Effects and Functions of an Electrocardiogram Under the Influence of Exercise

Abstract

 This lab examines the specific sections of a typical sinus rhythm through the use of an electrocardiogram (ECG). An ECG is a measuring tool that monitors and examines the heart’s electrical activity. To examine the significance of exercise on the heart, the ECG is utilized. Heart rate of 27 subjects were monitored while at rest and post exercise. It was found that as exercised increased, heart rate also increased due to the increased demand of oxygen throughout the body. Studies have shown that the electrocardiogram undergoes various changes as exercise occurs due to change in cardiac functions.

Introduction

 There are various measurements used to examine and measure the heart. A common measurement is by determining heart rate by finding the pulse of an individual. Heart rate indicates the heart beats per minute (bpm). Resting heart rate is averaged to be 50-90 bpm (Zakynthinaki, 2015). Typically, conditioned athletes and highly fit individuals have lower heart rates of 40-60 bpm, indicating a very high level of cardiovascular fitness. Athletes’ hearts are more efficient and therefore, do not need to pump as much blood to the body. During exercise, the body exerts more energy and oxygen uptake increases. This causes the heart to work harder and pump more blood to the body than at rest. Heart rate can increase up to 170-210 bpm during exercise (Franco et al., 2014). Maximal heart rate is the highest heart rate an individual can achieve by performing maximal effort to the point of complete exhaustion (Zakynthinake et al., 2015).

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Electrocardiography (ECG) is a measurement tool used to measure the heart’s electrical activity. By attaching electrodes to specific locations in the body, an electrocardiogram analyzes heart rate, as well as sinus rhythm of an individual at rest and post exercise. This is demonstrated through a graphical record of the physiological events occurring in the heart during atrial and ventricular depolarization and repolarization phases. The objective of this lab is to evaluate the key components of a normal sinus wave and the correlation between heart rate and exercise.

Methods and Materials

For this ECG lab, a group member was selected as the subject while another group member started up the Biopac software on the computer. The subject was sitting on a chair while the other group members sanitized (using special sanitized wipes) the surface where the electrodes would be attached. After electrodes were stuck on the inside of both ankles and the right wrist, a red lead was attached to the left ankle, a black lead on the right ankle, and a white lead attached on the right wrist. To measure the resting heart rate of the subject, a member controlling the Biopac software selected ‘calibration’ and recorded the ECG pattern of the subject for 15 seconds. The location of the seated recording was marked. The PR interval, PR segment, QRS complex, QT interval, and the ST segment of one sinus rhythm were recorded to be analyzed after. The leads were removed from the electrodes, while the electrodes remained attached to the subject.

The second part of the procedure required the group member to perform some sort of exercise that would elevate their heart rate. In this lab, 40 burpees were performed. After the exercise, the leads were quickly reattached by the group members to their respective electrodes and calibration was recorded by the member in charge of the Biopac software. The purpose of the second procedure was to measure the heart rate of the subject in beats per minute after exercise. The PR interval, PR segment, QRS complex, QT interval, and the ST segment of one sinus rhythm were once again recorded to compare with the resting heart rate values for % change. After all the required components were recorded, all the leads and electrodes were removed from the subject.

Results

 At rest, data from 27 subjects was collected on the electrocardiogram for heart rate (bpm), PR interval (secs), ST segment (secs). As seen in Table 1, the results displayed the beats per minute and the time it took for the various segments and intervals to occur. For the individual subject’s data, their heart rate more than doubled from rest to exercise. At rest, the individual’s Q-T interval had the longest time out of the four intervals and segments. Following the Q-T interval, the S-T segment had the second longest time followed by the QRS complex and the P-R interval respectively. Minimal difference in the individual’s QRS complex was observed between rest and exercise.

Table 1. Heart rate (bpm), P-R interval (secs), S-T segment (secs), Q-T interval (secs) and QRS complex (secs) of an individual subject at rest and exercise. Mean and standard deviation of heart rate (bpm), P-R interval (secs) and S-T segment (secs) of subjects (n=27) at rest and exercise.

After exercise, the heart rate (bpm), PR interval (secs), ST segment (secs) of 27 subjects were collected. According to Table 1, the individual’s Q-T interval took the longest followed by the S-T segment, P-R interval, and the QRS complex respectively. The P-R interval was faster by 12.71% and the S-T segment was faster by 21.96% after exercise than at rest.

