Investigating left myocardial function (P<0.0001). Interestingly, in another investigation

Investigating the Effects of Caffeine on Heart Rate

Literature review

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Caffeine, (1, 3, 7-trimethylxanthine),
is a stimulant to the central nervous system and its cardiovascular effects have
been a subject of study for many years. As the most commonly consumed psychoactive
drug, there is frequent concern that caffeine has an effect on blood pressure
and heart rate. Despite this, research on the cardiovascular effects has
produced generally inconsistent results.

Experimental studies on the
effects of caffeine on blood pressure show that caffeine consumption leads to
small rises in systolic and diastolic blood pressure.  A Mayo Clinic study on the effects of a
popular energy drink used 25 healthy young adults between the ages of 19 and 40
who were alternately given an energy drink or a placebo, after which blood
pressure and heart rate were measured. Results were compared between regular
caffeine drinkers and those who were not regular caffeine drinkers. The results
show that the subjects had a considerable rise in blood pressure and that the
increase was greater than 50% for those who were non habitual caffeine consumers
as compared to regular caffeine drinkers (Mayo Clinic, 2015).  In a similar study researching the effects of
energy drinks on myocardial function, rather than just the blood pressure and
heart rate being assessed, blood pressure, electrocardiographic and
echocardiographic examination were carried out on a larger sample of 35 healthy
subjects (Cameli et al., 2013). In
this study however, the placebo used did not contain the same level of sugar as
the energy drink used. The results showed that there was significant increase
in right and left myocardial function (P<0.0001). Interestingly, in another investigation on the cardiovascular effects of caffeine, as opposed to an increase, subjects showed a significant decrease in heart rate after 30 minutes (P<0.05), and no change in heart rate in the control group (Pilli et al., 2012). Results on blood pressure in the same study found a marked increase in diastolic and systolic blood pressure (P<0.05). This highlights the contradictory findings of epidemiological studies on this subject. All of these studies however had an incredibly small sample size and so the margin for error is broad. Exploring caffeine intake, a study at the University of California found that caffeine products could be beneficial for cardiovascular health. The study, which is the largest to date, was published in the Journal of the American Heart, and had a much more substantial sample size of 1388 participants. Their findings concluded that caffeine had no effect on heart rate (Dewland et al., 2016). All of the studies besides the one conducted by Mayo Clinic had a P value <0.05, which presents strong evidence against the null hypothesis and so the null hypothesis is rejected. Without the aid of statistical analysis, the findings of the Mayo Clinic study cannot be deemed as statistically significant.   Aim   The aim of this investigation is to determine whether increasing a person's caffeine intake affects their heart rate.   Hypothesis   Caffeine intake affects heartrate due to being a stimulant to the central nervous system, raising the heart rate in beats per minute. Prediction   Increasing the levels of caffeine intake increases the subject's heart rate.   Independent  Variable Caffeine intake (mg) Dependent  Variable Heart rate (BPM)           Method: The experiment involved a sample size of two subjects, one male and one female. Their initial resting heart rate was measured in beats per minute (BPM) using a blood pressure monitor and recorded in a table after which they drank 150ml of energy drink with a caffeine content of 50mg. After a fifteen minute interval, the heart rate was again measured and recorded to see any changes in heart rate. This was repeated another two times, each time the subject was made to drink 150ml of the energy drink and after a 15 minute interval the heart rate was again measured and recorded. Results: Effect of caffeine (mg) on Heart Rate (BPM)   Heart Rate (BPM) Subject Initial 50mg 100mg 150mg 1.  Female 100 109 118 126 2.  Male 119 126 133 136 Table 1. Table of caffeine intake (mg) against heart rate (BPM).                       Figure 1. Graph showing caffeine intake (mg) against heart rate (BPM).   