ECG

Student
Professor
Course
27th February, 2017
An Analysis of Electrocardiographic and Phonocardiography Recordings in Student Volunteers
Introduction
Electrocardiogram refers to the graphical representation of electrical potential across the heart. On the other hand, electrocardiography is the instrument with which an electrocardiogram is obtained (Moyer 2012). Since our body is a volume conductor, the electrical potentials generated in the myocardium (cardiac musculature) is reflected across different parts of the body. The electric potentials are recorded from different parts of body with the help of 12-lead system. A lead is a combination of two electrodes that is used to measure electrical potential across two different parts of a body. The 12-lead system that is used to obtain an electrocardiogram of an individual include, three standard leads, three augmented unipolar leads, and six precordial chest leads. An electrocardiogram reflects different waves and intervals. The waves and duration of an ECG represent different myocardial events. Such events are related to the transmission of electrical impulse over the myocardium and the resultant reaction of the myocardium to such impulse. However, the waves of an ECG represent electrical activities in the myocardium. Different types of diagnostic and prognostic features are aligned with the interpretation of electrocardiograms. On the other hand, ECGs significantly differ across healthy and diseased individuals and between athletes and non-athletes (Exeter et al.

2014).
The peacemaking activity of the SA (sinoatrial) and AV (atrioventricular) nodes form the physiological basis of an electrocardiogram. The impulse generated in the SA node spreads over the atrium and reaches the ventricles or the AV node through different intermodal fibers. The auricles start to depolarize under such impulse. On the other hand, the cardiac impulse reaching the AV node is carried through the Bundle of His and Purkinje fibers to get distributed over the ventricular myocardium. As a result, the wave of depolarization spreads over the ventricular myocardium from the apex to its base. Once the ventricular myocardium is depolarized, it undergoes contraction, which forms the basis of QRS complex of the ECG. The amplitude of different waves in an ECG reflects the volume of myocardium or the amount of depolarization in the respective cardiac musculature (Moyer 2012).
A standard electrocardiogram in a healthy individual comprises of four waves; P wave, QRS complex, T wave, and U wave. The P-wave signifies atrial contraction, while the QRS complex signifies ventricular contraction. On the contrary, the T wave signifies ventricular repolarization while the Q-wave signifies repolarization of the interventricular septum. However, the Q-wave is often inconspicuous and is absent in different individuals. Although T-wave denotes ventricular repolarization, however, it exhibits positive amplitude. This is because the apex of the heart repolarizes earlier than the base of the heart. Hence, when the apex of the heart starts to depolarize, the base of the heart is still depolarized. Hence, although the net depolarization of the ventricular myocardium is higher its repolarization, the amplitude of the T-wave is always positive. Since the wave of atrial repolarization is submerged within the QRS, such waves are not exhibited in an ECG. On the other hand, the distance between the different waves of an ECG signifies conduction abnormalities. For example, when the distance between P wave and R wave increases beyond 120ms, it signifies heart block. The distance between different waves of an ECG is referred as “intervals” (Moyer, 2012).
The present article represents an interpretive analysis of electrocardiograms that were recorded in student volunteers. The article explored one primary research question “whether the amplitude and duration of different waves in an ECG vary from person to person. The null hypothesis (Ho) contended that the amplitude and duration of different waves in an ECG do not vary from person to person. On the other hand, the alternative hypothesis contended that the amplitude and duration of different waves in an ECG vary from person to person.
Materials and Method
Study Design and Sampling
The present experiment was based on convenience sampling and quantitative analysis. The experiment was divided into four sections; interpretation of a standard ECG in an individual at rest, interpretation of three ECGs obtained from three different individuals for exploring the variations in ECG across different individuals, interpretation of ECG in the perspective of heart sounds, and assessment of ECG in alignment with phonocardiography. Three individuals were randomly selected for conducting the electrocardiographic and phonocardiogram recordings.
Procedure
The electrocardiograms and phonocardiography was conducted with standardized procedures. The amplitude, duration, and intervals of different waves in the electrocardiograms were analyzed. The analysis was based on the existing literature on different waves in electrocardiographic recordings of healthy and diseased individuals. Moreover, the waves of the ECG were also explored in relation to the status of exercise training in respective individuals.
Data Analysis and Interpretation
The data analysis included comparison of electrocardiographic recordings between the different study participants. On the other hand, the electrocardiographic and phonocardiography recordings of the study participants were also compared with the standard electrocardiograms of healthy and diseased individuals.
