Critical medicine topic-CMT 012

HYPERTROPHIC OBSTRUCTIVE

CARDIOMYOPATHY (HOCM):

HEMODYNAMICS

Introduction

The most important physiological abnormality in hypertrophic obstructive cardiomyopathy (HOCM) is diastolic dysfunction, not systolic dysfunction. Although there is dynamic obstruction of the left ventricular outflow tract (LVOT), left ventricular systolic function is usually preserved or even enhanced. Most symptoms arise because the stiff hypertrophied ventricle fills poorly during diastole.

Primary Hemodynamic Abnormality

The hallmark of HOCM is:

• Diastolic dysfunction due to impaired ventricular relaxation and reduced compliance.
• The hypertrophied myocardium becomes stiff.
• The ventricle cannot expand normally during diastole.

Mechanism of Diastolic Dysfunction

The stiff left ventricle causes:

• Impaired ventricular filling
• Reduced ventricular compliance
• Elevated left ventricular end-diastolic pressure (LVEDP)

This leads sequentially to:

• Increased left atrial pressure
• Increased pulmonary venous pressure
• Increased pulmonary capillary pressure

These pressure elevations produce:

• Exertional dyspnea
• Pulmonary congestion
• Exercise intolerance

Systolic Function

Unlike dilated cardiomyopathy:

• Left ventricular systolic function is normal or hyperdynamic.
• Contractility is increased.
• Ejection fraction is usually:
• Normal (>55%)
• Often supernormal (70–85%)

Therefore:

• HOCM is not a disease of systolic pump failure.

Left Ventricular Outflow Tract Obstruction

Many patients develop:

• Dynamic subaortic obstruction
• Pressure gradient between the left ventricle and aorta

This obstruction results from:

• Hypertrophied interventricular septum
• Systolic anterior motion (SAM) of the mitral valve

Ventricular Emptying

Despite LVOT obstruction:

• Ventricular emptying is unusually rapid.
• More than 80% of stroke volume is ejected during the early part of systole.
• Ejection is completed earlier than in normal individuals.

Thus:

• The ejection fraction remains normal or increased.
• LV systolic performance is preserved.

Relationship Between SAM and LVOT Gradient

There is a strong relationship between:

• Systolic anterior motion (SAM) of the mitral valve
• Degree of LVOT pressure gradient

However:

• A larger gradient does not necessarily mean more severe symptoms.
• Symptom severity and obstruction often correlate poorly.

Variability of Obstruction

The LVOT gradient is dynamic and varies with:

• Exercise
• Hydration status
• Preload
• Afterload
• Contractility

Consequently:

• The pressure gradient may change from day to day.
• Symptoms may also fluctuate.

 Cause of Symptoms

Most symptoms are due to:

Diastolic Dysfunction

Leading to:

• Dyspnea
• Fatigue
• Exercise intolerance

Dynamic LVOT Obstruction

Responsible for:

• Exertional syncope
• Post-exertional syncope
• Angina

 

Why Angina Occurs

Angina develops because of:

• Increased myocardial oxygen demand
• Reduced coronary perfusion during diastole
• Compression of intramyocardial coronary arteries by hypertrophied muscle
• LVOT obstruction increasing wall stress

Coronary arteries may be normal.

 

Why Syncope Occurs

Syncope is caused by:

• Dynamic LVOT obstruction reducing cardiac output during exercise
• Ventricular arrhythmias
• Abnormal blood pressure response to exercise

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Important Hemodynamic Features of HOCM

Parameter	Finding
Primary abnormality	Diastolic dysfunction
LV compliance	Decreased
LV relaxation	Impaired
LVEDP	Increased
Left atrial pressure	Increased
Pulmonary venous pressure	Increased
Pulmonary capillary pressure	Increased
Contractility	Hyperdynamic
Ejection fraction	Normal or increased
LVOT obstruction	Dynamic
Ventricular emptying	Rapid, early systolic
Symptoms correlate with gradient	Poorly

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Clinical Pearls

• Diastolic dysfunction is the principal physiological abnormality in HOCM.
• Left ventricular systolic function is usually normal or hyperdynamic.
• Ejection fraction is often normal or supranormal.
• Dynamic LVOT obstruction results from septal hypertrophy and systolic anterior motion (SAM) of the mitral valve.
• Symptoms correlate more closely with impaired diastolic filling than with the magnitude of the LVOT gradient.
• Exertional syncope and angina may occur due to dynamic outflow obstruction and myocardial ischemia.

Key Points

• Most important abnormality: Diastolic dysfunction.
• LV ejection fraction: Normal or increased.
• LV filling: Impaired due to a stiff hypertrophied ventricle.
• LVOT obstruction: Dynamic and variable.
• Symptoms: Mainly due to impaired diastolic filling, with syncope and angina partly attributable to dynamic systolic obstruction.

 .C.GANESAN M.D., 

PROFESSOR OF MEDICINE

DR.C.GANESAN M.D.,

PROFESSOR OF MEDICINE