Cardiac Physiology — Preload Afterload Contractility

Frank-Starling Mechanism (Preload-Dependent Performance)

• Sarcomere length-tension relationship: Optimal myocardial contractile force occurs when the ventricle is filled to approximately 2.2 μm sarcomere length, corresponding to maximal overlap between actin and myosin filaments
• At this optimal length, cross-bridge cycling is maximized; both underfilling (reduced overlap) and overfilling (excessive overlap with reduced cross-bridge formation) impair force generation
• Left ventricular end-diastolic volume (LVEDV) and left ventricular end-diastolic pressure (LVEDP) serve as clinical surrogates for preload; the relationship between these parameters and stroke volume defines the ventricular function curve
• Increased preload acutely improves CO; however, excessive preload causes pulmonary congestion as the ventricle operates on the flat portion of the Frank-Starling curve, where further volume increases produce minimal CO improvement and significant elevation in diastolic pressures

Afterload Physiology (Resistance to Ejection)

• Afterload is determined primarily by systemic vascular resistance (SVR), arterial compliance, blood viscosity, and ventricular geometry (via the Laplace relationship: wall stress = [pressure × radius] / [2 × wall thickness])
• Increased afterload reduces stroke volume (SV) and ejection fraction (EF) by decreasing the velocity of fiber shortening and the extent of fiber shortening
• The relationship is inverse: elevated afterload necessitates increased wall stress to generate sufficient force for ejection, consuming more myocardial oxygen and reducing mechanical efficiency
• In acute hypertension, the left ventricle compensates through increased contractility (via sympathetic activation) and increased preload; chronic elevation leads to left ventricular hypertrophy (LVH) with eventual systolic and diastolic dysfunction
• Afterload reduction improves CO in failing hearts by reducing wall stress and oxygen consumption, allowing greater shortening and improved mechanical efficiency

Contractility (Intrinsic Myocardial Function)

• Contractility is the force-generating capacity of the myocardium at a given preload and afterload, independent of loading conditions
• Cellular basis: Contractility is determined by calcium handling, myofilament sensitivity to calcium, and the number of functioning sarcomeres
• Inotropic state is modulated by:
• β-adrenergic stimulation: Increases intracellular cAMP → enhanced calcium release from sarcoplasmic reticulum (SR) → increased troponin C binding → stronger cross-bridge cycling
• Contractile protein composition: Expression of β-myosin heavy chain (slower, more efficient) versus α-myosin (faster, less efficient) affects force and speed
• Myofilament calcium sensitivity: Altered by phosphorylation of troponin I, myosin-binding protein C, and titin; hypertrophy and heart failure reduce sensitivity
• Negative inotropic states include myocardial ischemia, acidosis, hypoxia, hyperkalemia, anesthetics, and β-blockers
• Positive inotropic effects result from catecholamines, digoxin (via Na-K-ATPase inhibition → increased intracellular calcium), phosphodiesterase-3 inhibitors (milrinone), and calcium sensitizers (levosimendan; not available in US)

Integration of Parameters: The Cardiac Function Curves

• The ventricular function curve plots stroke volume or cardiac output against preload; the curve shifts upward with increased contractility and downward with decreased contractility
• Iso-contractility lines represent curves of constant contractility at varying preloads and afterloads
• In normal physiology, the heart operates near the plateau of the Frank-Starling curve, where small increases in preload produce minimal CO changes; this provides homeostatic stability
• In acute decompensation, the failing heart shifts rightward on the Frank-Starling curve (higher preload required for same CO) and downward (lower absolute CO at any given preload)

Factors Increasing Preload

• Volume expansion: Fluid overload, excessive intravenous hydration, renal dysfunction (sodium retention), cirrhosis (splanchnic vasodilation)
• Mitral regurgitation, aortic regurgitation: Backward flow increases ventricular filling
• Atrial fibrillation: Loss of atrial "kick" may initially be compensated by increased diastolic filling time, but decreased heart rate variability impairs preload optimization
• Venous return augmentation: Supine positioning, leg elevation, Valsalva release, pregnancy (increased circulating volume)

Factors Decreasing Preload

• Volume depletion: Hemorrhage, dehydration, excessive diuresis, gastrointestinal losses
• Decreased venous return: Standing (orthostasis), positive pressure ventilation, pulmonary embolism
• Sepsis: Distributive shock with peripheral vasodilation
• Mitral stenosis: Obstructs mitral valve flow → reduces LV filling
• Pericardial disease: Tamponade or restriction physically limits ventricular filling

