2- Heart rate, heart sound and murmurs



Babylon University

College of Medicine

Department of physiology

Dr. Ghafil Saihood Hassan 2016 – 2017.

2- Heart rate, heart sound and murmurs.

Objectives:

1. Describe the heart rate.

2. Explain the mechanism of the heart sounds.

3. Explain the mechanism of the murmurs.

Heart rate:

It is number of heart beat. The contraction of atria (atrial systole) is followed by contraction of the ventricle (ventricular systole). SA node able to generate action potential with rate of 60 –90 beats/minute in young adult. When SA node does not generate action potential, AV node generates it at rate of 40 – 60 beats /minute and Purkinje fiber at rate of 20 – 40 beats /minute. Increase in heart rate will reduces the duration of ventricular diastole and so reduce the time available for ventricular filling that will reduce the stroke volume.

Normal, and abnormal heart rate:

A normal heart rate is varies between 60 and 90 beats/min. which is called normocardia. A fast heart rate, more than 100 beats/min. is called tachycardia. The electrocardiogram is normal except that the rate of heartbeat is increased. The general causes of tachycardia are:

1-Increased body temperature.

2-Stimulation of the heart by the sympathetic nerves.

The term "bradycardia" means a slow heart rate, usually defined as less than 60 beats per minute (eg Athletes and vagal stimulation). See figure 7.

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Figure (7): sinus tachycardia and bradycardia (Hall, 2016)

Effects autonomic nervous system on heart rate:

Figure 8 shows the effect of sympathetic and parasympathetic on heart rate.

Stimulation of the sympathetic nervous system activates B1 receptors in the SA node by norepinephrine which increases inward Na ion current. That mean the SA node is depolarized to threshold more frequently (increase heart rate).

Stimulation of parasympathetic nervous system activates muscarinic receptors by acetylcholine (Ach) in the SA node which decreases inward Na ion current. That mean SA node is depolarized to threshold less frequently (decreased heart rate).

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Figure (8): Effect of autonomic nervous system on the heart (Ganong's review of medical physiology 2010)

Effect of Body temperature on heart rate: Increase temperature as occur in fever, increases permeability of the cardiac muscle membrane. During fever the heart rate increases approximately 18 beats/min. per one C°. Decrease temperature causes greatly decreased heart rate. Contractile strength of the heart often is enhanced by a moderate increase in temperature, but prolonged elevation of the temperature exhausts the metabolic system of the heart and causes weakness.

Factors increase heart rate:

1-Decreased activity of baroreceptors.

2- Inspiration.

3- Excitement.

4- Anger.

5- Painful stimuli.

6- Hypoxia.

7- Norepinephrine.

8- Exercise.

9- Epinephrine.

10- Fever.

11- Thyroid hormone

Factors decrease heart rate:

1- Increased activity of baro-receptors.

2- Expiration.

3- Grief.

4- Stimulation of pain fiber in trigeminal nerve.

5- Increased intracranial pressure.

Heart sound

When the stethoscope is placed on the chest wall over the heart, two sounds are normally heard during each cardiac cycle (1st & 2nd heart sounds). Opening of the valve does not hear because this is slowly developing process that normally makes no noise. When the valve close, the vanes of the valves and the surrounding fluid vibrate under the influence of the sudden pressure in all directions through the chest.

1-First heart sound: When the ventricles contract, first heart sound is heard by closure of the A-V valves. Vibration is low in pitch and relatively long. The valves bulge backward toward the atrium until the chordae tendineae abruptly stops the back bulging.

2-Second heart sound: When the aortic and pulmonary valves close at end of systole, they close rapidly and vibrate for short period of higher pitch.

The duration of the 1st. heart sound (0.14 second), is longer than the second heart sound (0.11 second); this is because the semilunar valves are tauter than AV valves, so they vibrate for short period than do AV valves. The first heart sound has a lower frequency (pitch) which is 25-45 Hz than second heart sound (50 Hz), this is because the tautness of the semilunar valves in comparison with much less taut AV valves. Second heart sound is single during expiration, while the interval between aortic and pulmonary valve closure during inspiration is frequently long enough for the second sound to be re-duplicated (physiological splitting) because during inspiration, the venous return increases, so there will be a delay in filling the right ventricle and delay in the closure of the pulmonary valve.

Third heart sound: It has duration of 0.1 second, not heard normally by stethoscope. It is physiological sound in children and in young adult. It occurs in the middle third diastole, caused by rapid ventricular filling and is probably due to vibration set up by the in- rush of blood. It is a low-pitched sound and can be heard after the S2. It is heard in normal heart; in children and in adult during exercise. It is also heard in anemia, and AV valve regurgitation.

Fourth heart sound: Normally not heard with stethoscope, but by phonocardiogram, because it has low frequency (20 Hz). It occurs immediately before the first heart sound at late diastole. This sound occurs due to atrial contraction. It is caused by inrush of blood into the ventricle. It is not heard in normal hearts but occurs during ventricular overload as in severe anemia, Thyroitoxicosis (hyperthyroidism) or in reduced ventricular compliance and in hypertension. See figure 9.

The third and fourth sounds are low frequency. They give a characteristic gallop rhythm. Both sounds are best heard with the bell of the stethoscope at the cardiac apex.

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Figure (9): Heart sounds (Ganong's review of medical physiology 2010)

Murmurs

Murmurs or bruits are abnormal sound heard in various parts of CVS. Normal blood flow is laminar and non-turbulent (silent) up to critical velocity. Above this velocity and beyond an obstruction, blood flow is turbulent (creates sound). Blood flow speeds when an artery or a heart valve is narrowed. The major cause of cardiac murmur is the disease of the heart valves. When the orifice of a valve is narrowed (stenosis), blood flow through it is accelerated and turbulent. When a valve is incompetent, blood flow backward through it (regurgitating) that accelerates flow. mechanical events of cardiac cycle. Murmur due to disease of a particular valve can generally be heard well when the stethoscope is over that valve. See figure 10.

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Figure (10): Turbulent blood flow (Ganong's review of medical physiology 2010)

The time of murmurs are listed below table.

Table (1): Show type of valve, abnormality and timing of murmur.

|Valve | Abnormality |Timing of murmur |

|Aortic or pulmonary |Stenosis |Systolic |

| |Insufficiency |Diastolic |

|Mitral or tricuspid |Stenosis |Diastolic |

| |Insufficiency |Systolic |

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