Conducting System of the Heart and Electrocardiogram

Transcription

1 Lab 6 Conducting System of the Heart and The next two weeks will involve an in-depth study of the Cardiovascular system. This week we will focus on the electrical conducting system within the heart and how EKGs record this electrical activity. Next week we will record an EKG in conjunction with blood pressure and pulse so that you can directly observe the relationship between the electrical and mechanical events that contribute to blood circulating through your body. The human heart is autorhythmic, since it will continue to beat if removed from the body (Think of Indiana Jones and the Temple of Doom!). Heart contractions are, therefore, not dependent upon the brain, rather the rhythm comes from within the heart itself. The heart is composed almost entirely of large, strong muscle fibers (myocardium), which are responsible for the pumping action of the heart. Other cardiac muscle cells are weakly contractile and produce or conduct the rhythm for the rest of the heart. A group of these weak muscle cells is located in the sinoatrial (SA) node and acts as the pacemaker for the heart. These cells rhythmically produce action potentials, which spread via gap junctions to fibers of both atria. The resulting contraction pushes blood into the ventricles. While adjacent atrial fibers are connected by gap junctions, the only electrical connection between the atria and the ventricles is via the atrioventricular (AV) node. The action potential spreads slowly through the AV node and then rapidly through the Bundle of His, the Bundle Branches and Purkinje fibers to excite both ventricles leading to their contraction. Figure 1 In this lab your will record EKG waves which are direct recordings of the electrical activity within the heart. These waves will be recorded on EKG paper. Objectives: 1) To read and interpret EKG wave forms plotted onto EKG paper. 2) To recognize the most common clinical conditions associated to failure of the conduction system of the heart based on an EKG. 1

2 Interpreting the EKG. Each component of the EKG reflects depolarization or repolarization of a portion of the heart. (Depolarization precedes contraction. Repolarization precedes relaxation). Waves: deflections above or below baseline Segments: sections of baseline between two waves Intervals: combinations of waves and segments Baseline---- Figure 2 P WAVE immediately precedes atrial contraction (systole) QRS COMPLEX immediately precedes ventricular contraction (systole) T WAVE immediately precedes ventricular relaxation (diastole). P-R INTERVAL represents the time between the onset of atrial depolarization and the onset of ventricular depolarization. Q-T INTERVAL represents the time between the onset of ventricular depolarization and the end the ventricular repolarization. T-P INTERVAL (not labeled) represents the time between each heart electrical cycle! It is important because it is the only interval that should vary in a normal person! There is no distinctly visible wave representing atrial repolarization in the ECG because it occurs during ventricular depolarization. Because the wave of atrial repolarization is relatively small in amplitude (i.e., has low voltage), it is masked by the much larger ventricular-generated QRS complex Activity 1: EKG at rest One person from each group should be chosen as the subject. This person will also be the subject for Activity 2: EKG after exercise. Your group should work together to make calculations from the EKG paper. Make sure that each person understands how to read and interpret the EKG waveform on the EKG paper. This is important for your midterm exam; but more so it will be important for some of you that go into the Cardio-area of Health Care. The EKG will be recorded using the machine at the front of the room. Only one person can be recorded from at a time and recording takes 1-2 minutes.so PLEASE PLEASE PLEASE be patient. 2

