User:DocElisa/Heart rate

Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute (bpm). Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during physical exercise, sleep or illness. Because an individual has 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).

Measuring heart rate

Wrist heart rate monitor

Heart rate monitor with a wrist receiver

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. 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:

1. The ventral aspect of the wrist on the side of the thumb (radial artery).
2. The ulnar artery.
3. The neck (carotid artery).
4. The inside of the elbow, or under the biceps muscle (brachial artery).
5. The groin (femoral artery).
6. Behind the medial malleolus on the feet (posterior tibial artery).
7. Middle of dorsum of the foot (dorsalis pedis).
8. Behind the knee (popliteal artery).
9. Over the abdomen (abdominal aorta).
10. The chest (apex of the heart), which can be felt with one's hand or fingers. It is also possible to auscultate the heart using a stethoscope.
11. The temple (superficial temporal artery).
12. The lateral edge of the mandible (facial artery).
13. The side of the head near the ear (posterior auricular artery).

ECG-RRinterval

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).

Various commercial heart rate monitors are also available. Some monitors, used during sport, consist of a chest strap with electrodes. The signal is transmitted to a wrist receiver for display.

Another way of determining the heart rate is by recording of the body vibrations: (seismocardiography).

Basal heart rate

The basal or resting heart rate (HR_rest) 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) however frequencies between 50 and 60 are yet considered normal and don't need concern. This is the firing rate of the heart sinoatrial node (SAN), where are located the faster heart pacemaker cells driving the self-generated rhythmic firing and responsible for the cardiac muscle automaticity.

Heart rate is not a stable value and it increases or decreases in response to the body 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.

Maximum

The maximum heart rate (HR_max) 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 HR_max 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 to estimate their individual maximum heart rate.

Formulae

Fox and Haskell formula; widely used.

Various formulas are used to estimate individual maximum heart rates, mostly based on age.

Tanaka, Monahan, & Seals

In 2001, the Journal of the American College of Cardiology published the formula:

• HR_max = 208 - (0.7 × age)

In a paper titled: "Age-predicted maximal heart rate revisited", Tanaka H, Monahan KD, and Seals DR, from the Department of Kinesiology and Applied Physiology, University of Colorado at Boulder, detailed both a meta-analysis (of 351 prior studies involving 492 groups and 18,712 subjects) and a laboratory study (of 514 healthy subjects). It 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 analysed maximum heart rates of 132 individuals recorded yearly over 25 years, and produced a linear equation very similar to the Tanaka formula—HR_max = 206.9 - (0.67 × age)—and a nonlinear equation—HR_max = 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 — HR_max = 163 + (1.16 × age) - (0.018 × age²).

Haskell and Fox

Notwithstanding the research of Tanaka, Monahan, & Seals, the most widely cited formula for HR_max (which contains no refererence to any standard deviation) is still:

HR_max = 220 - age

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 HR_max. Despite the widespread publication of this formula, research spanning two decades reveals its large inherent error (Sxy = 7–11 b/min). Consequently, the estimation calculated by HR_max = 220 - age has neither the accuracy nor the scientific merit for use in exercise physiology and related fields.

Robergs and Landwehr

A 2002 study of 43 different formulae for HR_max (including that of Haskell and Fox - see above) published in the Journal of Exercise Psychology concluded that:

1. no "acceptable" formula currently existed, (they used the term "acceptable" to mean acceptable for both prediction of VO₂, and prescription of exercise training HR ranges)
2. the least objectionable formula was:

HR_max = 205.8 - (0.685 × age)

This had a standard deviation that, although large (6.4 bpm), was considered acceptable for prescribing exercise training HR ranges.

Gulati formula (for women)

Research conducted at Northwestern University by Martha Gulati, et al., in 2010 suggested a maximum heart rate formula for women:

HR_max = 206 - (0.88 × age)

Lund Study

A study from Lund, Sweden gives reference values (obtained during bicycle ergometry) for men:

HR_max = 203.7 / (1 + exp (0.033 x (age - 104.3)))

and for women:

HR_max = 190.2 / (1 + exp (0.0453 x (age - 107.5)))

Other formulae

• HR_max = 206.3 - (0.711 × age)

(Often attributed to "Londeree and Moeschberger from the University of Missouri")

• HR_max = 217 - (0.85 × age)

(Often attributed to "Miller et al. from Indiana University")

Limitations

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 HR_max 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 HR_max 60 bpm apart (160 to 220): the range is extremely broad, and some say "The heart rate is probably the least important variable in comparing athletes."