Analysis: The results show a positive correlation, indicating that as the caffeine intake of each individual increased, the heart rate also increased. There was an increase in heart rate in both subjects in the range of 3-9 beats per minute after each caffeine intake. In Subject 1, the heart rate increased by nine beats per minute from the initial reading to the reading after a 100mg caffeine intake, after which it increased by eight beats per minute. In Subject 2, the heart rate increased by seven beats per minute from the initial reading to the reading after a 100mg caffeine intake, after which it increased by only three beats per minute . Both subjects showed a lower increase in heart rate for the reading taken after a 150mg caffeine intake as compared to the readings taken from the initial stage to after the 100mg caffeine intake. In Subject 1, from the initial reading to the final reading, there was a total increase of 26 beats per minute, from 100 BPM to 126 BPM. In Subject 2, the total increase was 17 beats per minute, from 119 BPM to 136 BPM. No anomalies were present in the data. Discussion In this experiment, the subjects were made to drink a total of 150mg of caffeine, in order to determine whether there was an increase in heart rate. Caffeine (C8H10N4O2 ), is a stimulant to the central nervous system and begins diffusing into the bloodstream almost immediately through the lining of the mouth and throat and then through the lining of the small intestine (Caffeine Informer, 2017) . Studies have shown that the effects of caffeine can be noticeable even only ten minutes after consumption however reaches the maximum concentration in the blood stream after approximately 45 minutes (Riggio, 2017). For this experiment, an initial reading of each subjects' resting heart rate was taken. For a healthy adult, the normal heart rate ranges between 60 BPM and 100 BPM (Mayo Clinic, 2015). The human heart rate is controlled by the sympathetic nervous system and the parasympathetic nervous system via the sinoatrial node, a group of cells within the wall of the right atrium which acts as a natural pacemaker (Toole, Toole, 2008). The sympathetic and parasympathetic nervous systems make up the bodies autonomous nervous system which regulates the subconscious actions of internal muscles and glands (NCBI, 2017). Both have opposing roles as the sympathetic nervous system stimulates effectors and speeds up activity and the parasympathetic system inhibits effectors and slows down activity. The nerve fibres which connect to the heart are known as the accelerans nerve and the vagus nerve. Sympathetic input to the heart is done via the accelerans nerve through the release of norepinephrine and parasympathetic input is done via the vagus nerve through the release of acetylcholine. Stimulation of these nerves ultimately increases and decreases heart rate (Medscape, 2017). Figure 2. The similar molecular structures of caffeine and adenosine. (StOlaf, 2016)   Figure 3. Caffeine acting as an adenosine antagonist. (utexas, 2018) In the investigation, both subjects were asked not to consume any caffeine, in order to ensure that the results would not be affected by any existing caffeine in their system as in the human body, caffeine has a half-life of around 4-6 hours (Caffeine Informer, 2017). After just one 50mg intake of caffeine it was possible to see an increase in heart rate in both subjects. Caffeine's ability to increase the heart rate is directly linked to the purine nucleoside, adenosine. Adenosine is a central nervous system modulator that is formed from the breakdown of adenosine triphosphate (ATP) (Klabunde, 2012). In the heart, adenosine receptors known as adenosine A? receptors are stimulated by adenosine, which creates a myocardial depressant effect, thus decreasing the heart rate. Adenosine receptors are G-coupled receptors which are a large group of proteins with the function of detecting molecules and activating internal transduction pathways in order to produce cellular responses. Caffeine, which has a similar shape to the adenosine molecule, acts as an adenosine receptor antagonist (Heller et al., 2008). This means that caffeine can bind to the adenosine A? receptor, blocking adenosine, so that it cannot decrease the heart rate. Instead, caffeine binds to the receptors and increases neural activity (Dubuc, 2017). This increase in conduction of electrical impulses stimulates the pituitary gland to release hormones which then go on to stimulate the adrenal glands to release adrenaline, also known as the "fight or flight" hormone. Adrenaline, also known as epinephrine, increases heart rate by binding to adrenergic receptors of the heart tissue (Hoyle, 2017). As well as this the increased activation of neural circuits also leads to the release of norepinephrine, also known as noradrenaline. Norepinephrine also raises the heart rate and the force of the hearts contractions via the acclerans nerve after its release from sympathetic nerves near the tissue of the Sino atrial node.   