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Heart rate is the speed of the heartbeat, specifically the number of heartbeats per unit of time. The heart rate is typically expressed as beats per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excrete carbon dioxide. Activities that can provoke change include physical exercise, sleep, anxiety, stress, illness, ingesting, and drugs.
The normal human heart rate ranges from 60–100 bpm. Bradycardia is a slow heart rate, defined as below 60 bpm. Tachycardia is a fast heart rate, defined as above 100 bpm at rest. When the heart is not beating in a regular pattern, this is referred to as an arrhythmia. These abnormalities of heart rate sometimes, but not always, indicate disease. 
Heart rate is measured by finding the pulse of the heart. This pulse rate can be found at any point on the body where the artery's pulsation is transmitted to the surface by pressuring it with the index and middle fingers; often it is compressed against an underlying structure like bone. (A good area is on the neck, under the corner of the jaw.) The thumb should not be used for measuring another person's heart rate, as its strong pulse may interfere with the correct perception of the target pulse.
The radial artery is the easiest to use to check the heart rate. However, in emergency situations the most reliable arteries to measure heart rate are carotid arteries. This is important mainly in patients with atrial fibrillation, in whom heart beats are irregular and stroke volume is largely different from one beat to another. In those beats following a shorter diastolic interval left ventricle doesn't fill properly, stroke volume is lower and pulse wave is not strong enough to be detected by palpation on a distal artery like the radial artery. It can be detected, however, by doppler.
Possible points for measuring the heart rate are:
A more precise method of determining pulse involves the use of an electrocardiograph, or ECG (also abbreviated EKG). Continuous electrocardiograph monitoring of the heart is routinely done in many clinical settings, especially in critical care medicine. On an ECG the heart rate is measured using the R wave to R wave interval (RR interval). Additionally pulse oximeters measure heart rate by pulse detection.
Heart rate monitors allow measurements to be taken continuously and can be used during exercise when manual measurement would be difficult or impossible (such as when the hands are being used).
The basal or resting heart rate (HRrest) is measured while the subject is relaxed but awake, in a neutrally temperate environment, and not having recently exerted himself or herself nor having been subject to a stress or even a surprise (for example the simple noise of a doorbell can augment the heart rate and blood pressure). The typical resting heart rate in adults is 60–80 beats per minute (bpm). For endurance athletes at the elite level, it is not unusual to have a resting heart rate between 33 and 50. This is the firing rate of the heart sinoatrial node (SAN), where the faster heart pacemaker cells driving the self-generated rhythmic firing and responsible for the cardiac muscle automaticity are located.
Heart rate is not a stable value and it increases or decreases in response to the body's need in a way to maintain an equilibrium (basal metabolic rate) between requirement and delivery of oxygen and nutrients. The normal SAN firing rate is affected by autonomic nervous system activity: sympathetic stimulation increases and parasympathetic stimulation decreases the firing rate.
The maximum heart rate (HRmax) is the highest heart rate an individual can achieve without severe problems through exercise stress, and depends on age. The most accurate way of measuring HRmax is via a cardiac stress test. In such a test, the subject exercises while being monitored by an ECG. During the test, the intensity of exercise is periodically increased through increasing speed or slope of the treadmill (if a treadmill is being used), continuing until certain changes in heart function are detected in the ECG, at which point the subject is directed to stop. Typical durations of such a test range from ten to twenty minutes.
Standard textbooks of physiology and medicine mention that heart rate (HR) is readily calculated from the ECG as follows: HR = 1,500/RR interval in millimeters, HR = 60/RR interval in seconds, or HR = 300/number of large squares between successive R waves. In each case, the authors are actually referring to instantaneous HR, which is the number of times the heart would beat if successive RR intervals were constant.
Conducting a maximal exercise test can require expensive equipment. People just beginning an exercise regimen are normally advised to perform this test only in the presence of medical staff due to risks associated with high heart rates. For general purposes, people instead typically use a formula, mostly based on age, to estimate their individual maximum heart rate. However, age-based predictive formulae are "largely useless" because of the "poor relationship between maximal heart rate and age" and large standard deviations around predicted heart rates.:ix, 108–112
From Tanaka, Monahan, & Seals (2001):
Their meta-analysis (of 351 prior studies involving 492 groups and 18,712 subjects) and laboratory study (of 514 healthy subjects) concluded that, using this equation, HRmax was very strongly correlated to age (r = −0.90). The regression equation that was obtained in the laboratory-based study (209 − 0.7 x age), was virtually identical to that of the meta-study. The results showed HRmax to be independent of gender and independent of wide variations in habitual physical activity levels.
