. Mechanism: RAAS blockade reduces vasoconstriction and aldosterone-mediated sodium retention.
• Second-line: Beta-blockers — Metoprolol succinate, carvedilol, or bisoprolol reduce heart rate, contractility, and blood pressure, decreasing myocardial oxygen demand. Typical dosing: metoprolol succinate 25–190 mg daily; carvedilol 3.125–25 mg twice daily.
• Third-line: Calcium channel blockers — Amlodipine or diltiazem reduce afterload through vasodilation. Typical dosing: amlodipine 2.5–10 mg daily.
• Acute severe hypertension: IV vasodilators including nitroglycerin, labetalol, or esmolol rapidly reduce afterload.

Management of Decreased Contractility (Expanded Rightward Loop with Depressed ESPVR)

• First-line: ACE inhibitors/ARBs + Beta-blockers + Aldosterone antagonists — Triple therapy forms the backbone of systolic heart failure management. ACE inhibitors/ARBs reduce afterload and prevent remodeling; beta-blockers reduce adrenergic drive and allow myocardial recovery; aldosterone antagonists improve outcomes in severe systolic dysfunction (RALES trial, EMPHASIS-HF trial).
• Second-line: SGLT2 inhibitors — Dapagliflozin and empagliflozin reduce

Complications of the underlying hemodynamic derangement

• Acute cardiogenic pulmonary edema (emergency): when the loop shifts rightward onto the steep portion of the EDPVR, small increments in volume produce large increases in end-diastolic pressure, which is transmitted to the pulmonary capillaries. Signaled by orthopnea, diffuse crackles, and hypoxemia with elevated BNP/NT-proBNP.
• Cardiogenic shock (emergency): a depressed ESPVR with a narrow, rightward-shifted loop means stroke volume falls despite maximal preload. Signaled by narrow pulse pressure, cool extremities, oliguria, lactate elevation. Per the ACC/AHA/HFSA 2022 heart failure guideline, hypoperfusion with congestion warrants inotropic support and consideration of temporary mechanical circulatory support.
• Subendocardial ischemia: afterload elevation raises wall stress (Laplace) and the pressure–volume area, the principal determinant of myocardial oxygen consumption, while elevated LVEDP compresses subendocardial vessels. Signaled by exertional angina and ST depression with angiographically normal coronaries in aortic stenosis or LVH.
• Malignant arrhythmia (emergency): chamber dilatation and fibrosis create reentrant substrate; ventricular fibrillation / pulseless VT are the shockable rhythms requiring immediate defibrillation per AHA ACLS.
• Loss of the atrial kick: new atrial fibrillation in a stiff, small-volume ventricle (HCM, restrictive cardiomyopathy, severe AS) abolishes the terminal filling segment and can precipitate abrupt pulmonary edema — an emergency favoring rate/rhythm control and anticoagulation per the ACC/AHA 2023 atrial fibrillation guideline.

Complications of therapy

• Over-diuresis: excessive leftward loop shift produces prerenal AKI, hypokalemia and hypomagnesemia (arrhythmogenic). Signaled by rising creatinine with hemoconcentration and orthostasis.
• Preload-dependent hypotension (emergency): nitrates or other vasodilators in severe aortic stenosis, obstructive HCM, RV infarction, or tamponade collapse filling; treat with volume and an alpha agonist, not more vasodilator.
• RAAS blockade: hyperkalemia and AKI with ACEI/ARB/ARNI plus MRA; ACEI/ARNI angioedema is an airway emergency, and ARNI must not overlap with an ACEI.
• Beta blocker started during decompensation: acute negative inotropy lowers the ESPVR and can precipitate shock — initiate only when euvolemic.
• Inotropes: dobutamine/milrinone raise oxygen consumption and provoke tachyarrhythmias and hypotension; digoxin toxicity presents with nausea, visual halos, and bradyarrhythmias.

• Width equals stroke volume, area equals stroke work: the horizontal width of the loop is EDV − ESV; the enclosed area is external work. The total pressure–volume area (stroke work plus potential energy) tracks myocardial oxygen consumption — which is why raising afterload is metabolically far more expensive than raising preload for the same stroke volume.
• ESPVR is the contractility line; EDPVR is the compliance line: the ESPVR slope (Emax) is the load-independent index of inotropy. A stem describing "contractility increased" must show a steeper, leftward ESPVR — not merely a wider loop.
• The classic distractor: pure preload increase widens the loop rightward with an unchanged ESV and unchanged ESPVR; pure afterload increase makes the loop taller and narrower with an increased ESV. Students mislabel the second as decreased contractility — the ESPVR line itself has not moved.
• Valvular loops examiners love: aortic stenosis — tall, narrow loop with high peak systolic pressure; mitral regurgitation — loss of the true isovolumetric contraction segment (volume falls as soon as pressure rises, so the left border is not vertical); aortic regurgitation — loss of the isovolumetric relaxation segment with a wide, rightward loop and large stroke volume.
• The association tested most: an S4 with a preserved ejection fraction and a steep EDPVR is diastolic (HFpEF) physiology — dyspnea from high filling pressure at normal volume; an S3 with a dilated, rightward loop is systolic dysfunction.
• Single best next step in acute decompensation with congestion: IV loop diuretic (furosemide), per the ACC/AHA/HFSA 2022 heart failure guideline; if congestion coexists with hypoperfusion, escalate to inotropic support rather than more diuresis.
• Do not truncate guideline-directed medical therapy for HFrEF: the ACC/AHA/HFSA 2022 guideline endorses four classes — ARNI (or ACEI/ARB), beta blocker, MRA, and SGLT2 inhibitor — each of which shifts the loop leftward by reducing afterload, adverse remodeling, and volume.
• Preload-dependent lesions: in severe aortic stenosis or obstructive HCM, nitroglycerin is the wrong answer; reducing preload/afterload collapses the loop and drops cardiac output.