Evaluation During the experiment, due to the time restrictions, the resting heart rate of each subject was measured after only 15 minutes after having consumed the energy drink due to time restrictions. Although caffeine absorbs almost immediately into the bloodstream and the effects can be visible after only 10-15 minutes, the caffeine is at its maximum in the bloodstream after 45 minutes. After 45 minutes approximately 99% of the caffeine has been absorbed into the blood (Riggio, 2017). This means that after the 15 minutes, the caffeine has not fully absorbed into the blood stream and so only a small amount of caffeine is actually able to produce an effect on the heart rate. If the experiment was to be carried out again, the best results would be obtained after having a 45 minute interval instead of a 15 minute interval. Another factor that should be taken into account is that the two subjects were of different age, sex and weight. All of these factors will affect the way in which caffeine effects the participant's heart rate. Generally speaking, a person of larger size will require a larger dose of caffeine in order to produce the same results as a person of smaller size (Medicinenet, 2017). As well as this, the experiment did not take into account the fact that both of the subjects have caffeine on a regular basis. Those who consume caffeine regularly are shown to have a higher tolerance to the effects of caffeine (Mayo Clinic, 2017). Due to a higher caffeine tolerance level, the heart rate of the subjects may not have been raised as much as if they were not regular caffeine drinkers. In order to produce more reliable results, the factors of age, sex, weight and caffeine tolerance would need to be controlled as much as possible. A factor which could affect the results in this experiment is the fact that the energy drink used contains a high level of sugar. There has been evidence to suggest that consuming a high level of sugar can also increase the heart rate. In a study on the effects of sugar on heart rate carried out by Kennedy and Scholey, participants showed an increase in heart rate after an intake of glucose as compared to subjects who were not given any. This could be due to the breakdown of glucose causing an increased metabolism which can increase heart rate and blood pressure (Williams, 2017). As the energy drink also contained high amounts of sugar as well as caffeine, it could be that the increase in heart rate could also be an effect of the sugar content and not the caffeine. If the experiment were to be carried out again, it would be essential to choose a sugar free caffeinated drink in order to ensure that the sugar content does not affect the reliability of the results. In addition to this, the energy drink also contains an organic compound known as taurine. Although research on the topic is obscure, some studies suggest that taurine can also have an effect on heart rate. In a study done by Koscis et al, "a correlation exists between taurine levels and heart rate, with the highest taurine levels found in species with the highest heart rates" (NCBI, 2017). In order to avoid taurine also affecting the subjects' heart rates it is necessary to ensure that the beverage used does not contain taurine and also other substances which could affect the heart rate other than caffeine. Although results were obtained from both of the subjects in the experiment, the sample size of the experiment was incredibly small. In order to conclude that caffeine does raise a person's heart rate, a much larger sample size would be required in order for the results to be representative and also significant. As well as this, the experiment was carried out only once, which means that there was not enough data in order to calculate a mean and indicate precision. It would be beneficial to repeat the experiment at least another two times in order to identify any anomalous results and also reduce the effects of any random errors. This would also indicate whether the heart rate measurements were precise. If any anomalous results were to occur, having more data would make them much easier to identify and discard. As the experiment was not repeated, it is not possible to deem whether the results are actually repeatable. Other limitations of the experiment included the lack of sophisticated equipment. The measurements in the experiment were carried out with basic blood pressure monitors however many of these were faulty. This resulted in sometimes having to take the reading more than once, leading to some intervals being slightly longer than 15 minutes. This affects the accuracy of the results as some of the results may not be the true value of resting heart rate after 15 minutes. As well as this, with regards to accuracy, there is evidence to suggest that the results may not be accurate. Namely, an average adult's resting heart rate is between 60-100 BPM (Laskowski, 2015). The initial reading of heart rate taken from both subjects was over the average. Also, readings of resting heart rate were not taken from either subject at any other time. Therefore, there is no data to compare whether the readings taken during the investigation were accurate.  