In 2007, researchers at the Oakland University analyzed maximum heart rates of 132 individuals recorded yearly over 25 years, and produced a linear equation very similar to the Tanaka formula, HRmax = 206.9 − (0.67 × age), and a nonlinear equation, HRmax = 191.5 − (0.007 × age2). The linear equation had a confidence interval of ±5–8 bpm and the nonlinear equation had a tighter range of ±2–5 bpm. Also a third nonlinear equation was produced: HRmax = 163 + (1.16 × age) − (0.018 × age2).
Notwithstanding the research of Tanaka, Monahan, & Seals, the most widely cited formula for HRmax (which contains no reference to any standard deviation) is still:
Although attributed to various sources, it is widely thought to have been devised in 1970 by Dr. William Haskell and Dr. Samuel Fox. Inquiry into the history of this formula reveals that it was not developed from original research, but resulted from observation based on data from approximately 11 references consisting of published research or unpublished scientific compilations. It gained widespread use through being used by Polar Electro in its heart rate monitors, which Dr. Haskell has "laughed about", as the formula "was never supposed to be an absolute guide to rule people's training."
While it is the most common (and easy to remember and calculate), this particular formula is not considered by reputable health and fitness professionals to be a good predictor of HRmax. Despite the widespread publication of this formula, research spanning two decades reveals its large inherent error, Sxy = 7–11 bpm. Consequently, the estimation calculated by HRmax = 220 − age has neither the accuracy nor the scientific merit for use in exercise physiology and related fields.
A 2002 study of 43 different formulas for HRmax (including that of Haskell and Fox – see above) published in the Journal of Exercise Psychology concluded that:
Research conducted at Northwestern University by Martha Gulati, et al., in 2010 suggested a maximum heart rate formula for women:
A study from Lund, Sweden gives reference values (obtained during bicycle ergometry) for men:
and for women:
Maximum heart rates vary significantly between individuals. Even within a single elite sports team, such as Olympic rowers in their 20s, maximum heart rates have been reported as varying from 160 to 220. Such a variation would equate to a 60 or 90 year age gap in the linear equations above, and would seem to indicate the extreme variation about these average figures.
Figures are generally considered averages, and depend greatly on individual physiology and fitness. For example an endurance runner's rates will typically be lower due to the increased size of the heart required to support the exercise, while a sprinter's rates will be higher due to the improved response time and short duration. While each may have predicted heart rates of 180 (= 220 − age), these two people could have actual HRmax 20 beats apart (e.g., 170-190).
Further, note that individuals of the same age, the same training, in the same sport, on the same team, can have actual HRmax 60 bpm apart (160–220): the range is extremely broad, and some say "The heart rate is probably the least important variable in comparing athletes."
Heart rate reserve (HRreserve) is the difference between a person's measured or predicted maximum heart rate and resting heart rate. Some methods of measurement of exercise intensity measure percentage of heart rate reserve. Additionally, as a person increases their cardiovascular fitness, their HRrest will drop, thus the heart rate reserve will increase. Percentage of HRreserve is equivalent to percentage of VO2 reserve.
This is often used to gauge exercise intensity (first used in 1957 by Karvonen).
Karvonen's study findings have been questioned, due to the following:
Heart rate recovery (HRrecovery) is the reduction in heart rate at peak exercise and the rate as measured after a cool-down period of fixed duration. A greater reduction in heart rate after exercise during the reference period is associated with a higher level of cardiac fitness.:114
Heart rates that do not drop by more than 12 bpm one minute after stopping exercise are associated with an increased risk of death. Investigators of the Lipid Research Clinics Prevalence Study, which included 5,000 subjects, found that patients with an abnormal HRrecovery (defined as a decrease of 42 beats per minutes or less at two minutes post-exercise) had a mortality rate 2.5 times greater than patients with a normal recovery.:114 Another study by Nishime et al. and featuring 9,454 patients followed for a median period of 5.2 years found a four-fold increase in mortality in subjects with an abnormal HRrecovery (≤12 bpm reduction one minute after the cessation of exercise).:114 Shetler et al. studied 2,193 patients for thirteen years and found that a HRrecovery of ≤22 bpm after one minute "best identified high-risk patients".:114 They also found that while HRrecovery had significant prognostic value it had no diagnostic value.:114
Training regimes sometimes use HRrecovery as a guide of progress and to spot problems such as overheating or dehydration.[better source needed] After even short periods of hard exercise it can take a long time (about 30 minutes) for the heart rate to drop to rested levels.