Factors Increasing Afterload

• Systemic hypertension: Sustained elevation in BP increases SVR
• Aortic stenosis: Fixed obstruction increases wall stress throughout systole
• Arterial vasoconstriction: Norepinephrine release (catecholamine surge, hypoxia, pain), angiotensin II, vasopressin in cardiogenic shock
• Increased blood viscosity: Severe polycythemia, hypergammaglobulinemia
• Aortic atherosclerosis and stiffness: Reduces arterial compliance, increases pulse pressure and impedance

Factors Decreasing Afterload

• Vasodilation: Sepsis, anaphylaxis, nitroprusside, hydralazine, ACE inhibitors
• Anemia: Reduced blood viscosity
• Hypothermia: Decreased metabolic demand
• Advanced cirrhosis: Splanchnic vasodilation

Factors Increasing Contractility

• Sympathetic activation: Catecholamines (norepinephrine, epinephrine) via β₁-adrenergic receptors
• Positive inotropic agents: Dobutamine, milrinone, digoxin, levosimendan
• Increased heart rate: Bowditch effect (increased frequency of contraction enhances force)
• Acute hypertrophy (short-term compensation)

Factors Decreasing Contractility

• Myocardial infarction: Loss of viable myocardium and regional wall motion abnormality
• Cardiomyopathy: Dilated (viral, genetic, alcoholic, peripartum), restrictive (amyloidosis, sarcoidosis), hypertrophic
• Chronic heart failure: Downregulation of β-adrenergic receptors, altered calcium handling, myofilament dysfunction
• Metabolic derangements: Acidosis (pH <7.2), severe hypoxia (PaO₂ <50 mmHg), hyperkalemia (K⁺ >6.5 mEq/L)
• Medications: β-blockers, non-dihydropyridine calcium channel blockers, antiarrhythmic agents (especially disopyramide)
• Myocarditis: Acute inflammatory damage
• Ischemia: Reversible loss of function; permanent if infarction occurs

Symptoms Related to Elevated Preload

• Dyspnea: Orthopnea (shortness of breath when supine), paroxysmal nocturnal dyspnea (PND), exertional dyspnea
• Peripheral edema: Ankle, sacral, or generalized swelling indicating elevated systemic venous pressure
• Abdominal distension and discomfort: Hepatomegaly with right upper quadrant tenderness

Symptoms Related to Elevated Afterload

• Angina pectoris: Chest discomfort with exertion due to increased myocardial oxygen demand imposed by high wall stress
• Exertional dyspnea and fatigue: Inability to increase cardiac output sufficiently to meet metabolic demands
• Syncope or presyncope: Particularly in aortic stenosis when contractility cannot overcome fixed obstruction

Symptoms Related to Decreased Contractility

• Fatigue and exercise intolerance: Inadequate cardiac output response to activity
• Dyspnea at rest and with exertion: Pulmonary edema from compensation via increased preload
• Orthostatic hypotension: Especially if accompanied by autonomic dysfunction or hypovolemia
• Cool extremities: Peripheral hypoperfusion from low cardiac output

Physical Examination Findings in Elevated Preload

• Elevated jugular venous pressure (JVP): >4 cm H₂O at 45° angle; indicates high right atrial pressure
• Hepatojugular reflux: Further elevation of JVP with right upper quadrant pressure, reflecting hepatic congestion
• Crackles (rales): Bilateral, predominantly at lung bases; indicate pulmonary edema
• S₃ gallop: Low-frequency diastolic sound heard best at apex with patient supine; results from rapid ventricular filling against stiff ventricle
• Peripheral edema: Pitting edema of lower extremities, sacrum in bedridden patients

Physical Examination Findings in Elevated Afterload

• Hypertension: Systolic and/or diastolic elevation
• Narrow pulse pressure: Reflects decreased stroke volume in severe afterload elevation
• Sustained apical impulse: Lateral displacement and hyperkinesis indicates LVH with preserved EF
• Fourth heart sound (S₄): Reflects forceful atrial contraction against stiff LV wall

Physical Examination Findings in Decreased Contractility

• Displaced apical impulse: Leftward and downward displacement (lateral to midclavicular line) indicates LV dilation
• Diminished S₁ intensity: Reflects reduced force of mitral/tricuspid valve closure
• Cardiomegaly: Enlarged cardiac silhouette on examination; diffuse impulse without localization
• Cool extremities, weak pulses: Manifestations of low cardiac output state
• Tachycardia and tachypnea: Compensatory mechanisms to maintain cardiac output

