Electrocardiogram

An electrocardiogram, or ECG (or EKG) is a recording of the electrical changes in the myocardium during the cardiac cycle.

Section 3, Chapter 15

P Wave

• Represents atrial depolarization

• Leads to atrial contraction

• Conduction of electrical impulse

from right to left and downward

QRS Complex

• Represents depolarization of

ventricles

• Leads to ventricular contraction

• This massive wave hides the atria

repolarization

Electrocardiogram

from right to left and downwardrepolarization

T Wave

• Represents repolarization of

ventricles

Electrocardiogram

Figure 15.22d. An ECG pattern with the corresponding systole and diastole shown above.

ECG of a regular heart rhythm at 75 beats per minute

Heart Arrhythmias:

normal

ECG of a regular heart rhythm at 75 beats per minute

Atrial Flutter. Atria fire 250-350 times per minute. For every QRS

complex there may be 4 or more P waves.

Bradycardia – cardiac rhythm less than 60 beats per minute.

Examples of Heart Arrhythmias. Arrows indicate p Wave.

Bradycardia – cardiac rhythm less than 60 beats per minute.

Tachycardia– cardiac rhythm greater than 100 beats per minute.

Atrial fibrillation. Instead of contracting, the atria become quivering

chambers. The ventricles respond only to impulses that make it to the

AV node.

Examples of Heart Arrhythmias, fibrillation

Ventricular fibrillation = Life threatening arrhythmia. Ventricles quiver, and are

unable to pump blood properly. Requires immediate defibrillation.

The heart rate is controlled intrinsically by the SA

node, but sympathetic and parasympathetic fibers

alter the rate at which the pacemaker fires.

Regulation of cardiac cycle

Cardiac Control Center

• Within Medulla Oblongata

• Receives sensory impulses from throughout the cardiovascular system and relays motor impulses to heart in response.

• Cardioinhibitor & cardioaccelerator reflex centers

• Cardioinhibitor reflex center

– Parasympathetic fibers from vagus nerves innervate SA & AV nodes.

– Acetylcholine (ACh) released from fibers decreases the firing rates of SA

& AV nodes.

– Heart rate decreases

• Cardioaccelerator reflex center

– Sympathetic fibers from accelerator nerves innervate SA & AV nodes.

– Norepinephrine released from fibers increases the firing rates of SA & AV

nodes.

– Heart rate and force of contraction increases

Cardioinhibitor & cardioaccelerator reflex centers alter the heart

rate in response to sensory impulses from receptors

Baroreceptors – monitor blood pressure

• Within aortic arch and carotid sinuses

• Rising blood pressure stimulates cardioinhibitor center

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Figure 15.24b Illustration of the baroreflex arc

End of Section 3, Chapter 15