Normal heart sounds as heard with a stethoscope
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While heart rhythm is regulated entirely by the sinoatrial node under normal conditions, heart rate is regulated by sympathetic and parasympathetic input to the sinoatrial node. The accelerans nerve provides sympathetic input to the heart by releasing norepinephrine onto the cells of the sinoatrial node, and the vagus nerve provides parasympathetic input to the heart by releasing acetylcholine onto sinoatrial node cells. Therefore, stimulation of the accelerans nerve increases heart rate, while stimulation of the vagus nerve decreases it.
Due to individuals having a constant blood volume, one of the physiological ways to deliver more oxygen to an organ is to increase heart rate to permit blood to pass by the organ more often. Normal resting heart rates range from 60–100 bpm. Bradycardia is defined as a resting heart rate below 60 bpm. However, heart rates from 50 to 60 bpm are common among healthy people and do not necessarily require special attention. Tachycardia is defined as a resting heart rate above 100 bpm, though persistent rest rates between 80–100 bpm, mainly if they are present during sleep, may be signs of hyperthyroidism or anemia (see below).
There are many ways in which the heart rate speeds up or slows down. Most involve stimulant-like endorphins and hormones being released in the brain, many of which are those that are 'forced'/'enticed' out by the ingestion and processing of drugs.
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This section discusses target heart rates for healthy persons and are inappropriately high for most persons with coronary artery disease.
For healthy persons, the Target Heart Rate or Training Heart Rate (THR) is a desired range of heart rate reached during aerobic exercise which enables one's heart and lungs to receive the most benefit from a workout. This theoretical range varies based mostly on age; however, a person's physical condition, sex, and previous training also are used in the calculation. Below are two ways to calculate one's THR. In each of these methods, there is an element called "intensity" which is expressed as a percentage. The THR can be calculated as a range of 65–85% intensity. However, it is crucial to derive an accurate HRmax to ensure these calculations are meaningful (see "Maximum heart rate" section above).
Example for someone with a HRmax of 180 (age 40, estimating HRmax As 220 − age):
The Karvonen method factors in resting heart rate (HRrest) to calculate target heart rate (THR), using a range of 50–85% intensity:
Example for someone with a HRmax of 180 and a HRrest of 70:
An alternative to the Karvonen method is the Zoladz method, which derives exercise zones by subtracting values from HRmax:
Example for someone with a HRmax of 180:
A number of investigations indicate that faster resting heart rate has emerged as a new risk factor for mortality in homeothermic mammals, particularly cardiovascular mortality in human beings. Faster heart rate may accompany increased production of inflammation molecules and increased production of reactive oxygen species in cardiovascular system, in addition to increased mechanical stress to the heart. There is a correlation between increased resting rate and cardiovascular risk. This is not seen to be "using an allotment of heart beats" but rather an increased risk to the system from the increased rate.
An Australian-led international study of patients with cardiovascular disease has shown that heart beat rate is a key indicator for the risk of heart attack. The study, published in The Lancet (September 2008) studied 11,000 people, across 33 countries, who were being treated for heart problems. Those patients whose heart rate was above 70 beats per minute had significantly higher incidence of heart attacks, hospital admissions and the need for surgery. University of Sydney professor of cardiology Ben Freedman from Sydney's Concord hospital, said "If you have a high heart rate there was an increase in heart attack, there was about a 46 percent increase in hospitalizations for non-fatal or fatal heart attack."
Standard textbooks of physiology and medicine mention that heart rate (HR) is readily calculated from the ECG as follows:
Tachycardia is a resting heart rate more than 100 beats per minute. This number can vary as smaller people and children have faster heart rates than average adults.
Physiological condition when tachycardia occurs are
Pathological conditions when tachycardia occurs are:
Bradycardia was defined as a heart rate less than 60 beats per minute when textbooks asserted that the normal range for heart rates was 60 - 100 bpm. The normal range has since been revised in textbooks to 50 - 90 bpm for a human at total rest. Setting a lower threshold for bradycardia prevents misclassification of fit individuals as having a pathologic heart rate. The normal heart rate number can vary as children and adolescents tend to have faster heart rates than average adults. Bradycardia may be associated with medical conditions such as hypothyroidism.
Trained athletes tend to have slow resting heart rates, and resting bradycardia in athletes should not be considered abnormal if the individual has no symptoms associated with it. For example Miguel Indurain, a Spanish cyclist and five time Tour de France winner, had a resting heart rate of 28 beats per minute, one of the lowest ever recorded in a healthy human. Martin Brady achieved the world record for the slowest heartbeat in a healthy human with a heart rate of just 27 bpm in 2005 (a record that still stands).
Arrhythmias are abnormalities of the heart rate and rhythm (sometimes felt as palpitations). They can be divided into two broad categories: fast and slow heart rates. Some cause few or minimal symptoms. Others produce more serious symptoms of lightheadedness, dizziness and fainting.