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

Electrocardiogram

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

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

Atrial Flutter. Atria fire 250-350 times per minute. For every QRS complex there may be 4 or more P waves.

Heart Arrhythmias:

normal

Bradycardia – cardiac rhythm less than 60 beats per minute.

Tachycardia– cardiac rhythm greater than 100 beats per minute.

Examples of Heart Arrhythmias. Arrows indicate p Wave.

Atrial fibrillation. Instead of contracting, the atria become quivering chambers. The ventricles respond only to impulses that make it to the AV node.

Ventricular fibrillation = Life threatening arrhythmia. Ventricles quiver, and are unable to pump blood properly. Requires immediate defibrillation.

Examples of Heart Arrhythmias, fibrillation

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