Laboratory 15 Electrocardiograph Lab (ECG) and Heart Sounds

Transcription

1 Laboratory 15 Electrocardiograph Lab (ECG) and Heart Sounds Objectives: Identify the waveforms in a normal ECG. Relate the waveforms to atrial and ventricular depolarization and repolarization and to the activity of the conduction system. Relate the ECG waveforms to the normal mechanical events of the cardiac cycle. Given the heart rate, calculate the length of one cardiac cycle. Predict how changes in heart rate (HR) and/or stroke volume (SV) will affect cardiac output. Discriminate between the two classic heart sounds and relate them to events in the ECG and cardiac cycle. 1. Heart Sounds: Most of us are familiar with a doctor or nurse using a stethoscope to listen to heart sounds. Several heart abnormalities, including heart mumers, can be diagnosed this way. What do you hear when you listen to the heart? Typically, you will hear two distinct sounds often referred to as Lub- Dub. The first heart sound (S1), or lub, is caused when the atrioventricular valves close, or the beginning of systole. If you listen carefully, you can distinguish two individual sounds within the first sound. This is due to the mitral valve closing just before the tricuspid valve closing. Closing of the semilunar valves cause the second heart sound (S2), or dub and occurs at the end of systole. The sound produced by the closing of each valve can be best heart, or auscultated, at different locations on the body. Figure 1 shows the areas that are optimal for listening to the sounds for each valve. Figure 1. Positions for maximal auscultation for each heart valve. Familiarize yourself with the parts of a stethoscope before using one for cardiac auscultation. Figure 2 shows a typical stethoscope. A stethoscope has a headpiece with ear tips that fit in your ears. The eartips are designed to be tilted forward into your ears. The other end, or chestpiece, has a wide diaphragm and a narrower bell. The diaphragm is usually used for listening to high pitched sounds like heart and lung sounds whereas the bell is used for lower frequency sounds like intestinal sounds. 47

2 However, the bell can be useful for hearing certain heart murmers. The bell and the diaphragm turn relative to each other to control which one is collection the sound waves. Activity: Obtain a stethoscope and clean the ear pieces using an alcohol wipe. Put the earpieces in your ear. Ensure that the diaphragm is open. Place the diaphragm on each of the four locations shown in figure 1. Listen for similarities and differences in the sound from each location. 2. ECG Overview: An electrocardiograph (ECG) is a graphic record of the electrical currents generated in and transmitted through the Figure 2. Parts of a Stethoscope. heart. It is actually a composite of the action potentials generated by all of the conductive cells of the heart at a given time. In a healthy heart the waves tend to be consistent in size and timing. Changes in the size and timing of the waves can be indicative of problems with the heart s conduction system. Caution: Remember that what we are doing in lab is an abbreviated 3 lead ECG as vs. the normal 12 leads used in the clinic so abnormalities observed in lab are NOT diagnostic. Methods have been developed to analyze the heart rate automatically, by reading the ECG signal trace and performing certain types of signal processing in order to extract the heart rate information. In analyzing the ECG, the most important parameter to focus on is the QRS complex. The ECG signal is split up in the literature into different waves, each with its own nomenclature. The QRS complex stands for the key portion of the ECG wave that involves the depolarization of the right and left ventricles. The QRS complex thus contains the peak of the pulse, and indicates that a heartbeat has occurred. QRS detection is the most important parameter in the determination of heart rate variability. Although not every QRS complex contains three separate Q, R and S waves, any conventional combination of these can be considered a QRS complex. However, in order to understand the ECG reading Figure 3. A typical ECG tracing. and analyze it, it is necessary 48