3 Procedure: Remove all metal jewelry; it could interfere with the recording. You must be VERY STILL during recording. EKG are electrical signals and so are EMGs..you don t want the EKG to record motor signals to skeletal muscle.only electrical signals to cardiac muscle. We will use the 3-lead technique to record EKG. Clinically you would use a 12-lead technique which would be MUCH higher resolution and can be used to diagnose many more issues with the cardiac electrical system. You will place a lead on the left and right clavicle and a lead on the left lower leg. Be SURE to clean the area with an alcohol swab (to remove skin oils) and then rough the area with a paper towel (skin should be slightly red). EKGs leads are very sensitive and require excellent contact between the skin and the electrode gel. The lead should be new enough that you DO NOT need to add additional gel. Clip the RA, LA, and LL lead to the tab on the EKG leads. Record for 1-2 minutes and take the print out back to your lab bench. Do the following activities using your group s EKG records: On EKG paper the boxes and dots mean the following: 1 big box = 200 ms = 0.2 s 1 dot (small box) = 40 ms = 0.04 s; There are 5 dots within a box! 5 BOXES = 1 second 1. Label P, QRS and T waves on a representative tracing 2. Compute average heart rate in beats/min using R-R interval. Normal resting rate = bpm. Measure R-R interval (number of boxes from PEAK TO PEAK) Compute seconds/beat by converting number of boxes into seconds. (Each small box = 0.04s. Each large box = 0.2s). Use the sec/beat value that you just calculated and plug it into the formula below to find the Beats/min. You should calculate EACH R-R interval and find the beats/min for several heart beat cycles (at least 5) and average those values to find the AVERAGE HEART RATE! 3. Measure the P-R interval, and compute its duration. (Normal duration is about sec.). PR is from the beginning of P to the beginning of the Q deflection (refer to Fig. 2). 4. Measure the QRS-complex duration from the beginning of the Q deflection to the end of the S deflection (Normal duration is sec.) 3

4 Measure duration on horizontal axis Each small box = 0.04 s (40 ms) Each large box = 0.2s (200 ms) Five large boxes = 1 sec 5. Measure the Q-T interval. From the beginning of the Q deflection to the end of the T wave. Normally this should not be MORE than ½ of the RR interval (i.e. ventricular Depolarization/Repolarization should not take more than ½ of a full cardiac cycle!) 6. Measure the T-P interval duration. From the end of T to the beginning of P (Normal duration VARIES with heart rate!!!!). T-P interval is the time BETWEEN successive electrical cardiac cycles. It is the only number that should change. This is so because the electrical events WITHIN a cardiac cycle are precisely timed and fixed at those precise times (from P to T). Therefore the ONLY way to change heart rate is to alter the TP interval, that is the amount of time between heart beat! Activity 2: EKG after exercise For each group, the person that was the subject for Activity 1: EKG at rest should also be the subject for this activity. Instruct the subject to exercise vigorously (for example, to go down the stair and come back to the room). As soon as the subject finishes the exercise as the subject to sit down, place the electrodes as quick as possible, and obtain an EKG. If the subject is at rest by the time you start your recording you will have to repeat the experiment. Follow the procedure described under Experiment 1 to obtain your EKG trace and to measure the waves on your EKG. When you are finished with the measurements of your resting and exercise EKGs, obtain copies of the EKGs from the instructor. Determine what parameters stay the same and which ones change. 4

5 Normal Parameters Associated with EKGs Figure 3 Normal Parameters Summary Heart rate = beats per minute (bpm) P-R Interval = sec QRS-complex duration = sec Q-T Interval = no more than ½ R-R Interval EKG Abnormalities Irregular Heart Rates Normal heart rates fall between bpm (indicated SA-node depolarizations each minute). A heart rate that is slower than this range (i.e. longer R-R interval) is called sinus bradycardia. A heart rate that is faster than this range (i.e. shorter R-R interval) is called sinus tachycardia. Heart or AV Blocks The P wave represents the wave of depolarization that spreads from the SA node throughout the atria, and is usually 0.08 to 0.1 seconds ( ms) in duration. The brief isoelectric (zero voltage) period after the P wave represents the time in which the impulse is traveling within the AV node where the conduction velocity is greatly retarded. The P-R interval represents the time between atrial depolarization and the onset of ventricular depolarization, which normally ranges from 0.12 to 0.20 seconds in duration. An abnormality in the P-R interval indicates a problem with conduction of the depolarization from the SA-node to the AV-node via the intermodal pathways. There are three different degrees (types) of heart/av blocks, presented in increasing order of severity. First degree (partial) heart block A P-R interval >0.2 sec but with all EKG elements present each cycle (P-wave, QRS complex, and T-wave) indicates a blockage in the intermodal pathways that is slowing the conduction of the depolarization from the SA-node to the AV-node. 5

