Diastolic Myocardial Stiffness

Diastolic Myocardial Stiffness is the passive mechanical resistance of the ventricular wall to stretch during diastolic filling, determined by the intrinsic elastic properties of the myocardium and its extracellular matrix rather than by any active, calcium-dependent process, and constituting, together with active relaxation, one of the two principal determinants of how readily the ventricle fills at any given filling pressure.

Passive versus Active Diastolic Properties

Distinguishing Stiffness from Relaxation

Diastolic myocardial stiffness refers specifically to the passive elastic resistance of already-relaxed cardiac tissue to further stretch, in contrast to the rate of active relaxation, which describes how quickly the myocardium transitions from the contracted to the fully relaxed state; a ventricle can therefore exhibit normal relaxation kinetics but abnormal (increased) passive stiffness, or the reverse, with distinct underlying mechanisms and distinct clinical implications.

The Passive Pressure-Volume Relationship

Passive stiffness is characterized by the diastolic pressure-volume relationship measured after active relaxation is complete, a curve that is markedly nonlinear, with pressure rising only gradually at low filling volumes but increasingly steeply as the ventricle approaches the upper limit of its passive compliance.

where the stiffness coefficient β characterizes the exponential relationship between incremental changes in diastolic pressure (dP) and volume (dV) at any given operating pressure P.

Cellular and Molecular Determinants

Titin-Based Passive Tension

The giant sarcomeric protein titin, spanning from the Z-disc to the thick filament, functions as a molecular spring that generates passive restoring tension when the sarcomere is stretched, and is a principal determinant of myocyte-level passive stiffness; titin exists in multiple splice isoforms of differing length and compliance, with the relative expression of a more compliant (N2BA) versus a stiffer (N2B) isoform substantially influencing overall passive myocyte stiffness.

Titin Phosphorylation

Beyond isoform composition, titin's passive stiffness is further modulated by phosphorylation at multiple sites by protein kinase A, protein kinase G, and calcium/calmodulin-dependent protein kinase II, providing an additional, more rapidly adjustable layer of regulation superimposed on the baseline stiffness set by isoform expression.

Cytoskeletal Contributions

Intermediate filament proteins such as desmin and microtubule networks contribute additional, generally smaller, components of myocyte passive stiffness, mechanically linking the sarcomere to the surrounding cytoskeleton and cellular membrane.

Extracellular Matrix Contributions

Collagen Network

The extracellular collagen matrix surrounding cardiomyocytes, principally type I and type III collagen organized into a supportive network connecting adjacent myocytes and myofibril bundles, contributes substantially to overall tissue-level stiffness, particularly at higher filling pressures where the collagen network becomes the dominant load-bearing element as titin-based myocyte stiffness approaches its own compliance limit.

Fibrosis and Collagen Cross-Linking

Increased collagen deposition (fibrosis) and increased enzymatic cross-linking of existing collagen fibers, both of which occur in response to chronic pressure overload, inflammation, and aging, substantially increase tissue-level passive stiffness independent of any change in cardiomyocyte titin composition, representing a distinct structural pathway to increased diastolic stiffness.

Physiological Regulation

Beta-Adrenergic and Nitric Oxide Signaling

Protein kinase A and protein kinase G-mediated titin phosphorylation, engaged respectively by beta-adrenergic and nitric oxide-cyclic GMP signaling pathways, generally reduce titin-based passive stiffness, providing a physiological mechanism by which sympathetic activation and endothelial nitric oxide signaling can acutely improve diastolic compliance to accommodate increased venous return during exercise or other states of increased circulatory demand.

Load-Dependent Behavior

Because the diastolic pressure-volume relationship is markedly nonlinear, the effective stiffness experienced by the ventricle at any moment depends on its operating point along this curve: a ventricle operating at low filling volume experiences relatively low effective stiffness, while the same ventricle operating at high filling volume experiences substantially greater effective stiffness even without any underlying change in the tissue's intrinsic material properties.

Pathological Increases in Stiffness

Hypertrophic Remodeling

Concentric ventricular hypertrophy, typically driven by chronic pressure overload, increases wall thickness and myocyte titin-based stiffness, and is frequently accompanied by increased interstitial fibrosis, together substantially increasing passive stiffness and requiring elevated filling pressures to achieve normal diastolic volumes.

Infiltrative and Fibrotic Disease

Conditions that deposit abnormal material within the myocardial interstitium, such as amyloidosis, or that produce extensive replacement fibrosis following myocardial infarction, can produce markedly increased diastolic stiffness through mechanisms largely independent of the titin-based regulatory pathways operating in otherwise structurally normal myocardium.

Titin Isoform Shifts in Disease

Shifts toward the stiffer N2B titin isoform, or reduced titin phosphorylation due to impaired nitric oxide-cyclic GMP signaling (as occurs in some forms of heart failure with preserved ejection fraction), directly increase myocyte passive stiffness and are increasingly recognized as important, potentially treatable, contributors to clinical diastolic dysfunction distinct from fibrosis-based mechanisms.

Clinical Consequences

Elevated Filling Pressures

Because increased diastolic stiffness requires higher filling pressure to achieve any given diastolic volume, patients with increased myocardial stiffness characteristically develop elevated left atrial and pulmonary venous pressures at comparatively normal or even reduced ventricular volumes, producing symptoms of pulmonary congestion despite preserved systolic ejection fraction, the hemodynamic hallmark of heart failure with preserved ejection fraction.