Nonobstructive HCM: Beyond Rate Control Toward Myocardial Architecture
The recent trial of bisoprolol and verapamil in nonobstructive hypertrophic cardiomyopathy (HCM) raises a question more fundamental than which conventional drug should be preferred. It challenges the clinical assumption that slowing the heart is necessarily beneficial when the limiting problem may be the structure and energetic efficiency of the myocardium itself.
In symptomatic nonobstructive HCM, bisoprolol reduced peak oxygen consumption, whereas verapamil largely preserved it, underscoring a phenomenon that has been visible for decades but rarely integrated into management: when stroke‑volume reserve is structurally limited, suppressing heart‑rate response can directly worsen exercise cardiac output and functional capacity. These findings do not mean that beta‑blockers have no role in patients who require them for atrial fibrillation rate control, malignant arrhythmias, or severe hypertension. But they do show that, in a ventricle constrained by hypertrophy, disarray, fibrosis, extracellular‑matrix stiffening, and microvascular dysfunction, routine heart‑rate suppression is not a solution to the primary disease; it removes the last remaining mechanism by which such a ventricle can augment output without addressing any of the architectural causes of its limitation.
Beyond-the-Pump.pdf“Nonobstructive” Does Not Mean Unconstrained
The absence of a left-ventricular outflow-tract gradient describes only what is not present: a major obstruction at the ventricular exit. It does not describe the severe intramyocardial limitations that may remain.
Nonobstructive HCM can include a thickened ventricular wall, a relatively small cavity, cardiomyocyte hypertrophy, myocyte disarray, interstitial and replacement fibrosis, abnormal intramyocardial vessels, and impaired microvascular perfusion. These features differ among patients, but together they can produce a ventricle that contracts vigorously at rest while having limited capacity to increase useful output during physical activity.
This is why a normal or elevated ejection fraction can be misleading. Ejection fraction describes the percentage of ventricular volume expelled. It does not establish whether the absolute stroke volume is adequate, whether the output can rise with exercise, or how much metabolic energy the heart expends to generate that output.
Why Heart-Rate Suppression May Fail
During exercise, cardiac output depends on both heart rate and stroke volume:
In a thick-walled, small-cavity ventricle, stroke-volume augmentation may be limited. Filling may be restricted by impaired relaxation, stiffness, fibrosis, and reduced compliance. Under these conditions, an increase in heart rate is not necessarily an excessive or pathological response. It may be the principal remaining means by which the heart raises output.
Beta-blockade can therefore create a mismatch. It lowers the capacity to increase heart rate while leaving the structural limits on filling and stroke volume unchanged. The intended benefit—more time for filling—may be insufficient if the ventricle cannot use that time effectively because the chamber is mechanically constrained.
The relevant clinical question is not whether a drug lowers heart rate. It is whether it improves the patient’s ability to sustain useful cardiac output during ordinary activity and exercise.
HCM as an Architectural Disease
HCM begins with abnormalities in sarcomeric biology in many patients, but its clinical expression is not confined to altered contractile proteins. Over time, the disease becomes an architectural disorder of the entire myocardial wall.
Hypertrophied cardiomyocytes increase the metabolic burden of contraction. Myocyte disarray reduces the orderly alignment through which individual cellular forces become coordinated chamber deformation. Fibrosis alters stiffness and creates heterogeneous regions that can interfere with relaxation, filling, and mechanical coordination. Small-vessel remodeling and microvascular dysfunction restrict oxygen delivery to tissue with already high metabolic requirements.
These processes act together. The myocardium may remain capable of generating force, but it becomes less effective at converting that force into efficient ventricular work.
The important distinction is between contractility and performance. A hyperdynamic heart is not necessarily an efficient heart. It may contract strongly while producing limited forward-flow reserve at a high metabolic cost.
The ECM Problem
The extracellular matrix is not passive material between cardiomyocytes. It is part of the infrastructure that holds cells in mechanical relationship, distributes wall stress, transmits force, organizes capillary support, and contributes to the elastic properties of the ventricular wall.
In a healthy heart, this organization supports coordinated contraction and recoil. In HCM, pathological remodeling can turn the same scaffold into a source of stiffness, mechanical heterogeneity, and inefficient force transmission.
