Lesson 5. ELECTROCARDIOGRAPHY I Components of the ECG

Transcription

1 Lesson 5 ELECTROCARDIOGRAPHY I Components of the ECG Computer # 29 Joe Smith Nick Jones Jill Johnson Kate Anderson Tuesday Lab Section September 20, 2007

2 Hypothesis The heart rate will be lowest when the subject is supine. The heart rate will increase when the subject sits up compared to supine. Post-exercise, the heart rate will be highest compared to seated and supine. In looking at the effect of respiration, the heart rate will increase during inhalation and the heart rate will decrease during expiration. This effect will be greater during deep breathing compared to regular breathing. Specific Aims We will learn how to record a three-lead electrocardiogram (ECG). We will record the ECG while our subject is supine, seated, and after exercise. We will compare the ECG trace of our subject during deep breathing relative to regular breathing, emphasizing the influence of inhalation and exhalation on heart rate. We will analyze the ECG traces to gain an understanding of the relationship between the electrical and mechanical events of the cardiac cycle. Background The heart consists of four chambers, two atria and two ventricles. [Briefly describe blood flow path in heart] The mechanical pumping of the heart depends on electrical stimulation that causes the cardiac muscles to contract. The electrical activity of the heart follows a regular pattern that defines the cardiac cycle. The electrical signal originates in the sinoatrial (SA) node [Describe the path that the electrical signal follows from the SA node to the Purkinje fibers] As action potentials move through the cells, the cells depolarize and contract. Once the cells have contracted, a repolarization signal follows the same pathway and causes the cells to relax (Widmaier 2006). The frequency of firing from the SA node, corresponding to a change in heart rate, can be influenced by several factors. [Briefly describe the effect of autonomic nervous system and the resting respiratory cycle on heart rate] An electrocardiogram (ECG) is used to record the electrical activity of the heart. In a three-lead ECG, electrodes are placed on the wrist and ankle to record the electrical signal. Echoes of the depolarization and repolarization of the heart are sent through the rest of the body, which can be detected by placing a pair of very sensitive receivers (electrodes) on other parts of the body (Pflanzer 2006).

3 An ECG trace has a distinct waveform pattern that correlates to particular electrical events in the heart. The P wave results from atrial depolarization [Describe the electrical event that corresponds to each wave]. Methods The experiment was performed as described in Lesson 5 of the Biopac Student Lab Manual (Pflanzer 2006). The three-lead electrode set (SS2L), three disposable electrodes, and the lab mat were used in this experiment. Our subject was [give the subject profile information]. We placed the white lead on the electrode on the right anterior forearm at the wrist, the red lead was placed on the electrode just above the left inner ankle, and the black lead was placed on the electrode on the medial surface of the right leg, just above the ankle as shown in Figure 1. right forearm WHITE lead right leg BLACK lead (ground) left leg RED lead Figure 1: Electrode lead configuration for lead II ECG. Reproduced from Pflanzer 2006 without permission. Our subject was supine on the lab mat and relaxed for 5 minutes prior to data collection. After calibration of the system, we began our experiment. There were 5 data conditions tested. The first four conditions each had an ECG recording of 20 seconds. The first condition tested was supine with regular breathing, a marker was inserted for each inhalation and exhalation. The second condition tested was [describe the other test conditions]. The subject did 30 push-ups in the lab room to elevate her heart rate. We then recorded the ECG of the subject for one minute post-exercise.

4 Results For the first test condition of supine with regular breathing, we analyzed the components of the ECG waveform as given in Table 1. The QT interval, corresponding to ventricular systole, and the T-R interval, corresponding to ventricular diastole, are given in Table 2. The heart rate for supine with regular breathing was measured in terms of duration and beats per minute () as given in Table 3, with an average heart rate of 77. For supine, deep breathing, the heart rate was found to increase to 84 during inhalation and decreased to 73 as shown in Table 4. The subject s heart rate increased after sitting up from a supine position. The average heart rate for seated regular breathing was found to be 82 (Table 5). When the subject breathed deeply, the heart rate increased to 87 during inspiration and decreased to 80 (Table 6). Table 1: Components of the ECG waveform for supine, regular breathing ECG Duration (seconds) Amplitude (mv) Component Cycle 1 Cycle 2 Cycle 3 Mean Cycle 1 Cycle 2 Cycle 3 Mean P wave PR interval PR segment QRS complex QT interval ST segment T wave Table 2: Ventricular systole and diastole for Supine, Resting, Regular Breathing Ventricular Readings Cycle 1 Cycle 2 Cycle 3 Mean QT Interval (corresponds to Ventricular Systole) 0.3 End of T wave to subsequent R wave (corresponds to Ventricular Diastole) 0.5