Finally, another aspect in which the investigation was lacking was that there was no control group. Having a control group is also necessary to ensure that the independent variable is the only one affecting the results. In this case, the control group would drink a non-caffeinated beverage, which would act as a placebo and the experiment would be conducted using the same method as participants consuming the caffeinated beverage. Conclusion The results obtained from the experiment support the hypothesis that increasing levels of caffeine increase an individual's heart rate. For both participants of the experiment, as the caffeine intake increased, the resting heart rate also increased. After an overall intake of 150mg of caffeine, the subjects showed an increased heart rate in the range of 17-26 beats per minute. Further Research Further research into the cardiovascular effects of caffeine                                           References Anderson, H. (2018). The Effects of Caffeine on Adenosine. online LIVESTRONG.COM. Available at: Accessed 21 Jan. 2018. (2018). Caffeine Metabolism. online Available at: Accessed 21 Jan. 2018. Corti, R. (2018). Coffee Acutely Increases Sympathetic Nerve Activity and Blood Pressure Independently of Caffeine Content: Role of Habitual Versus Nonhabitual Drinking. Dixit, S., Stein, P., Dewland, T., Dukes, J., Vittinghoff, E., Heckbert, S. and Marcus, G. (2018). Consumption of Caffeinated Products and Cardiac Ectopy. Dubuc, B. (2018). THE BRAIN FROM TOP TO BOTTOM. online Available at: Accessed 21 Jan. 2018. Gander, K. (2018). Coffee may not be as bad for your heart as you might think. online The Independent. Available at: Accessed 21 Jan. 2018. Garden, H., Science, P. and Compounds, C. (2018). How Caffeine Works. online HowStuffWorks. Available at: Accessed 21 Jan. 2018. Green, P., Kirby, R. and Suls, J. (2018). The effects of caffeine on blood pressure and heart rate: A review. Griffin, S. (2018). How to Induce Adrenaline Release. online LIVESTRONG.COM. Available at: Accessed 21 Jan. 2018. Hoyle, M. (2018). Why Adrenaline Speeds up Heart Rate. online LIVESTRONG.COM. Available at: Accessed 21 Jan. 2018. (2018). Energy Drinks Raise Resting Blood Pressure, Dramatic In Those Not Used To Caffeine. online Available at: Accessed 21 Jan. 2018. Klabunde, R. (2018). CV Pharmacology | Adenosine. online Available at: Accessed 21 Jan. 2018. Laskowski, E. (2018). 2 easy, accurate ways to measure your heart rate. online Mayo Clinic. Available at: Accessed 22 Jan. 2018. Medscape. (2018). Cardiovascular Effects of Coffee: Is It a Risk Factor?. online Available at: Accessed 21 Jan. 2018. Menci, D., Righini, F., Cameli, M., Lisi, M., Benincasa, S., Focardi, M. and Mondillo, S. (2018). Acute Effects of an Energy Drink on Myocardial Function Assessed by Conventional Echo-Doppler Analysis and by Speckle Tracking Echocardiography on Young Healthy Subjects. (2018). Energy drinks linked to more heart, blood pressure changes than caffeinated drinks alone | American Heart Association. online Available at: Accessed 21 Jan. 2018. Norian, E., Barzegari, A., Mahdirejei, H., Sujodi, A. and Eslami, L. (2018). Cite a Website - Cite This For Me. online Available at: Accessed 21 Jan. 2018. Riggio, G. (2018). How Long Does the Caffeine from Coffee Stay in Your System?. online LIVESTRONG.COM. Available at: Accessed 21 Jan. 2018.   Sadava, D., Hillis, D., Heller, H., Purves, B. and Orians, G. (2008). Life. Sunderland (Massachusett): Sinauer Associates. Scientific American. (2018). How does caffeine affect the body?. online Available at: Accessed 21 Jan. 2018. Taelman, J., Vandeput, S., Spaepen, A. and Van Huffel, S. (2018). Influence of Mental Stress on Heart Rate and Heart Rate Variability. Toole, G. and Toole, S. (2008). AQA A2 biology. Cheltenham: Nelson Thornes. University of California. (2018). Regular caffeine use does not result in extra heartbeats, study shows. online Available at: Accessed 21 Jan. 2018. Williams, J. (2018). Why Eating Sugar Raises Your Heart Rate. online LIVESTRONG.COM. Available at: Accessed 21 Jan. 2018.