EKG Lab

Exercise 31 The Intrinsic Conduction System The ability of cardiac musele to beat is intrinsic it does not depend on impulses from the nervous system to initiate its con traction and will continue to contract rhythmically even if all nerve connections are severed. The intrinsic conduction system of the heart consists of cardiac pacemaker cells. The intrinsic conduction system ensures that heart muscle depolarizes in an orderly and sequential manner, from atria to ventricles and that the heart beats as a coordinated unit. The components of the intrinsic conduction system include the following (Figure 31.1): The sinontrial (SA) node, located in the right atrium just inferior to the entrance to the superior vena cava The atrioventricular (AV) node in the lower atrial septum at the junction of the atria and ventricles The AV bundle thundle of Hir and right and left bundle branches located in the interventricular septum Purkinje fibers The subendocardial conducting network, also called Note that the atria and ventricles are separated from one another by a region of electrically inert connective tissue, No the depolarization wave can he transmitted to the ventricles only via the tract between the AV node and AV bundle. Thus. any damage to the AV node-bundle pathway partially or totally insulates the ventricles from the influence of the SA node. Superior vena cava 1 The sinoatrial (SA) node (pacemaker) generates impulses. Internodal pathway Right atrium 2 The impulses pause (0.1 sec) at the atrioventricular (AV) node. ③ The atrioventricular (AV) bundle conducts the impulses to the bundle branchea. 4 The bundle branches conduct the impulses through the interventricular septum. The subendocardial conducting network depolarizes the contractile cells of both ventricles. Figure 31.1 The intrinsic conduction system of the heart. Impulses travel through the heart in order 1 to 5 following the yellow pathway. Dashed-line arrows indicate ransmission of the impulse from the SA node through the atria. Solid yellow arrow indicates ransmission of the impulse from the SA node to the AV node via the internodal pathway. Electrocardiography The conduction of impulses through the heart generales elec-trical currents that eventually spread throughout the body. These impulses can be detected on the body's surface and The graphic recording of the electrical changes occurring during the cardiac cycle is called an electrocardiogram (ECG or EKG) (Figure 31.2). A typical ECG has three recognizable waves. or deflections: the P wave, the ORS comples, and the T wave. For analysis, the ECG is divided into segments and intervals. A segment is a region between two waves. For example, the S-T segment is the region between the end of the S deflection and the start of the T wave. An interval is a region that contains a segment and one or more waves. For example, the Q-T interval includes the S-T segment as well as the QRS complex and the T wave. Boundaries for waves as well as some commonly mea-sured segments and intervals are described in Table 31.1. The deflection waves of an ECG correlate to the depolarization and repolarization of the heart's chambers (Figure 31.3.p. 452). (a) QRS complex Ventricular depolarization Ventricular repolarization Atrial depolarization (b) S-T S Segment P-R Q Interval Q-T Interval 0.2 0.6 0.8 Time (s) 0.4 Figure 31.2 The normal electrocardiogram. (a) Regular sinus rhythm. (b) Waves, segments, and intervals of a normal ECG. Table 31.1 Boundaries of Each ECG Component Feature P wave P-R interval ORS complex S-T segment Q-T interval Twave T-P segment R-R interval Boundaries Start of P deflection to return to baseline Start of P deflection to start of O deflection Start of O deflection to 5 return to baseline End of 5 deflection to start of T wave Start of O deflection to end of T wave Start of T deflection to return to baseline End of T wave to start of next P wave Peak of R wave to peak of next R wave Abnormalities of the deflection waves and changes in the time intervals of the ECG are useful in detecting myocardial infarcts (heart attacks) or problems with the conduction system of the heart. Table 31.2 summarizes some examples of abnormal elec-trocardiogram tracings and their possible clinical significance. A heart rate over 100 beats/min is referred to as tachycardia; a rate below 60 beats/min is bradycardia. Although neither con-dition is pathological, prolonged tachycardia may progress to fibrillation, a condition of rapid uncoordinated heart contrac-tions. Bradycardia in athletes is a positive finding: that is, it indi-cates an increased efficiency of cardiac functioning. Because stroke volume (the amount of blood ejected by a ventricle with each contraction) increases with physical conditioning, the heart can contract less often per minute and still meet circula-tory demands. Twelve standard leads are used to record an ECG for diagnostic purposes. Three of these are bipolar leads that measure the voltage difference between the arms, or an arm and a leg, and nine are unipolar leads. Together the 12 leads provide a fairly comprehensive picture of the electrical activ-ity of the heart. For this investigation, four electrodes are used (Figure 31.4, p. 452), and results are obtained from the three standard limb leads (also shown in Figure 31.4). Several types of physiographs Table 31.2 Examples of Abnormal ECGs and Possible Clinical Significance Finding Enlarged & wave Prolonged P-R interval Prolonged Q-T interval (when compared to the R-R interval) S-T segment elevated from baseline Possible clinical significance Enlarged ventricles. First-degree heart block. The signal from the SA node to the AV node is delayed longer than normal. Increased risk of ventricular arrhythmias. This interval corresponds to the beginning of ventricular depolarization through ventricular repolarization. Myocardial infarction (heart attack).