Results
434340203835ECG at rest
ECG Amplitude and Durations P wave 0.061 mV 0.08 seconds
QRS complex 0.025 mV 0.085 seconds
T wave 0.149 mV 0.14 seconds
ECG intervals (R-R intervals) and Heart rate 1st pair 1.08 seconds 55.6 beats/minute
2nd pair 1.085 seconds 55.1 beats/minute
3rd pair 0.929 seconds 64.6 beats/minute
ECG and Variations
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Discussion and Conclusion
The cardiac cycle represents the atrial and ventricular events that occur during one heartbeat. The events in a cardiac cycle are categorized as the duration of a respective event. The total duration of cardiac cycle in a healthy adult is 0.8 sec. During a cardiac cycle, the atria and ventricles contract and relax in an alternate manner. The duration of atrial systole is 0.01 seconds, while the duration for atrial diastole is 0.07 seconds. Likewise, the duration of ventricular systole is 0.03 seconds, while the duration for ventricular diastole is 0.07 seconds. Hence, the duration of ventricular events synchronizes with the duration of atrial events. The amplitude of different waves in an electrocardiographic recording of a healthy individual reflects the series of events in the myocardium. The ‘P’ wave represents atrial depolarization and its amplitude is usually less than 2.5mV.
In the present case study, the ‘P’ wave measured 0.061 mV in amplitude. Hence, it can be inferred that the mass of atrial myocardium was within normal limits and there were no signs of atrial hypertrophy. The ‘QRS’ wave represents ventricular depolarization and its amplitude is usually less than 0.12mV to 0.26mV (as recorded by different leads in an ECG) of a healthy adult individual. In the present case study, the ‘P’ wave measured 0.025 mV in amplitude. Hence, it can be inferred that the mass of the ventricular myocardium was within normal limits and there were no signs of ventricular hypertrophy. Finally, the ‘T’ wave represents ventricular repolarization and its amplitude varies between 0.1mV to 0.2mV in different leads of an ECG. In the present case study, the ‘T’ wave measured 0.149mV in amplitude. Such findings suggest that ventricular repolarization was initiated in an appropriate portion of the myocardium. These assumptions are aligned with the observations of P wave and QRS complex. This is because auricular or ventricular hypertrophies could have increased the magnitude of ‘T’ wave beyond 0.2 mV.
The QRS complex is usually larger because the mass of the ventricular myocardium is always higher than the mass of the auricular myocardium. Since the amount of myocardial tissue is greater in the ventricles, the cardiac impulse takes time to spread over the ventricular musculature. As a result, the amount of depolarization in the ventricular musculature is significantly higher than the amount of depolarization in the atrial musculature. Hence, the QRS complex is larger than the P wave. The R-R interval in an ECG depicts the heart rate of an individual. In the present case study, the R-R interval varied in successive heart beats. Hence, the respective individual exhibited variability in heart rate. A mild variability in heart rate is acceptable across normal healthy individuals.
The variability in heart rate stems from the psychological and physiological status of an individual. For example, an increased activation of the sympathetic nervous system leads to increased heart rate (Hill et al. 2011). On the other hand, a postural change of an individual from the supine to standing position also increases one’s heart rate. Such fluctuations are aligned to ensure the homeostasis of cardiac output. For example, a postural change of an individual from the supine to standing position reduces the blood pressure in the respective individual. A reduction in blood pressure could be attributed to reduced stroke volume that occurs due to venous pooling. On the other hand, blood pressure is a function of cardiac output and peripheral resistance. Likewise, cardiac output is a product of stroke volume and heart rate. To ensure auto regulation of blood flow or cardiac output the heart rate increases. To recall, cardiac output is a product of stroke volume and heart rate. Increased heart rate is confirmed by a reduction in the duration of the R-R interval and vice versa.
Although the resting heart rate in a non-athletic and healthy individual vary between 60 and 90 beats per minute; however; athletes exhibit a much lower heart rate. A reduction in heart rate below 60 beats minute is referred as bradycardia, and when such heart rate is observed across athletes, it is referred as “Athlete’s bradycardia.” Athletes Bradycardia is a physiological compensatory mechanism that stems from an increase in ventricular volume (Corrado et al. 2008). An increase in ventricular volume is referred as ventricular hypertrophy. Athletes exhibit ventricular hypertrophy in their right ventricles. Such adaptations are attributed to the cardiovascular changes that accompany aerobic training.
