Reference: Marieb, E. N., Mitchell, S. J., & Smith, L. A. (2013). Human anatomy & physiology laboratory manual (11th ed.). [Fetal Pig version]. San Francisco, CA: Pearson

Reference: Marieb, E. N., Mitchell, S. J., & Smith, L. A. (2013). Human anatomy & physiology laboratory manual (11th ed.). [Fetal Pig version]. San Francisco, CA: Pearson
Heart contraction results from a series of depolarization waves that travel through the heart preliminary to each beat. Because cardiac muscle cells are electrically connected by gap junctions, the entire myocardium behaves like a single unit, a functional syncytium. The Intrinsic Conduction System The ability of cardiac muscle to beat is intrinsic?it does not depend on impulses from the nervous system to initiate its contraction 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 sinoatrial (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 (bundle of His) and right and left bundle branches, located in the interventricular septum; and the subendocardial conducting network, also called Purkinje fibers (Figure 31.1). Note that the atria and ventricles are separated from one another by a region of electrically inert connective tissue, so the depolarization wave can be 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. Figure 31.1 The intrinsic conduction system of the heart. Impulses travel through the heart in order to following the yellow pathway. Dashed-line arrows indicate transmission of the impulse from the SA node through the atria. Solid yellow arrow indicates transmission of the impulse from the SA node to the AV node via the internoda Electrocardiography The conduction of impulses through the heart generates electrical currents that eventually spread throughout the body. These impulses can be detected on the body?s surface and recorded with an instrument called an electrocardiograph. 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 deflection waves: the P wave, the QRS complex, 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 measured 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. 462). 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. 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 Boundaries P wave Start of P deflection to return to baseline P-R interval Start of P deflection to start of Q deflection QRS complex Start of Q deflection to S return to baseline S-T segment End of S deflection to start of T wave Q-T interval Start of Q deflection to end of T wave T wave Start of T deflection to return to baseline T-P segment End of T wave to start of next P wave R-R interval Peak of R wave to peak of next R wave Table 31.2 summarizes some examples of abnormal electrocardiogram 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 condition is pathological, prolonged tachycardia may progress to fibrillation, a condition of rapid uncoordinated heart contractions. Bradycardia in athletes is a positive finding; that is, it indicates 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 more slowly and still meet circulatory 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 activity of the heart. For this investigation, four electrodes are used (Figure 31.4, p. 462), and results are obtained from the three standard limb leads (also shown in Figure 31.4). Several types of physiographs or ECG recorders are available. Your instructor will provide specific directions on how to set up and use the available apparatus if standard ECG apparatus is used (Activity?A). Instructions for use of BIOPAC?apparatus (Activity 1B) follow (pp. 464?468). Table 31.2 Examples of Abnormal ECGs and Possible Clinical Significance Finding Possible clinical significance Enlarged R wave Enlarged ventricles. Prolonged P-R interval First-degree heart block. The signal from the SA node to the AV node is delayed longer than normal. Prolonged Q-T interval (when compared to the R-R interval) Increased risk of ventricular arrhythmias. This interval corresponds to the beginning of ventricular depolarization through ventricular repolarization. S-T segment elevated from baseline Myocardial infarction (heart attack). Figure 31.3 The sequence of depolarization and repolarization of the heart related to the deflection waves of an ECG tracing. Figure 31.4 ECG recording positions for the standard limb leads. Prepare for lab: Watch the Pre-Lab Video >Study Area>Pre-Lab Videos Understanding the Standard Limb Leads As you might expect, electrical activity recorded by any lead depends on the location and orientation of the recording electrodes. Clinically, it is assumed that the heart lies in the center of a triangle with sides of equal lengths (Einthoven?s triangle) and that the recording connections are made at the corners of that triangle. But in practice, the electrodes connected to each arm and