Heart rate caffeine, effects

True. Caffeine is a mild stimulant that in moderate dosage does little harm and provides a lift . When taken in excess it can have an adverse effect on heart rate. 

Dodd et al.48 tested 17 moderately trained males for V02 max and time to exhaustion on a bicycle ergometer. Experimental trials involved the administration of 3- or 5-mg/kg caffeine 1 h prior to testing. Caffeine had no effect on exercise performance. Since nearly half of the subjects were caffeine naive (<25 mg/d), while the other half were caffeine tolerant (>310 mg/d), the researchers were able to conclude that even though caffeine had no significant effects on performance, it did produce a variety of physiologically significant effects (heart rate and expired ventilation volume) in the caffeine naive group. 

Smith, D., Tong, J. and Leigh, G., Combined effects of caffeine and tobacco on the components of choice reaction time, heart rate and hand steadiness. Perc and Motor Skills 45(2), 635-639, 1977. 

MacDougall, J. M., Musante, L., Castillo, S., and Acevedo, M.C., Smoking, caffeine, and stress Effects on blood pressure and heart rate in male and female college students. Health Psychology 7, 461-478, 1988. 

Rizzo, A. A., Stamps, L. E., Fehr, L. A., Effects of caffeine withdrawal on motor performance and heart rate changes. International Journal of Psychophysiology 6(l), 9-14, 1988. 

Physical effects of high doses of ketamine include decreased respiration and heart rate, increased blood pressure, and the possibility of vomiting and convulsions. These can lead to cardiac and respiratory arrest, coma, and death. The risk of ketamine overdose is much greater when it is mixed with other drugs such as alcohol, Ecstasy, caffeine, or cocaine. Overdoses of ketamine have been reported when people boost the drug (take another dose before the first dose wears off) to prolong its psychedelic effects. 

The caffeine naive individual may notice some changes in heart rate following consumption of a strong cup of coffee. Most caffeine users have developed a tolerance to the cardiovascular effects, but these effects may occur if there is elevated consumption. 

Death from SNA Intoxication appears to Involve both cardiovascular and respiratory systems. 2 SNA Increases the effects of pentobarbital on both respiration and heart rate. The depressant effects of SNA on the myocardium and on the Purklnje system are antagonized effectively by epinephrine, but not by Metrazol (pentylenetetrazol) and caffeine. Artificial respiration restored cardiac stability and regularity. It was suggested that SNA Interferes with oxygen utilization In animals. The circulatory and respiratory responses to SNA were considered to be mediated through the medulla oblongata. 2... 

In addition to being a stimulant, caffeine also has many effects outside of the nervous system. Caffeine is a cardiovascular stimulant, causing the heart rate to increase while constricting blood vessels, both of which can cause increases in blood pressure. Caffeine is also a diuretic (i.e., it increases urination), so it can sometimes cause dehydration and low calcium levels. If taken during pregnancy, the amount of caffeine should be limited to 300 mg per day, as birth problems have been reported in pregnant women who consume more than this amount. 

Norephedrine and ephedrine mimic and stimulate the release of the adrenal hormones norepinephrine and epinephrine. Norephinephrine raises heart rate and epinephrine stimulates carbohydrate metabolism resulting in an increased metabolic rate, fatty acids release from lipocytes (fat cells), and a protein sparing effect. Caffeine simply prolongs the effect. 

The contradictions that abound in the experimental findings about tolerance to caffeine are probably a result of poor research methods, such as not specifying caffeine-use patterns in subjects (Curatolo Robertson, 1983). In general, tolerance probably does develop to caffeine s effects on renal function, sleep, and other physiological functions, such as blood pressure and heart rate. On the other hand, little tolerance seems to develop to caffeine s stimulant effects (Hogan, Hornick, Bouchoux, 2002). 

Caffeine causes several centrally mediated side effects including nervousness, irritability, and sleeplessness. Caffeine also acts on kidneys to increase diuresis. Convulsions and increased heart rate can occur with particularly high doses of the drug. 

Caffeine exerts a noticeable inotropic effect on the myocardium and a positive chronotropic effect particularly on the sinoatrial mode which ultimately result into a transient observed heart-rate, force of contraction, working of the heart, and above all the cardiac output. However, it is largely belived that the vasoconstriction of the cerebral blood vessels by caffeine remarkably contributes a lot to its exceptional capability to relieve headaches. 

Experimental studies in healthy subjects, on the way xanthine drugs possibly interact with adenosine, have shown that caffeine and theophylline (but not enprofylline) reduced the increased heart rate and the changes in blood pressure caused by infusions of adenosine, and attenuated adenosine-induced vasodilatation. Theophylline also attenuated adenosine-induced respiratory effects and chest pain. Similarly, an adenosine infusion antagonised the haemodynamic effects of a single dose of theophylline in healthy subjects, but did not reduce the metabolic effects (reductions in plasma potassium and magnesium). ... 

Caffeine causes the release of catecholamines, such as adrenaline, into the blood, which could account for the increases in heart rate and blood pressure that are seen. The blood pressure rise may be exaggerated in the presence of non-selective beta blockers, which block vasodilatation leaving the alpha (vasoconstrictor) effects of adrenaline unopposed. This will also oppose the actions of the beta blockers. 

Due to the CNS stimulant effects of caffeine, use of caffeine-containing products is cautioned in persons with heart disorders, as excessive caffeine consumption may increase heart rate or exacerbate arrhythmias or psychological disorders, as caffeine may aggravate depression or induce anxiety (Brinker 2001). 

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