Exercise training improves venous return and end-diastolic volume in the respective individual. As a result, the volume of the right ventricle significantly higher in athletes compared to their non-athletic counterparts. Hence, athletes exhibit more ventricular filling compared to their non-athletic counterparts. Therefore, to meet the physiological and metabolic demands of aerobic exercise, an athlete can achieve a desired cardiac output by increasing their stroke volume compared to their non-athletic counterparts (Drezner et al. 2012). Hence, athletes do not have to increase their heart rate to achieve the desired cardiac output unlike their non-athletic counterparts. To recall, cardiac output is a product of stroke volume and heart rate. Hence, a low heart rate in athletes promote cardiac filling and ensures higher stroke volume compared to their non-athletic counterparts. Additionally, exercise training leads to cholinergic stimulation of the SA (sino-atrial) node. To recall, SA node is referred as the pacemaker cells of the myocardium. Cholinergic stimulation leads to increased secretion of acetylcholine from the post-ganglionic nerve fibers those impinge on the SA node. Post-ganglionic cholinergic fibers are inhibitory in action. As a result, the peacemaking activity or heart rate is lowered in athletes compared to their counterparts who do not undergo aerobic training (Brosnan et al. 2014).
The present experiment indicated that the amplitude and duration of different waves of a ECG significantly vary between individuals. For example, the amplitude of the P wave was almost double in Danna compared to amplitude of P wave in Josh. Likewise the duration of P wave was lowest in Van compared to Josh and Danna. Hence, Danna exhibited atrial hypertrophy compared to Josh. On the contrary, Van exhibited lower duration in P wave. Such findings confirm that the atrial musculature of Van depolarized earlier than josh or Danna. Likewise, the amplitude of R-wave significantly differed between Danna and Van. Such findings suggest that the amount of ventricular musculature in Van was higher than Danna or Josh. Hence, Van exhibited ventricular hypertrophy higher than Josh and Danna. However, the duration of QRS complex was lowest in Danna. Such findings confirm that the ventricular myocardium of Danna depolarized much earlier than the ventricular myocardium of Josh or Van. The amplitude of T-wave was higher in Josh compared to Van compared to others. However, the duration of T-wave was significantly lower in Danna compared to Josh and Van. Hence, ventricular musculature in Danna exhibited early repolarization compared to Josh and Van. The variations in heart rate cannot be inferred from the experimental data. This is because the experimental data did not reflect the duration of R-R intervals.
Heart sounds are primarily caused due the closure and opening of the atrioventricular and semilunar valves. The “lub” sound immediately occurs following QRS complex because it marks the end of ventricular contraction and the opening of the semilunar valves. On the other hand, the ventricular diastole occurs and the “dub” sound is heard after the T wave because T wave marks the start of the ventricular repolarization. To recall, ventricular repolarization marks the initiation of ventricular diastole. Although the heart sounds are synchronized between each cardiac cycle, however; it might be subjected to temporal variations. Such variations are attributed to the spread of cardiac impulse or stenosis in the atrioventricular or semilunar valves. Moreover, such differences are also attributed to the physiological and metabolic demands of the body. For example, a change in heart rate could alter the frequency of the “lub” and “dub” sounds in the phonocardiogram. The present experiment on ECG and phonocardiography reflected that the duration, amplitude, and intervals of different waves in an ECG significantly vary across individuals. Moreover, the study also reflected that the duration, amplitude, and intervals of different waves in an ECG of a specific individual are also subjected to temporal variations.
References
Brosnan, M., La Gerche, A., Kalman, J., Lo, W., Fallon, K., Macisaac, A., Prior, D. 2013. The Seattle criteria increase the specificity of preparticipation ECG screening among elite athletes. Br J Sports Med.  48: 1144–1150
Corrado, D., Basso, C., Schiavon, M., Pelliccia, A., Thiene, G. 2008.  Pre-participation screening of young competitive athletes for prevention of sudden cardiac death. J Am Coll Cardiol. 52:1981–1989
Drezner, J., Asif, I., Owens, D., Prutkin, J., Salerno, J., Fean, R., Rao, A., Stout, K., Harmon, K. 2012. Accuracy of ECG interpretation in competitive athletes: the impact of using standardised ECG criteria. Br J Sports Med.  46: 335–340
Exeter, D.J., Elley, C.R., Fulcher, M.L., Lee, A.C., Drezner, J.A., Asif, I.M. 2014. Standardised criteria improve accuracy of ECG interpretation in competitive athletes: a randomised controlled trial. Br J Sports Med.  48:1167–1171
Hill, A.C., Miyake, C.Y., Grady, S., Dubin, A.M. 2011. Accuracy of interpretation of preparticipation screening electrocardiograms. J Pediatr.  159 :783–788
 Moyer VA.2012. “Screening for coronary heart disease with electrocardiography: U.S. Preventive Services Task Force recommendation statement.” Annals of Internal Medicine. 157 (7): 512–518