to the left leg are considered to connect to the triangle corners. The standard limb leads record the voltages generated between any two of the connections. A recording using lead I (RA-LA), which connects the right arm (RA) and the left arm (LA), is most sensitive to electrical activity spreading horizontally across the heart. Lead II (RA-LL) and lead III (LA-LL) record activity along the vertical axis (from the base of the heart to its apex) but from different orientations. The significance of Einthoven?s triangle is that the sum of the voltages of leads I and III equals that in lead II (Einthoven?s law). Hence, if the voltages of two of the standard leads are recorded, that of the third lead can be determined mathematically. Activity 1A Recording ECGs Using a Standard ECG?pparatus Preparing the Subject If using electrodes that require gel, place the gel on four electrode plates and position each electrode as follows after scrubbing the skin at the attachment site with an alcohol swab. Attach an electrode to the anterior surface of each forearm, about 5 to 8 cm (2 to 3?n.) above the wrist, and secure them with rubber straps. In the same manner, attach an electrode to each leg, approximately 5 to 8 cm above the ankle. Disposable electrodes may be placed directly on the subject in the same areas. Attach the appropriate tips of the patient cable to the electrodes. The cable leads are marked RA (right arm), LA (left arm), LL (left leg), and RL (right leg, the ground). Making a Baseline Recording Position the subject comfortably in a supine position on a cot, or sitting relaxed on a laboratory chair. Turn on the power switch, and adjust the sensitivity knob to 1. Set the paper speed to 25 mm/sec and the lead selector to the position corresponding to recording from lead I (RA-LA). Set the control knob at the RUN position and record the subject?s at-rest ECG from lead I for 2 to 3 minutes or until the recording stabilizes. The subject should try to relax and not move unnecessarily, because the skeletal muscle action potentials will also be recorded. Stop the recording and mark it ?lead I.? Repeat the recording procedure for leads II (RA-LL) and III (LA-LL). Each student should take a representative portion of one of the lead recordings and label the record with the name of the subject and the lead used. Identify and label the P, QRS, and T waves. The calculations you perform for your recording should be based on the following information: Because the paper speed was 25 mm/sec, each millimeter of paper corresponds to a time interval of 0.04 sec. Thus, if an interval requires 4 mm of paper, its duration is 4 mm ?0.04 sec/mm = 0.16 sec. Calculate the heart rate. Obtain a millimeter ruler and measure the R to R interval. Enter this value into the following equation to find the time for one heartbeat. ???????m/beat 3 0.04 sec/mm 5 ???????ec/beat Now find the beats per minute, or heart rate, by using the figure just calculated for seconds per beat in the following equation: 60 sec/min 4 (sec/beat) 5 beats/min Measure the QRS complex, and calculate its duration. ???????Measure the Q-T interval, and calculate its duration. ???????Measure the P-R interval, and calculate its duration. ???????Are the calculated values within normal limits? ???????At the bottom of this page, attach sections of the ECG recordings from leads I through III. Make sure you indicate the paper speed, lead, and subject?s name on each tracing. Also record the heart rate on the tracing. Why This Matter Atrial Fibrillation (AF) During atrial fibrillation, or AF, the atria spasm instead of contracting as a coordinated unit, which leads to pooling of blood in the atria. Atrial fibrillation is a result of damage to the intrinsic conduction system. During afib, the atria generate as many as 500 action potentials per minute, much faster than the usual 100 action potentials per minute generated by the SA node. Multiple signals flood the AV node, but it can?t repolarize fast enough to pass on all of these action potentials. The rate of contraction of the ventricles may increase or decrease, contributing to the irregular, but often rapid, heartbeat. The underlying cause of AF is usually other heart conditions, such as hypertension and coronary heart disease. ?Recording the ECG for Running in Place Make sure the electrodes are securely attached to prevent electrode movement while recording the ECG. Set the paper speed to 25 mm/sec, and prepare to make the recording using lead I. Record the ECG while the subject is running in place for 3 minutes. Then have the subject sit down, but continue to record the ECG for an additional 4 minutes. Mark the recording at the end of the 3 minutes of running and at 1 minute after cessation of activity. Stop the recording. Calculate the beats/min