Critical medicine topic-CMT 011

 HYPERTROPHIC CARDIOMYOPATHY (HCM):

DYNAMIC CHANGES IN THE LEFT VENTRICULAR

OUTFLOW TRACT (LVOT) MURMUR

A hallmark of hypertrophic cardiomyopathy (HCM) is the dynamic nature of left ventricular outflow tract (LVOT) obstruction. The intensity of the systolic murmur varies depending on changes in:

·        Myocardial contractility

·        Preload (ventricular filling)

·        Afterload (systemic vascular resistance)

The murmur becomes louder when the LV cavity becomes smaller, increasing LVOT obstruction.

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Mechanisms That Increase the Murmur

Three physiological changes accentuate the systolic murmur:

1.   Increased myocardial contractility

2.   Decreased preload

3.   Decreased afterload

All three increase the LVOT pressure gradient.

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Dynamic Maneuvers That Increase the Murmur

1. Sudden Standing from Squatting (Most Helpful Bedside Maneuver)

During Squatting

·        ↑ Venous return (↑ preload)

·        ↑ Aortic pressure (↑ afterload)

·        ↑ LV cavity size

·        ↓ LVOT obstruction

·        ↓ Murmur intensity

Sudden Standing

·        ↓ Venous return

·        ↓ LV cavity size

·        ↑ LVOT obstruction

·        ↑ Pressure gradient

·        Louder systolic murmur

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2. Valsalva Maneuver (Strain Phase)

Produces:

·        ↓ Preload

·        ↓ Afterload

·        Smaller LV cavity

·        Increased dynamic obstruction

Result: Louder HCM murmur.

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3. Increased Contractility

The murmur becomes louder with:

·        Exercise

·        Tachycardia

·        Hypovolemia

·        Post-extrasystolic potentiation (following premature atrial contraction)

·        Isoproterenol infusion

·        Digitalis therapy

Mechanism:

·        Stronger ventricular contraction

·        Increased systolic anterior motion (SAM) of the mitral valve

·        Greater LVOT obstruction

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4. Amyl Nitrite

Produces marked:

·        ↓ Preload

·        ↓ Afterload

Result:

·        Increased LVOT gradient

·        Louder systolic murmur

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Mechanisms That Decrease the Murmur

The murmur becomes softer with:

·        Increased preload

·        Increased afterload

·        Decreased contractility

These changes enlarge the LV cavity and reduce dynamic obstruction.

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Dynamic Maneuvers That Decrease the Murmur

1. Müller Maneuver

(Deep inspiration against a closed glottis)

Effects:

·        ↑ Preload

·        ↑ Afterload

·        Larger LV cavity

·        Reduced LVOT obstruction

Result: Softer murmur.

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2. Valsalva Maneuver (Overshoot Phase)

After release:

·        ↑ Venous return

·        ↑ Afterload

·        Larger LV cavity

Result:

·        Decreased LVOT gradient

·        Softer murmur

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3. Phenylephrine

Effects:

·        ↑ Systemic vascular resistance (↑ afterload)

·        Larger LV cavity

·        Reduced obstruction

Result: Decreased murmur intensity.

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4. Beta-Adrenoceptor Blockers

Effects:

·        ↓ Heart rate

·        ↓ Contractility

·        Improved ventricular filling

·        Reduced LVOT obstruction

Result: Softer murmur.

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5. Isometric Handgrip

Effects:

·        ↑ Afterload

·        ↓ Dynamic LVOT obstruction

Result: Decreased murmur intensity.

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Summary Table

Intervention	Preload	Afterload	Contractility	Effect on HCM Murmur
Sudden standing	↓	↓	—	↑ Murmur
Squatting	↑	↑	—	↓ Murmur
Valsalva (strain)	↓	↓	—	↑ Murmur
Valsalva (release/overshoot)	↑	↑	—	↓ Murmur
Müller maneuver	↑	↑	—	↓ Murmur
Exercise	—	—	↑	↑ Murmur
Tachycardia	↓ Filling	—	↑	↑ Murmur
Hypovolemia	↓	—	—	↑ Murmur
Isoproterenol	↓	↓	↑	↑ Murmur
Digitalis	—	—	↑	↑ Murmur
Amyl nitrite	↓	↓	—	↑ Murmur
Phenylephrine	—	↑	—	↓ Murmur
Beta-blockers	↑ Filling	—	↓	↓ Murmur
Isometric handgrip	—	↑	—	↓ Murmur

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Clinical Pearls

·        HCM murmur is dynamic, unlike the fixed murmur of valvular aortic stenosis.

·        Standing and the Valsalva strain phase make the murmur louder by reducing LV volume.

·        Squatting, handgrip, and phenylephrine make the murmur softer by increasing LV cavity size or afterload.

·        Beta-blockers are first-line therapy because they reduce contractility and LVOT obstruction.

·        The standing-from-squatting maneuver is the most useful bedside test for demonstrating dynamic LVOT obstruction.

 

DR.C.GANESAN M.D.,

PROFESSOR OF MEDICINE