Target rate

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 HR_max to ensure these calculations are meaningful (see above).

Example for someone with a HR_max of 180 (age 40, estimating HR_max As 220 − age):

65% Intensity: (220 − (age = 40)) × 0.65 → 117 bpm

85% Intensity: (220 − (age = 40)) × 0.85 → 153 bpm

Karvonen method

The Karvonen method factors in resting heart rate (HR_rest) to calculate target heart rate (THR), using a range of 50–85% intensity:

THR = ((HR_max − HR_rest) × % intensity) + HR_rest

Example for someone with a HR_max of 180 and a HR_rest of 70:

50% Intensity: ((180 − 70) × 0.50) + 70 = 125 bpm

85% Intensity: ((180 − 70) × 0.85) + 70 = 163 bpm

Zoladz method

An alternative to the Karvonen method is the Zoladz method, which derives exercise zones by subtracting values from HR_max:

THR = HR_max − Adjuster ± 5 bpm

Zone 1 Adjuster = 50 bpm

Zone 2 Adjuster = 40 bpm

Zone 3 Adjuster = 30 bpm

Zone 4 Adjuster = 20 bpm

Zone 5 Adjuster = 10 bpm

Example for someone with a HR_max of 180:

Zone 1(easy exercise): 180 − 50 ± 5 → 125 − 135 bpm

Zone 4(tough exercise): 180 − 20 ± 5 → 155 − 165 bpm

Heart rate reserve

Heart rate reserve (HRR) 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 HR_rest will drop, thus the heart rate reserve will increase. Percentage of HRR is equivalent to percentage of VO₂ reserve.

HRR = HR_max − HR_rest

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:

• The study did not use VO₂ data to develop the equation.
• Only six subjects were used, and the correlation between the percentages of HRR and VO₂ max was not statistically significant.

Recovery heart rate

Recovery heart rate is the heart rate measured at a fixed (or reference) period after ceasing activity, typically measured over a one minute period.

A greater reduction in heart rate after exercise during the reference period indicates a better-conditioned heart. Heart rates that do not drop by more than 12 bpm one minute after stopping exercise are associated with an increased risk of death.

Training regimes sometimes use recovery heart rate as a guide of progress and to spot problems such as overheating or dehydration. 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.

Abnormalities

Tachycardia

Main article: Tachycardia

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

1. Exercise
2. Pregnancy
3. Emotional conditions such as anxiety or stress.

Pathological conditions when tachycardia occurs are:

1. Fever
2. Anemia
3. Hypoxia
4. Hyperthyroidism
5. Hypersecretion of catecholamines
6. Cardiomyopathy
7. Valvular heart diseases
8. Acute Radiation Syndrome

Bradycardia

Main articles: Bradycardia and Athletic heart syndrome

Bradycardia is defined as a heart rate less than 60 beats per minute although it is seldom symptomatic until below 50 bpm when a human is at total rest. This number can vary as children and small adults 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. Whilst 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)

Arrhythmia

Main article: Cardiac dysrhythmia

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.

As a risk factor

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:

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. However, because the above formula is almost always mentioned, students determine HR this way without looking at the ECG any further.

Bibliography

• O'Rouke, R.A., Fuster, V. (2001). Hurst's The Heart (10 (International edition) ed.). McGraw-Hill. ISBN 0-07-116296-8.{{cite book}}: CS1 maint: multiple names: authors list (link)

• Berne, Robert (2004). Physiology. Elsevier Mosby. p. 276. ISBN 0-8243-0348-2. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: ref duplicates default (link)

• Sherwood L. (2008). Human Physiology, From Cells to Systems. Cengage Learning. ISBN 9780495391845. Retrieved 2013-03-10.