during the third minute of running, at 1 minute after exercise, and at 4 minutes after exercise. Record below: ???????beats/min while running in place ???????beats/min at 1 minute after exercise ???????beats/min at 4 minutes after exercise Compare this recording with the previous recording from lead I. Which intervals are shorter in the ?running? recording? ???????Does the subject?s heart rate return to resting level by 4 minutes after exercise? ???????Recording the ECG During Breath Holding Position the subject comfortably in the sitting position. Using lead I and a paper speed of 25 mm/sec, begin the recording. After approximately 10 seconds, instruct the subject to begin breath holding, and mark the record to indicate the onset of the 1-minute breath-holding interval. Stop the recording after 1 minute, and remind the subject to breathe. Calculate the beats/minute during the 1-minute experimental (breath-holding) period. Beats/min during breath holding: ???????Compare this recording with the lead I recording obtained under resting conditions. What differences do you see? ???????Attempt to explain the physiological reason for the differences you have seen. (Hint: A good place to start might be to check hypoventilation or the role of the respiratory system in acid-base balance of the blood.) ????????????????????????????Activity 1B Electrocardiography Using BIOPAC?In this activity, you will record the electrical activity of the heart under three different conditions: (1) while the subject is lying down, (2) after the subject sits up and breathes normally, and (3) after the subject has exercised and is breathing deeply. In order to obtain a clear ECG, it is important that the subject: Remain still during the recording. Refrain from laughing or talking during the recording. When in the sitting position, keep arms and legs steady and relaxed. Remove metal watches and bracelets. Setting Up the Equipment Connect the BIOPAC?unit to the computer and turn the computer ON. Make sure the BIOPAC?unit is OFF. Plug in the equipment (as shown in Figure 31.5 ): Figure 31.5 Setting up the BIOPAC?unit. Plug the electrode lead set into Channel 1. Leads are shown plugged into the MP36/35 unit. Electrode lead set?CH 1 Turn the BIOPAC?unit ON. Place the three electrodes on the subject (as shown in Figure 31.6 ), and attach the electrode leads according to the colors indicated. The electrodes should be placed on the medial surface of each leg, 5 to 8 cm (2 to 3 in.) superior to the ankle. The other electrode should be placed on the right anterior forearm 5 to 8 cm above the wrist. The subject should lie down and relax in a comfortable position with eyes closed. A chair or place to sit up should be available nearby. Start the Biopac Student Lab program on the computer by double-clicking the icon on the desktop or by following your instructor?s guidance. Select lesson L05-ECG-1 from the menu, and click OK. Type in a filename that will save this subject?s data on the computer hard drive. You may want to use the subject?s last name followed by ECG-1 (for example, SmithECG-1), then click OK. Because we are not recording all available lesson options, click the File Menu, choose Lesson Preferences, choose Heart Rate Data, and click OK. Choose Do Not Calculate and click OK. Click the file menu and choose Lesson Preferences again; select Lesson Segments and click OK; click the box for Deep Breathing to deselect it; and then click OK. Figure 31.6 Placement of electrodes and the appropriate attachment of electrode leads by color. Calibrating the Equipment Examine the electrodes and the electrode leads to be certain they are properly attached. The subject must remain supine, still, and relaxed. With the subject in a still position, click Calibrate. This will initiate the process whereby the computer will automatically establish parameters to record the data. The calibration procedure will stop automatically after 8 seconds. Observe the recording of the calibration data, which should look similar to the data example (Figure 31.7). If the data look very different, click Redo Calibration and repeat the steps above. If the data look similar, proceed to the next section. Don?t click Done until you have completed all 3 segments. Recording Segment 1: Subject Lying Down To prepare for the recording, remind the subject to remain still and relaxed while lying down. Click Continue and when prepared, click Record and gather data for 20 seconds. At the end of 20 seconds, click Suspend. Observe the data, which should look similar to the data example (Figure 31.8). If the data look very different, click Redo and repeat the steps above. Be certain to check attachment of the electrodes and leads, and remind the subject not to move, talk, or laugh. If the data look similar, move on to