6 Second degree (partial) heart block This is indicated in the EKG by a QRS complex occurring only after every other P-wave. In other words, it takes two P-waves (two SA-node depolarizations) to sufficiently excite the AV-node to trigger ventricular contractions. Third degree (full) heart block This is indicated in the EKG by a lack of temporal relationship between the P-wave and the QRS-complex. The blockage is so severe that the SA-node depolarization never reaches the AV-node, which results in uncoordinated contractions between the atria and ventricles. Instead, the atrial and ventricular depolarizations are independently controlled by their individual pacemakers (the SA-node for the atria and the AV-node for the ventricles). Bundle Branch Blocks and Premature Ventricular Contraction (PVC) The AV-node (ventricular) depolarization rapidly spreads via the Bundle of His, Bundle branches and the Purkinje fibers (see Fig. 1) to contract both ventricles. The duration of QRS complex, which represents this ventricular depolarization, is normally 0.06 to 0.1 seconds. This relatively short duration indicates that ventricular depolarization normally occurs very rapidly. A prolonged QRS complex duration (> 0.1 sec) indicates a ventricular problem. One possibility is that the conduction of the AV-node depolarization is impaired by a blockage in the Bundle of His-Bundle branch-purkinje fiber pathway. This is called a bundle branch block. The bundle branch block slows the normally fast conduction of the ventricular depolarization, which extends the duration of the QRS complex. Another possibility is that an abnormal pacemaker site (meaning one that is not located within the SA-node) is active within the ventricles. This abnormal pacemaker is called an ectopic foci (see Fig. 4). A depolarization from an ectopic foci does not travel along the fast Bundle of His-Bundle branch-purkinje fiber pathway. This slower conduction of the depolarization extends the duration of the QRS complex. A prolonged QRS complex caused by an ectopic foci could also cause a heart arrhythmia with extra QRS complexes, with a normal QRS complex from a AV-node-generated ventricular depolarization and a prolonged QRS complex generated by the ectopic foci. NOTE: a QRS complex generated by an ectopic foci will not be preceded by a P-wave! This condition is called a premature ventricular contraction (PVC). Figure 4 6

7 Myocardial Infarction A myocardial infarction (heart attack) occurs when a coronary artery becomes occluded by a blood clot, causing at least some of the cardiac muscle to die. There are two types of myocardial infarctions. The more sever type of heart attack is the ST segment elevation myocardial infarction (STEMI). This occurs when the coronary artery is completely blocked off and large portions of the cardiac muscle begin to die. This problem is indicated on an EKG by an elevated ST segment following the QRS complex (which is normally isoelectric) (see Fig 7 below). A less severe (but still want to avoid) myocardial infarction is called a non-st segment elevation myocardial infarction (NSTEMI). In NSTEMI, the blood clot only partly occludes the artery and a smaller portion of the cardiac muscle begins to die. NSTEMI lacks the elevated ST segment. Instead, the ST segment of the EKG is either depressed or the T-wave is inverted (see Fig. 5). 5). Figure 5 7

8 VENTRICULAR FIBRILLATION (Fig. 6) In ventricular fibrillation, waves of depolarization travel in multiple directions all over the ventricular muscle. As a result, there is no cardiac output. Without prompt intervention (i.e. defibrillation via the paddles ), this arrhythmia is usually fatal. Figure 6 ATRIAL FIBRILLATION (Fig. 7) In atrial fibrillation, the two atria quiver rather than beat effectively, so the blood is not completely pumped out of them. The result is that the blood may pool or clot. If one of these clots leaves the heart and becomes lodged in an artery in the brain, a stroke results. Atrial fibrillation is the cause of about 15% of strokes. Figure 7 Activity 3: Abnormal EKG Based on the EKG abnormalities described above identify the abnormal patterns on the EKGs provided by your instructor. You can do this while waiting to use the EKG machine. 8