3 to label each part of the complex. Once they are identified, they can be employed to characterize heartbeats. Once the QRS has been detected, the location of the QRS in time can be annotated as a beat, and a sequence of beat annotation over time can be charted. A heart rate algorithm that takes the sequence of beat annotations, measures the time difference between beats to calculate a heart rate at every moment in time can then analyze this sequence of beat annotations. Now, with the necessary heart rate information extracted, basic thresholds can be put on a heart rate variability system in order to determine if the patient has exited a normal state. If the heart rate is below 60 bpm or above 100 bpm for a normal adult, an alarm can be raised to alert the medical personnel that the patient requires particular attention. 3. Overview of using AD Instruments PowerLab : Please read and follow these guidelines! Bioinformatics can be very frustrating to troubleshoot, but if used properly, our ADInstruments PowerLab kits represent incredibly powerful learning tools. To open the experiment: 1. Click the Windows logo in the bottom- left corner, then select All Programs 2. Open the folder ADInstruments LabTutor Student Login 3. This will automatically open LabTutor via Internet Explorer and prompt you for a User Name and Password. This will be provided by your instructor! 4. Select your course. Be sure to select both the appropriate course number (Biol 242) and section 5. You can see any labs you ve already started listed under In Progress. Find the heading Available, and select the experiment titled: TCC_BIOL ECG & Pulse. 6. Make sure the PowerLab is connected and turned on. At each lab bench, one student should drive (ie work the mouse and LabTutor interface) while one test subject just applies the electrodes and relaxes during the whole process. If you have more than two members at your bench, trade off driving so that everyone can familiarize themselves with the PowerLab equipment and LabTutor interface (you will use it again later this quarter!). Your PowerLab equipment should be set up, but will need to be attached to the subject, and then calibrated. Follow the LabTutor instructions to attach the pulse transducer and reusable clamp electrodes. TIP #1. Read ALL instructions! If ever you are unsure about how to obtain a trace, modify your trace, etc, make sure you have read all instructions and followed the blue links for more information before asking for your instructor or lab technicians for assistance. TIP #2. Instructions in the AD Instrument instructions are very literal. Follow them exactly. TIP #3. Often you can begin to go through the lab but find you cannot record. This is due to not having actually clicked on the start button. Go back to the first page of the lab in the Lab Tutor Software and click the Start button on the upper right of the graph. 49

4 4. Tips for Obtaining Good ECG data: TIP #1: During Ex. 1 you will measure your ECG and pulse at rest and examine aspects of their relationship. Be sure your traces in both channels are clear and are recognizable as ECG and Pulse, respectively, before proceeding to the analysis or report. If your trace contains artefacts (ie noise, interference) or your signal is weak or absent, adjust the electrodes and/or pulse transducer accordingly and take a new reading. TIP #2. When attaching the finger pulse transducer, be sure it is comfortably snug. Making it too tight will lead to a sloppy, noisy trace, while making it too loose will transmit a weak signal or none at all. Find the happy medium. TIP #3. When recording a trace, the subject should be seated and relaxed. Sometimes it helps for the subject to quietly read and avoid looking at the screen. Report: Depending on your section, the report can either be printed or ed to your instructor. Your instructor will let you know their preference. Attributions: Figure 1: Du, S., & Calderon, D. (2011, April 15). How ECG Signals Are Analyzed. Retrieved from the Connexions Web site: Figure 2: La Pierre, D. (2009, Aug). [Positions of maximal auscultation]. Sharinghealth.ca. Retrieved July 16, 2012 from Figure 3: Du, S., & Calderon, D. (2011, April 15). How ECG Signals Are Analyzed. Retrieved from the Connexions Web site: EKG text: EKG Overview: Adapted from ELEC 301 Projects Fall 2009 Collection by: Rice University ELEC 301. How ECG Signals Are Analyzed Module by: Sharon Du, Dan Calderon. Available through Creative Commons licensing from Connexions. 50

5 Laboratory 15 ECG and Heart Sounds Name Section 1. Describe the difference in heart sounds between the four locations shown in Figure In table 1, in the AD Instrument Data, what does the " time" value in Table 1 represent in terms of the circulatory system? 3. Today in lab, you measured an electrical signal (ECG) produced by the heart. In your own words, describe as completely as you can the structures and processes that produces the QRS complex in the ECG you measured today. 51

6 4. List the physiological processes that take place between the QRS complex and the arrival of the pulse wave at the finger. In other words, what happens to connect the electrical events of the QRS wave to the physical event of a pulse in your fingertip. 5. Why do heart rate and pulse pressure increase during exercise? Why does it increase during panic, or during a "fight or flight" response? 6. Do the measurements of each cycle fall within the normal duration values listed in Table 2? If not, explain why your measurements fall outside the "normal" range. 7. Describe what happens in the cardiac cycle in the following situations. a. Describe what happens in the cardiac cycle immediately before the P wave. b. Describe what happens in the cardiac cycle during the P wave. 52

7 c. Describe what happens in the cardiac cycle immediately after the P wave (P-R segment, not to be confused with P-R interval). d. Describe what happens in the cardiac cycle during the QRS wave. e. Describe what happens in the cardiac cycle immediately after the QRS wave (S-T segment). f. Describe what happens in the cardiac cycle during the T wave. 8. Given your mean Q-Q interval, determine your heart rate. How can you, given a heart rate, determine the length of one cardiac cycle (Q-Q interval)? 9. What is an easier method to calculate your heart rate? 10. How do increases or decreases in heart rate (HR) affect cardiac output? 53

8 11. Explain why ventricular contraction (systole) and the 'lub' sound occur immediately after the QRS complex. 12. Explain why ventricular relaxation (diastole) and the 'dub' sound occur after the T wave. 54