Physiology_Lab_Practical 2_7.2

An abnormally long P-R interval indicates a condition called

1st degree heart block

On initial examination, a pt was found to have a P-R interval of 0.24second. A year later, the pt had a resting pulse of about 40 per minute. A year after that, the patient collapsed with a pulse of 20 per min and required the insertion of an artificial pacemaker. Explain what caused this sequence of events.

At the time of the initial examination a P-R interval of 0.24 sec defines a first-degree AV node block. A year later the AV conduction block has progressed with the pt now in third-degree AV node block as demonstrated by the appearance of ectopic pacemakers in the ventricles, the very slow rate of beat, and the lack of response to the demands of exercise. An artificial pacemaker may be installed to compensate for the complete conduction block.

Describe the pathway of conduction from the atria to the ventricles and correlate this conduction with the ECG waves.

Depolarization spreads through the atria producing the P wave. After a delay at the AV node, the depolarization then spreads down the bundle of His to the Purkinje fibers and the ventricular myocardium. This produces the QRS wave. Repolarization of the ventricles in the reverse direction produces the T wave. In third-degree, or complete AV node block the ventricles are not depolarized by impulses from the atria. The atria continue to beat according to the pace set by the SA node(supraventricular) while the ventricles beat according to the pace set by an ectopic pacemaker within the ventricles, responsible for the slower rate and the lack of response to the demands of exercise that raise the atrial pacing only

The electrical synapses between adjacent myocardial cells are called

Gap junctions

What happens to the beating of the atria and ventricles during third-degree AV node block? Why does this occur?

In third-degree AV node block none of the impulses from the atria reach the ventricles. No longer subservient to the SA node rhythm, the ventricle fibers beat at a much slower rate (20-45 beats per min) and do not respond to any demands from the cardiovascular control center in the medulla oblongata. The atria however, are still responding to the medulla and will continue to beat at a normal pace (sinus rhythm)

Suppose a person had paroxysmal atrial tachycardia, producing "palpitations" Describe this condition, and propose ways that the condition might be treated

Paroxysmal atrial tachycardia is also known as supraventricular tachycardia in which an ectopic focus above the ventricles results in spontaneous rapid running of the heart (150-250 beats per min) that begins and ends abruptly. The ectopic focus extrasystoles often produce a subjective feeling in the chest, called palpitations. This condition is often treated successfully with digitalis. Digitalis decreases the excitability of the AV node and thus maintains the ventricular rate within the normal range, despite the excitability that may be occurring in the atria

Which ECG wave must occur before the ventricles can contract?

QRS

The pacemaker region of the heart is the ___________________

SA node

Compare supraventricular tachycardia with ventricular tachycardia in terms of its nature, ECG pattern, and seriousness

Supraventricular tachycardia is characterized by an abrupt rapid atrial beat that drives a rapid ventricular beat for short intervals for periods of time. The ECG is normal in appearance and the ventricles are able to pump blood during this brief condition. In ventricular tachycardia however, the ventricles are able to pump blood during this brief condition. In ventricular tachycardia however, the ventricles are not paced by the atria but rather by excitable ectopic forci in the ventricles that cuase the ECG to have widened and distorted QRS complexes that can mask normal P waves. Ventricular tachycardia is more serious because it can degenerate into ventricular fibrillation and because the ventricular pumping is very erratic

Which ECG wave must occur before the ventricles can relax?

T wave

The abnormally fast rte of beat is called ___________, an abnormally slow rate is called ____________

Tachycardia; bradycardia

What property makes the normal pacemaker region o the heart function as a pacemaker? Explain.

The SA node fibers serve as the pacemaker of the heart because they open special ion channels and undergo spontaneous depolarization faster than any other fibers in the heart. Consequently, the SA node sets the pace, generating action potentials that are rapidly conducted to other myocardial cells via intercalated discs (gap junctions) Although capable of spontaneous depolarization, other myocardial cells depolarize at a slower rate and thus normally follow the SA node rhythm. The depolarization cells of the atria respond first by contracting as a unit, followed shortly by the ventricles that similarly depolarize and contract. Furthermore, the SA node pace is influenced by autonomic neurotransmitters acetylcholine (slows) and norepinephrine (speeds), and by epinephrine(speeds)

Indicate the electrical events that produce each of these waves a) P wave b)QRS wave c)T wave

a) depolarization of atria b)depolarization of ventricles c)repolarization of ventricles

The conducting tissue of the heart located in the interventricular septum is the

bundle of his

Leads I,II,and III are collectively called the

standard limb