The PR Interval and Ml Loudness

1. Why does a short P-R interval cause a loud M1?

ANS: The P controls the timing of atrial contraction, which raises left atrial (LA) pressure. The force of the contraction opens the mitral valve further at the end of diastole. The R controls the timing of ventricular contraction. If the P-R interval is short, ventricular contraction occurs so quickly after the atrium has contracted that the LA has not had time to relax (short X descent). Therefore, atrial pressure is still at a high level when the pressure in the LV exceeds it enough to close the mitral valve [2]. This means that the ventricle has a long time to contract before it overcomes the relatively high LA pressure. Therefore, the LV has time to accelerate to a rapid dP/dt part of its pressure curve by the time it closes the mitral valve. (See the accompanying figure.)

A. If the P-R interval is short, the LV contracts before the left atrium has had a chance to relax and drop its pressure. Therefore, the LV pressure will not exceed left atrial pressure until it has contracted for a long enough time to accelerate to a stage of rapid pressure rise by the time the mitral leaflets are closed. This produces an abrupt deceleration of forward flow and a loud sound.

B. If the P-R interval is long, the LV contracts later than at A, so that the left atrium has had time to drop to a low pressure when the LV pressure exceeds it. The pressure crossover point is on the slow part of the LV acceleration curve, and the valves are closed at a relatively slow rate, producing a soft sound.

A. If the P-R interval is short, the LV contracts before the left atrium has had a chance to relax and drop its pressure. Therefore, the LV pressure will not exceed left atrial pressure until it has contracted for a long enough time to accelerate to a stage of rapid pressure rise by the time the mitral leaflets are closed. This produces an abrupt deceleration of forward flow and a loud sound.

B. If the P-R interval is long, the LV contracts later than at A, so that the left atrium has had time to drop to a low pressure when the LV pressure exceeds it. The pressure crossover point is on the slow part of the LV acceleration curve, and the valves are closed at a relatively slow rate, producing a soft sound.

2. Why does a long P-R interval cause a soft Mj?

ANS: The delayed LV contraction gives the LA pressure a chance to drop to low levels (deep X descent) by the time the LV begins to contract. Thus LV pressure will exceed LA pressure at the very early slow part of its acceleration curve [2].

Note: a. It has usually been taught that the reason for the loud Ml with a short P-R is that the mitral leaflets are wide open at the time of the LV contraction onset—i.e., atrial relaxation, which is known to be able to close mitral valves by a kind of suction effect [3], has not had much time to act. The analogy of a wide-open door making more noise when it closes than does a slightly open door is often used but is not valid unless the door is made to accelerate as it closes.

b. The paradox of a long P-R interval and a loud S1 is seen in mitral stenosis (MS) and also in Ebstein's anomaly. In the latter, the Ml may actually be very soft, but the second component of the S1 may be loud, short, and clicking because it is caused by a closure of a large deformed anterior leaflet of the tricuspid valve. Because this leaflet has been likened to a large sail flapping in the breeze, this loud T1 has been called a sail sound. This sound often increases with inspiration and is usually associated with a very late tricuspid opening snap.

c. In sudden severe aortic regurgitation the mitral valve may be closed in mid-diastole and is associated with a soft or inaudible S1.

d. A soft S1 in the presence of an S2 that is of normal loudness suggests first-degree A-V block.

3. Which situations can be diagnosed by hearing the effect of a changing P-R interval on the M1?

ANS: Any atrioventricular (AV) dissociation, as in complete AV block or some ventricular tachycardias. (If the ventricular tachycardia has retrograde VA conduction into the atria, there will be no AV dissociation.)

This medium-frequency (MF) phonocardiogram is from a patient with complete AV block, i.e., the P waves and QRS complexes are independent, thus causing the P-R intervals to vary. Note the loud Sj after the short P-R (first one) and the soft one after the long P-R both at the apex and second left interspace (2 LIS).

This medium-frequency (MF) phonocardiogram is from a patient with complete AV block, i.e., the P waves and QRS complexes are independent, thus causing the P-R intervals to vary. Note the loud Sj after the short P-R (first one) and the soft one after the long P-R both at the apex and second left interspace (2 LIS).

Note: Type 1 second-degree AV block (Wenckebach periods) also has gradually longer P-R intervals until complete AV block occurs and a beat is dropped. The gradually longer P-R interval has been said to cause a softer and softer first sound until a pause occurs. However, if the longest P-R is very long (e.g., about 360 ms), the valves may reopen owing to continued pulmonary venous return, and so produce a slight increase in loudness. Often, though, the P-R changes are so small that no perceptible M1 changes occur.

4. How does left bundle branch block (LBBB) affect the loudness of the Ml?

ANS: The onset of LV contraction may be delayed so that the effect of a long P-R interval is produced.

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Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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