the next recording segment. Recording Segment 2: After Subject Sits Up, with Normal Breathing Tell the subject to be ready to sit up in the designated location. With the exception of breathing, the subject should try to remain motionless after assuming the seated position. If the subject moves too much during recording after sitting up, unwanted skeletal muscle artifacts will affect the recording. Figure 31.7 Example of calibration data. Click Continue and when prepared, instruct the subject to sit up. Immediately after the subject assumes a motionless state, click Record, and the data will begin recording. At the end of 20 seconds, click Suspend to stop recording. Observe the data, which should look similar to the data example (Figure 31.9). If the data look very different, have the subject lie down, then click Redo. Be certain to check attachment of the electrodes, then repeat steps 1?4 above. Do not click Record until the subject is motionless. If the data look similar, move on to the next recording segment. Recording Segment 3: After Subject Exercises, with?eep Breathing Click Continue, but don?t click Record until after the subject has exercised. Remove the electrode pinch connectors from the electrodes on the subject. Have the subject do a brief round of exercise, such as jumping jacks or running in place for 1 minute, in order to elevate the heart rate. As quickly as possible after the exercise, have the subject resume a motionless, seated position. Reattach the pinch connectors. Once again, if the subject moves too much during recording, unwanted skeletal muscle artifacts will affect the data. After exercise, the subject is likely to be breathing deeply but otherwise should remain as still as possible. Figure 31.8 Example of ECG data while the subject is lying down. Figure 31.9 Example of ECG data after the subject sits up and breathes normally. Immediately after the subject assumes a motionless, seated state, click Record, and the data will begin recording. Record the ECG for 60 seconds in order to observe post-exercise recovery. After 60 seconds, click Suspend to stop recording. Observe the data, which should look similar to the data example (Figure 31.10). If the data look very different, click Redo and repeat the steps above. Be certain to check attachment of the electrodes and leads, and remember not to click Record until the subject is motionless. When finished, click Done and then Yes. Remove the electrodes from the subject. A pop-up window will appear. To record from another subject, select Record from another subject and return to step 5 under Setting Up the Equipment. If continuing to the Data Analysis section, select Analyze current data file and proceed to step 2 of the Data Analysis section. Data Analysis If just starting the BIOPAC?program to perform data analysis, enter Review Saved Data mode and choose the file with the subject?s ECG data (for example, SmithECG-1). Use the following tools to adjust the data in order to clearly view and analyze four consecutive cardiac cycles: Click the magnifying glass (near the I-beam cursor box) to activate the zoom function. Use the magnifying glass cursor to click on the very first waveforms until there are about 4 seconds of data represented (see horizontal time scale at the bottom of the screen). Select the Display menu at the top of the screen, and click Autoscale Waveforms (or click Ctrl Y). This function will adjust the data for better viewing. Click the Adjust Baseline button. Two new buttons will appear; simply click these buttons to move the waveforms Up or Down so they appear clearly in the center of the display window. Once they are centered, click Exit. Figure 31.10 Example of ECG data after the subject exercises. Note that the first two pairs of channel/measurement boxes at the top of the screen are set to Delta T and bpm. Channel Measurement Data CH 1 Delta T ECG CH 1 bpm ECG ???????Analysis of Segment 1: Subject Lying Down Use the arrow cursor and click the I-beam cursor box for the ?area selection? function. First measure Delta T and bpm in Segment 1 (approximately seconds 0?20). Using the I-beam cursor, highlight from the peak of one R wave to the peak of the next R wave (as shown in Figure 31.11 ). Observe that the computer automatically calculates the Delta T and bpm for the selected area. These measurements represent the following: Delta T (difference in time): Computes the elapsed time between the beginning and end of the highlighted area bpm (beats per minute): Computes the beats per minute when the area from the R wave of one cycle to the R wave of another cycle is highlighted Record these data in the Segment 1 Samples chart under R to R Sample 1 (round to the nearest 0.01 second and 0.1 beat per minute). Using the I-beam cursor, highlight two