Causes and Associated Conditions

1. When should you consider an S4 pathological?

ANS: You should consider any S4 possibly pathological, even in a young subject, until proved otherwise. This means that before you call it physiological in the younger age group, you should probably hear a physiological S3 and a physiologically split S2, feel a normal apex beat, hear a venous hum in the neck (see Chapter 14), and obtain a normal ECG, chest X-ray, and echocardiogram to rule out a hyper-trophic cardiomyopathy.

2. What condition generates a strong enough atrial contraction to produce an audible S4?

ANS: Any condition in which the ventricle is "stiffer" than normal, i.e., in which the ventricle has decreased distensibility or compliance.

3. How does loss of distensibility of a ventricle cause a strong atrial contraction, i.e., how does the atrium "know" it must contract more strongly when the ventricle has lost compliance?

ANS: While the mitral valve is open, pressures are almost equal in both atrium and ventricle. This common diastolic chamber may be called an atrioventricle. If the LV is poorly distensible, as with left ventricular hypertrophy (LVH), the pressure rise due to filling of the atrioventricle from the pulmonary veins is steep. By the time the P wave and its subsequent atrial contraction occur, the atrial pressure is so high that, via a strong Starling effect, the atrium will contract with greater energy than normal.

4. What conditions cause a decreased compliance of the ventricle?

ANS: Those in which the ventricle is a. Thickened by LVH, as when it is laboring under a chronic pressure load, e g, hypertension (the most common cause of LVH in which an S4 is heard) and in hypertrophic cardiomyopathies.

b. Stiffened by replacement of myocardium by fibrous tissue, e.g., an old myocardial infarction.

c. Stiffened by ischemia due to angina or acute infarction. (Subtotal or total acute coronary occlusions in dogs increases the stiffness of the LV [2].)

Note: A patient whose filling pressure is elevated by a strong atrial contraction will be less dyspneic than one who has the same high filling pressure due to a high V wave, because a high A wave starting from a low

A low V wave and a disproportionately high A wave are seen on the left. These give a lower mean left atrial pressure and therefore less dyspnea than do both a high V wave and a relatively poor atrial contraction, as seen on the right.

left atrial pressure will result in a lower mean left atrial pressure than will a high V wave falling to a high left atrial pressure.

5. What causes an atrium in sinus rhythm to be too weak to help out a ventricle?

ANS: a. Atrial damage.

b. Too little blood in the atrium, e.g., due to diuresis.

c. The influence of agents that decrease inotropism, such as beta blockers.

6. How can the S4 assist in the diagnosis of constrictive pericarditis or tamponade? Explain.

ANS: An S4 is not heard in constriction or tamponade. This is because:

a. The ventricle is unable to expand at the end of diastole due to the constriction.

b. The atrium may be so tethered by the constrictive process that it cannot contract well.

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