5 Table 3: Heart rate for Supine, Resting, Regular Breathing Cardiac Cycle Measurement Mean Range T (seconds) Table 4: Heart rate for Supine, Deep Breathing Rhythm Cycle 1 Cycle 2 Cycle 3 Mean Inspiration Expiration Table 5: Heart rate for Seated, Regular Breathing Heart Rate Cycle 1 Cycle 2 Cycle 3 Mean Table 6: Heart rate for Seated, deep breathing Rhythm Cycle 1 Cycle 2 Cycle 3 Mean Inspiration Expiration Following exercise, the subject s heart rate increased again to 125 (Table 7. Post-exercise, the QT interval, corresponding to ventricular systole, and the T-R interval, corresponding to ventricular diastole, were measured and are given in Table 8. Both

6 intervals showed a decrease in duration compared to the supine condition (verify in Tables 2 and 8). A summary of the conditions tested and the resulting heart rates is listed in Table 9. Table 7: Heart rate after exercise Heart Rate Cycle 1 Cycle 2 Cycle 3 Mean Table 8: Ventricular systole and diastole after exercise Ventricular Readings Cycle 1 Cycle 2 Cycle 3 Mean QT Interval (corresponds to Ventricular Systole) End of T wave to subsequent R wave (corresponds to Ventricular Diastole) Table 9 Data summary for heart rate Condition Supine, regular breathing Supine, deep breathing, inhalation Supine, deep breathing, exhalation Seated, regular breathing Seated, deep breathing, inhalation Seated, deep breathing, exhalation After exercise start of recording After exercise end of recording Mean () Range/ Std Dev () Discussion In this section, you should: (a) Discuss the results of your experiment. What do your results mean? Do they make sense and why? There should be some physiological concepts addressed in this part of your discussion. What would you expect to happen based on the experimental procedure? Here you should also compare your data trends to those presented for a "normal, healthy individual" in Vander/Widmaier.

7 For example: Heart rate was found to increase after exercise. During exercise, there is an increased demand for oxygen by the skeletal muscles. The flow of blood is increased to meet the oxygen requirements by an increase in heart rate (Widmaier 2006). (b) You should answer ALL of the questions at the end of each lesson in the BIOPAC manual. The questions should be answered in order and written in paragraph form. Do not format as Q&A. Some questions will fit easily into the discussion of your results, and some are an extension of the data collected and will need to be presented after you have discussed your results. Even if you addressed some of these questions in Background, you need to include them in your Discussion. For example: Each of our ECG traces had one P wave for every QRS complex. The P waves were bell-shaped and had a lower amplitude compared to T wave. The T wave also had a bell-shaped waveform. (c) What are some of the limitations of the study? What are some possible sources of error/errata? The errors and limitations should be relevant to what you observed in the lab and specific. For example: One source of error noted for the exercise data we collected was due to the fact that the electrode began to peel off of the subject s wrist after exercise. We had to delay recording to re-adhere the electrode with medical tape, which may have reduced the increase in heart rate we expected to observe. A limitation of our study was that we were only able to test one subject. This did not give us a large sample of data from which to base our conclusions. Conclusions Our hypothesis was correct. We found that heart rate was lowest when the subject was supine. The heart rate increased when the subject was seated compared to supine. Postexercise, the heart rate was highest compared to seated and supine. In looking at the effect of respiration, the heart rate was increased during inhalation and decreased during expiration. This effect was greater during deep breathing compared to regular breathing. References Iaizzo, P. (2005) General Features of the Cardiovascular System. From: Handbook of Cardiac Anatomy, Physiology, and Devices. Edited by P.A. Iaizzo. Humana Press Inc, Totowa, NJ. Pages Pflanzer, Richard, J.C. Uyehara, and William McMullen. (2006) Lesson 5: Electrocardiography I, Components of the ECG. Biopac Student Lab Manual. BIOPAC Systems, Inc., Santa Barbara, CA. p

8 Widmaier, E.P; H. Raff; and K.T. Strang. (2006) Vander s Human Physiology: the mechanisms of body function, Tenth Edition. McGraw-Hill, New York. p ,