other pairs of R to R areas in this segment. Record the data in the same chart under Samples 2 and 3. ???????Calculate the means of the data in this chart. Next, use the zoom, Autoscale Waveforms, and Adjust Baseline tools described in step 2 to focus in on one ECG waveform within Segment 1. (See the example in Figure 31.12 ). Once a single ECG waveform is centered for analysis, click the I-beam cursor box to activate the ?area selection? function. Using the highlighting function and Delta T computation, measure the duration of every component of the ECG waveform. (Refer to Figure 31.2b and Table 31.1 for guidance in highlighting each component.) Figure 31.11 Example of highlighting from R wave to R wave. Figure 31.12 Example of a single ECG waveform with the first part of the P wave highlighted. Segment 1 Samples for Delta T and bpm Measure Channel R to R Sample 1 R to R Sample 2 R to R Sample 3 Mean Delta T CH 1 ????bpm CH 1 ????Highlight each component of one cycle. Observe the elapsed time, and record this data under Cycle 1 in the Segment 1 Elapsed Time chart. Scroll along the horizontal axis at the bottom of the data to view and analyze two additional cycles in Segment 1. Record the elapsed time for every component of Cycle 2 and Cycle 3 in the Segment 1 Elapsed Time chart. In the same chart, calculate the means for the three cycles of data and record. ???????Analysis of Segment 2: Subject Sitting Up and Breathing Normally Scroll along the horizontal time bar until you reach the data for Segment 2 (approximately seconds 20?40). A marker with ?Seated? should denote the beginning of this data. As in the analysis of Segment 1, use the I-beam tool to highlight and measure the Delta T and bpm between three different pairs of R waves in this segment, and record the data in the Segment 2 Samples chart on p. 468. Analysis of Segment 3: After Exercise with Deep Breathing Scroll along the horizontal time bar until you reach the data for Segment 3 (approximately seconds 40?100). A marker with ?After exercise? should denote the beginning of this data. As before, use the I-beam tool to highlight and measure the Delta T and bpm between three pairs of R waves in this segment, and record the data in the Segment 3 Samples chart on p. 468. Component Cycle 1 Cycle 2 Cycle 3 Mean P wave ????P-R interval ????QRS complex ????S-T segment ????Q-T interval ????T wave ????T-P segment ????R-R interval ????Using the instructions for steps 7?9 in the section Analysis of Segment 1, highlight and observe the elapsed time for each component of one cycle, and record these data under Cycle 1 in the Segment 3 Elapsed Time chart on p. 468. When finished, Exit from the file menu to quit. Compare the average Delta T times and average bpm between the data in Segment 1 (lying down) and the data in Segment 3 (after exercise). Which is greater in each case? ??????????????Segment 2 Samples for Delta T and bpm Measure Channel R to R Sample 1 R to R Sample 2 R to R Sample 3 Mean Delta T CH 1 ????bpm CH 1 ????Segment 3 Samples for Delta T and bpm Measure Channel R to R Sample 1 R to R Sample 2 R to R Sample 3 Mean Delta T CH 1 ????bpm CH 1 ????What is the relationship between the elapsed time (Delta?) between R waves and the heart rate? ???????What event does the period between R waves correspond to? ???????Is there a change in heart rate when the subject makes the transition from lying down (Segment 1) to a sitting position (Segment 2)? ???????Examine the average duration of each of the ECG components in Segment 1 and the data in Segment 3. In the Average Duration chart, record the average values for Segment 1 and the data for Segment 3. Draw a circle around those measures that fit within the normal range. Compare the Q-T intervals in the data while the subject is at rest versus after exercise; this interval corresponds closely to the duration of contraction of the ventricles. Describe and explain any difference. Segment 3 Elapsed Time for ECG Components (seconds) Component Cycle 1 P wave ?P-R interval ?QRS complex ?S-T segment ?Q-T interval ?T wave ?T-P segment ?R-R interval ???????????????Compare the duration in the period from the end of each T wave to the next P wave while the subject is at rest versus after exercise. Describe and explain any difference. ??????????????A patient presents with a P-R interval three times longer than the normal duration. What might be the cause of this abnormality? ??????????????Average Duration for ECG Components ECG component Normal duration (seconds) Segment 1 (lying down) Segment 3 (post-exercise) P wave 0.07?0.18 ??P-R interval 0.12?0.20 ??QRS complex 0.06?0.12 ??S-T segment <0.20 ??Q-T interval 0.32?0.38 ??T wave 0.10?0.25 ??T-P segment 0?0.40 ??R-R interval varies ? ?l pathway.

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