Clinical Assessment of Preload

• Physical examination remains the cornerstone: JVP, hepatomegaly, edema, crackles, S₃
• Chest X-ray: Pulmonary edema manifests as Kerley B lines (horizontal lines at lung periphery from interlobular septal edema), bat wing opacities (central alveolar edema), pleural effusions
• Echocardiography: Measures left ventricular end-diastolic dimension (LVEDD) and volume; assessment of diastolic function (E/A ratio, deceleration time, tissue Doppler indices) provides indirect preload assessment
• Right heart catheterization: Gold standard for preload assessment; provides direct measurement of pulmonary capillary wedge pressure (PCWP), central venous pressure (CVP); normal PCWP = 6-12 mmHg
• Biomarkers: Elevated BNP (>100 pg/mL) or NT-proBNP (>125 pg/mL) reflect volume overload and ventricular stretch

Clinical Assessment of Afterload

• Blood pressure measurement: Systolic and diastolic BP, pulse pressure (SBP - DBP; normal 30-40 mmHg)
• Systemic vascular resistance (SVR) calculation: SVR = (MAP - CVP) / CO × 80; normal = 800-1200 mmHg·min/L (normal CI and CVP assumed: SVR ≈ MAP / CO × 80)
• Echocardiography: Wall stress estimation using Laplace relationship; assessment of left ventricular mass index (LVMI) indicates chronic afterload elevation with hypertrophy (LVMI >115 g/m² in men, >95 g/m² in women)
• Arterial compliance: Aortic stiffness assessed via pulse wave velocity (PWV) or augmentation index on specialized hemodynamic monitoring

Clinical Assessment of Contractility

• Ejection fraction (EF): Primary measure of systolic function; normal >50%; reduced if <40%; defines HFrEF (heart failure with reduced EF)
• Measured via echocardiography (preferred first-line), cardiac MRI (superior accuracy), radionuclide ventriculography, or left ventriculography at catheterization
• Stroke volume and cardiac output: Direct measurement at catheterization (Fick method or thermodilution); calculation via echocardiography (SV = VTI × CSA)
• dP/dt max: Rate of left ventricular pressure change measured at catheterization; reflects contractile velocity; normal >1000 mmHg/s; <800 mmHg/s indicates depressed contractility
• Tissue Doppler imaging (TDI) and strain imaging: Global longitudinal strain (GLS) via speckle-tracking echocardiography detects subclinical systolic dysfunction earlier than EF; normal GLS = -18 to -20%
• Biomarkers of myocardial injury: Troponin elevation indicates acute myocardial necrosis (MI); CK-MB less specific
• Cardiac MRI with late gadolinium enhancement: Defines scar burden and tissue characterization in cardiomyopathy

Hemodynamic Profiles (Forrester Classification)

• Profile I (Dry, Warm): PCWP ≤18, CI ≥2.2 → normal physiology
• Profile II (Wet, Warm): PCWP >18, CI ≥2.2 → elevated preload, preserved contractility (flash pulmonary edema, acute decompensation)
• Profile III (Dry, Cold): PCWP ≤18, CI <2.2 → low contractility, appropriate preload compensation (compensated cardiogenic shock)
• Profile IV (Wet, Cold): PCWP >18, CI <2.2 → cardiogenic shock with pulmonary edema (worst prognosis)

Diagnostic Criteria for Heart Failure Phenotypes

• HFrEF: EF ≤40%; responsive to neurohormonal blockade
• HFmrEF (mid-range): EF 41-49%; intermediate prognosis; some benefit from ARNI/ACEi
• HFpEF (preserved): EF ≥50% with elevated filling pressures (E/e' >14, elevated PCWP, elevated BNP); often accompanied by diastolic dysfunction and afterload intolerance

Preload Management

Volume Overload (Elevated Preload)

• Diuretics (first-line):
• Loop diuretics (furosemide, torsemide, bumetanide): Inhibit Na-K-2Cl cotransporter in thick ascending limb; most potent; used in acute decompensation and chronic maintenance
• Initial dosing: 40 mg furosemide IV/PO; titrate to maintain euvolemia and urine output 0.5-1 L/day
• Complications: Hypokal