Figure 1. Average heart rate in beats per minute and standard deviation of 27 subjects while at rest and post exercise (40 burpees)

Figure 2. Average P-R Interval in seconds and standard deviation of 27 subjects while at rest and post exercise (40 burpees)

Figure 3. Average S-T Segment in seconds and standard deviation of 27 subjects while at rest and post exercise (40 burpees)

Figure 4. P-R interval (secs) and HR (bpm) scatter plot of subjects (n=27) after exercise

Figure 5. S-T segment (secs) and HR (bpm) scatter plot of subjects (n=27) after exercise

Figure 6. P-R interval (secs) and S-T segment (secs) scatter plot of subjects (n=27) after exercise

To determine that there was a real difference and the results were not simply due to chance, a T-test was conducted using a student T-test calculator online. Heart rate at rest and exercise is significant at p < .05. The P-R interval at rest and exercise is not significant at p <.05. The S-T segment is not significant at p < .05.

Discussion

 The heart consists of two pairs (left and right) of atria and ventricles. A valve on each side separates the atrium and the ventricle. As mentioned in the introduction, the electrocardiogram measures the electrical activity caused by these action potentials in the heart. The heart can pump blood automatically and independently due to the depolarization and repolarization of action potentials (Silverthorn, 2018). When membrane potential from resting potential is increased, depolarization occurs; in contrast, the return of the membrane potential to resting potential following depolarization is referred to as repolarization (Silverthorn, 2018).

As shown in Table 1, the ECG pattern is made up of many components. Specifically, a P wave refers to atrial depolarization, the QRS complex refers to ventricular depolarization, and a T wave refers to ventricular repolarization (Silverthorn, 2018). The PR interval exhibits the time an impulse takes to reach the ventricles in the sinus node, while the QT interval exhibits the duration of the ventricles depolarizing then repolarizing (Silverthorn, 2018). The PR interval represents the atria-ventricular nodal delay and the ST segment represents the time where the ventricles are emptying and contracting (Silverthorn, 2018). The TP interval exhibits the time the ventricles are being relaxed and refilled with blood (Silverthorn, 2018).

  From the data, since heart rate (bpm) of 27 subjects increased significantly (60.49%) from rest to exercise, it can be concluded that the heart pumps harder and heart rate increases during exercise. The rate of increase depends on the intensity of the exercise done. From the lab, it can be speculated that P-R interval gets faster (12.71% faster from this lab) during exercise and S-T segment gets faster (21.96%) as well. This implies that during or after exercise, the time that it takes for an impulse to reach the ventricles in the sinus node is faster (P-R interval) and is also faster for the ventricles to empty and contract (S-T segment). The QRS complex of the individual subject had a 0% change, demonstrating that the QRS complex remains the same or changes minimally at rest and exercise.

 Al-Zati and colleagues conducted a study on firefighters who are constantly required to exert physical and mental effort on the job (2014). Electrodes and leads were attached to the firefighters while asked to perform physically and mentally taxing tests to simulate their demands on the field. The electrocardiogram results were analyzed and data showed that most of the firefighters displayed a normal sinus rhythm during the baseline test (Al-Zati et al., 2014). However, during and after the physically and mentally demanding tests, many firefighters reached or exceeded their maximum heart rate. Similar to findings in this lab, a faster P-R interval and a minimal change in the QRS complex were found in the firefighters during and after exercise (Al-Zati et al., 2014). However, a difference was found in the ST segment. This lab’s data showed that ST segment time goes down but Al-Zati and colleagues demonstrated an upslope in their ST segment (2014).

A limitation that might have contributed to error in the data was human error. For example, incorrect electrode placement might have detected inaccurate sinus rhythms. Not attaching the leads right after exercise might have also contributed to error, as sinus rhythm have to be measured immediately after the exercise was performed.

Electrocardiography is a very useful tool to determine heart rate and the heart’s sinus rhythm. Research has shown that there is a tendency for heart rate to increase as activity level increases. This is due to a greater need of oxygen and blood supply in the body.

References

• Franco, V., Callaway, C., Salcido, D., McEntire, S., Roth, R., & Hostler, D. Characterization of electrocardiogram changes throughout a marathon. European Journal of Applied Physiology, 114(8), 1725-1735, 2014.
• Silverthorn, D. Human Physiology: An Integrated Approach. Pearson Education, Inc., Chapter 14, pages 432-472, 2018.
• (Silverthorn, 2018)
• Zakynthinaki, M. Modeling Heart Rate Kinetics. PLoS ONE. 10 (4): 1-17, 2015.