Fibrosis should therefore not be understood only as an increase in collagen quantity. Its consequences depend on where it is located, how it is cross-linked, how it changes fiber orientation, how it disrupts neighboring myocytes, and how it affects the microvascular environment.
This has therapeutic implications. A treatment may improve symptoms or reduce neurohormonal stress while leaving the deeper architectural disorder intact. It may slow a process without restoring the mechanical organization on which efficient function depends.
Limits of Present Therapy
Current medications remain clinically valuable, but their limits should be stated clearly.
Beta-blockers can be essential when a patient needs rate control, arrhythmia suppression, blood-pressure treatment, or management of specific symptoms. Yet they do not remove fibrosis, correct cellular disarray, restore normal wall mechanics, or rebuild microvascular reserve. In patients dependent on chronotropic compensation, they may worsen exercise capacity.
Verapamil and diltiazem may be reasonable alternatives in selected individuals, especially when their hemodynamic and heart-rate effects are better tolerated. However, they are not structural therapies. They do not reverse established replacement fibrosis or restore the original organization of the myocardium.
Myosin inhibitors represent an important advance, particularly in obstructive HCM. By modifying excessive actin-myosin interaction, they can substantially improve obstruction-related physiology. Yet they should not be treated as a complete solution for nonobstructive disease. They do not directly reorganize fibrotic tissue, normalize myocardial alignment, or restore a damaged microvascular network.
The unmet need is not merely another symptom-directed drug. It is treatment that addresses the structural substrate producing impaired reserve.
A Better Clinical Framework
Nonobstructive HCM should be evaluated as a disorder of reserve rather than only as a disorder of resting measurements.
A patient’s assessment should integrate:
LV cavity size and stroke-volume reserve
Exercise capacity, especially peak
Peak heart-rate response and chronotropic competence
Left-atrial size and indicators of filling pressure
Global and regional strain, and torsion or untwisting where available
Cardiac MRI evidence of focal scar and diffuse interstitial remodeling
Perfusion or microvascular assessment when clinically appropriate
Biomarkers such as NT-proBNP, interpreted in combination with imaging and exercise data
This approach does not require that every patient receive every advanced test. It requires recognizing that no single measure—especially not ejection fraction or resting heart rate—adequately represents the functional state of a structurally abnormal myocardium.
The Next Therapeutic Direction
The future should move toward phenotype-specific treatment. Some patients may benefit from rate reduction; others may depend on preserved chronotropic reserve. Some may be dominated by arrhythmia, some by microvascular ischemia, some by diastolic stiffness, and some by advanced fibrosis with limited stroke-volume reserve.
This demands therapies that can act earlier in the remodeling process and, ultimately, more directly on the pathological myocardial environment. Relevant targets include maladaptive fibroblast activation, collagen deposition and cross-linking, microvascular dysfunction, inflammatory and oxidative pathways, abnormal cardiomyocyte–matrix signaling, and the loss of coordinated ventricular mechanics.
The long-term goal should be more ambitious than reducing wall thickness or suppressing symptoms. It should be preservation—and where possible restoration—of the organized relationship among cardiomyocytes, extracellular matrix, capillaries, and ventricular geometry.
A Testable Proposal
The recent findings support a practical hypothesis: patients with the greatest burden of structural constraint will be most vulnerable to pharmacological loss of chronotropic reserve.
That risk profile may include a small LV cavity, impaired filling, fibrosis, reduced perfusion reserve, abnormal strain or torsion, elevated filling-pressure markers, and poor stroke-volume augmentation during exercise. Prospective trials should test whether these measures predict a fall in peak or functional status during beta-blocker therapy.
Such trials should judge treatment by integrated performance: exercise capacity, symptoms, cardiac output reserve, imaging-defined remodeling, and metabolic burden—not by heart rate alone.
Final Perspective
The new nonobstructive HCM data are important because they expose the limits of an old therapeutic reflex. Slowing the heart may be useful in some circumstances, but it cannot substitute for understanding why the heart fails to increase output.
The central problem in many patients may not be excessive rate or excessive force. It may be a structurally constrained myocardium that has lost the mechanical and metabolic reserve required for exertion.
That is why the next stage of HCM therapy must extend beyond rate control. It must address the architecture of the disease: the cardiomyocytes, their alignment, the extracellular matrix, the microvasculature, and the geometry through which all of them become a functioning heart.
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Mykola Iabluchanskyi together with Andriy Yabluchanskiy
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