Complications of deranged loading conditions

• Flash pulmonary edema (emergency): an abrupt rise in afterload (hypertensive crisis, acute aortic or mitral regurgitation) or preload pushes the ventricle onto the flat portion of the Frank-Starling curve; signaled by hypoxemia, diffuse crackles, and bat-wing opacities on chest radiograph.
• Cardiogenic shock (emergency): contractility failure with wet-cold Forrester profile IV; signaled by cool mottled extremities, narrow pulse pressure, oliguria, and rising lactate. The ACC/AHA/HFSA heart failure guideline supports early invasive hemodynamic monitoring and mechanical circulatory support when medical therapy fails.
• Subendocardial ischemia: high wall stress (Laplace) raises oxygen demand while elevated LVEDP compresses subendocardial vessels and shortens diastolic perfusion time; signaled by demand-related troponin elevation and ST depression without epicardial occlusion.
• Cardiorenal syndrome: venous congestion raises renal afterload and lowers transrenal perfusion gradient; signaled by rising creatinine despite volume overload.
• Functional mitral regurgitation and atrial fibrillation: chronic dilation displaces papillary muscles and stretches the atria, creating a regurgitant volume-overload spiral.

Complications of therapy

• Loop diuretics: hypokalemia, hypomagnesemia, contraction alkalosis, and prerenal azotemia; hypokalemia predisposes to torsades and potentiates digoxin toxicity. Rapid high-dose IV administration can cause ototoxicity.
• Overdiuresis or nitrates in preload-dependent states (emergency): profound hypotension in right ventricular infarction, tamponade, severe aortic stenosis, or hypertrophic cardiomyopathy — treat with volume, not more vasodilator.
• Nitroprusside: cyanide/thiocyanate accumulation, especially in renal impairment; signaled by altered mental status and unexplained anion-gap acidosis — an emergency requiring drug cessation and antidotal therapy.
• Inotropes (dobutamine, milrinone): increased cAMP and calcium loading raise myocardial oxygen consumption and arrhythmia risk; ACC/AHA/HFSA restricts them to low-output states or palliation, not routine use.
• Digoxin toxicity: nausea, confusion, yellow-green visual halos, and atrial tachycardia with AV block; treat significant toxicity with digoxin-specific Fab fragments.
• RAAS blockade (ACEI/ARB/ARNI/MRA): hyperkalemia, azotemia, angioedema; a 36-hour washout is required between an ACE inhibitor and sacubitril/valsartan.
• Beta blockers: negative inotropy can precipitate shock if initiated during acute decompensation.

• Ejection fraction is load-dependent — the classic distractor: EF falls with increased afterload and rises with increased preload even when intrinsic contractility is unchanged. The load-independent index of contractility is the slope of the end-systolic pressure-volume relationship (ESPVR), or end-systolic elastance. In mitral regurgitation, ejection into the low-pressure atrium makes EF overestimate true contractility — an EF below about 60% already signals significant dysfunction (ACC/AHA 2020 valvular guideline).
• Pressure-volume loop signatures: increased preload widens the loop rightward with a higher stroke volume; increased afterload raises end-systolic volume and shortens the loop; increased contractility steepens the ESPVR and drops end-systolic volume.
• Maneuvers are preload/afterload questions in disguise: Valsalva strain and standing decrease preload → murmurs of hypertrophic cardiomyopathy and mitral valve prolapse get louder, nearly all others softer. Handgrip increases afterload → mitral regurgitation, ventricular septal defect, and aortic regurgitation murmurs intensify; hypertrophic cardiomyopathy softens. Squatting increases both preload and afterload.
• Inferior STEMI with hypotension after nitroglycerin: suspect right ventricular infarction — the best next step is a right-sided ECG (V4R) plus IV fluid loading; nitrates and diuretics are contraindicated because the RV is preload-dependent.
• The S₃ versus S₄ association examiners test: S₃ reflects rapid filling into a dilated, volume-overloaded ventricle (high preload, systolic dysfunction); S₄ reflects atrial contraction against a stiff, hypertrophied ventricle (chronic afterload excess).
• Digoxin: Na⁺/K⁺-ATPase inhibition → rising intracellular Na⁺ → less Na⁺/Ca²⁺ exchange → more sarcoplasmic Ca²⁺. It improves symptoms but has not been shown to reduce mortality; hypokalemia potentiates toxicity.
• Guideline-directed therapy for HFrEF is four classes (ACC/AHA/HFSA 2022): ARNI (or ACEI/ARB), beta blocker, mineralocorticoid receptor antagonist, and SGLT2 inhibitor. Diuretics relieve congestion but do not alter mortality.