Heart rate

Heart rate is a term used to describe the frequency of the cardiac cycle. It is considered one of the four vital signs. Usually it is calculated as the number of contractions (heart beats) of the heart in one minute and expressed as "beats per minute" (bpm). See "Heart" for information on embryofetal heart rates. The heart beats up to 120 times per minute in childhood. When resting, the adult human heart beats at about 70 bpm (males) and 75 bpm (females), but this rate varies among people. However, the reference range is nominally between 60 bpm (if less termed bradycardia) and 100 bpm (if greater, termed tachycardia). Resting heart rates can be significantly lower in athletes. The infant/neonatal rate of heartbeat is around 130-150 bpm, the toddler's about 100–130 bpm, the older child's about 90–110 bpm, and the adolescent's about 80–100 bpm.

The body can increase the heart rate in response to a wide variety of conditions in order to increase the cardiac output (the amount of blood ejected by the heart per unit time). Exercise, environmental stressors or psychological stress can cause the heart rate to increase above the resting rate.

The pulse is the most straightforward way of measuring the heart rate, but it can be deceptive when some heart beats do not have much cardiac output. In these cases (as happens in some arrhythmias), the heart rate may be considerably higher than the pulse rate.

Auscultation is also a method of heart rate measurement.

Control of heart rate

The heart contains two cardiac pacemakers that spontaneously cause the heart to beat. These can be controlled by the autonomic nervous system and circulating adrenaline. The vagus nerve (which is pneumogastric nerve or cranial nerve X) which governs heart rate can be controlled through breathing.^{[citation needed]}

Measuring heart rate

The pulse rate (which in most people is identical to the heart rate) can be measured at any point on the body where an artery is close to the surface. Such places are wrist (radial artery), neck (carotid artery), elbow (brachial artery), and groin (femoral artery). The pulse can also be felt directly over the heart.(remember, never use your thumb to measure your pulse rate.)
Producing an electrocardiogram, or ECG (also abbreviated EKG), is one of the most precise methods of heart rate measurement. Continuous electrocardiographic monitoring of the heart is routinely done in many clinical settings, especially in critical care medicine. Commercial heart rate monitors are also available, consisting of a chest strap with electrodes. The signal is transmitted to a wrist receiver for display. Heart rate monitors allow accurate 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).
It is also possible to measure heart rate acoustically, by listening to the sounds the heart makes while beating. These sounds can be listened to using a stethoscope.

Heart rate variability

Main article: Heart rate variability

Heart rate variability (HRV) is the variation of beat-to-beat intervals. A healthy heart has a large HRV, while decreased or absent variability may indicate cardiac disease. HRV also decreases with exercise-induced tachycardia. HRV has been the focus of increased research to use it as a physiological marker to classify different pathological disorders.

One aspect of heart rate variability can be used as a measurement of fitness, specifically the speed at which one's heart rate drops upon termination of vigorous exercise. The speed at which a person's heart rate returns to resting is faster for a fit person than an unfit person. A drop of 20 beats in a minute is typical for a healthy person. A drop of less than 12 beats per minute after maximal exercise has been correlated with a significant increase in mortality ^[1].

Maximum heart rate

Maximum heart rate (also called MHR, or HR_max) is the maximum heart rate that a person should achieve during maximal physical exertion. Research indicates it is most closely linked to a person's age; a person's HR_max will decline as they age. Some research indicates the speed at which it declines over time is related to fitness—the more fit a person is, the more slowly it declines as they age.

HR_max is utilized frequently in the fitness industry, specifically during the calculation of target heart rate when prescribing a fitness regimen. A quick way to estimate MHR is to subtract your age from 220, but HR_max can vary significantly between same-aged individuals so direct measurement using a heart rate monitor (and with medical supervision or at least permission and advice) should be used by those seeking maximum safety and effectiveness in their training.^[2] People who have participated in sports and athletic activities in early years will have a higher MR than those less active as children.^{[citation needed]}

Measuring HR_max

The most accurate way of measuring HR_max for an individual is via a cardiac stress test. In such a test, the subject exercises while being monitored by an electrocardiogram (ECG). During the test, the intensity of exercise is periodically increased (if a treadmill is being used, through increase in speed or slope of the treadmill) until the subject can no longer continue, or 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 10 to 20 minutes.

Conducting an accurate maximal exercise test requires expensive equipment, and should only be performed in the presence of medical staff due to risks associated with high heart rates. Instead, people typically use predictive formulae to estimate their individual Maximum Heart Rate. The most common formula encountered is:

HR_max = 220 − age (can vary)

This is often attributed to various sources, including "Fox and Haskell". While the most common (and easy to remember and calculate), this particular formula is not considered by some to be a good predictor of HR_max.

A 2003 study [1] of 43 different formulae for HR_max (including the one above) concluded the following:

1) No "acceptable" formula currently existed, (they used the term "acceptable" to mean acceptable for both prediction of $V_{\mathrm {O} _{2}max}$ , and prescription of exercise training HR ranges)

2) The most accurate formula of those examined was:

HR_max = 205.8 − (0.685 * age)

This was found to have a standard of error, that although large (6.4 bpm), still deemed to be acceptable, for the use of prescribing exercise training HR ranges.

Other often cited formulae are:

HR_max = 206.3 − (0.711 * age)

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

HR_max = 217 − (0.85 * age)

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

Sally Edwards, CEO of Heart Zones proposes a set of gender specific formula for predicting Maximum Heart Rate.^[3]

For males: 210 - 1/2 your age - 5% of total body weight (in pounds) + 4 = HR_max

For females: 210 - 1/2 your age - 5% of total body weight (in pounds) + 0 = HR_max

Training Heart rates

(Warm up) — 50–60% of maximum heart rate: The easiest zone and probably the best zone for people just starting a fitness program. It can also be used as a warm up for more serious walkers. This zone has been shown to help decrease body fat, blood pressure and cholesterol. It also decreases the risk of degenerative diseases and has a low risk of injury. Fats are 85% of food energy burned in this zone.

Fitness Zone (Fat Burning) — 60–70% of maximum heart rate: This zone provides the same benefits as the healthy heart zone, but is more intense and burns more total food energy. The percent of food energy from fat is still 85%.

Aerobic Zone (Endurance Training) — 70–80% of maximum heart rate: The aerobic zone will improve your cardiovascular and respiratory system and increase the size and strength of your heart. This is the preferred zone if you are training for an endurance event. More food energy is burned with 50% from fat.

Anaerobic Zone (Performance Training) — 80–90% of maximum heart rate: Benefits of this zone include an improved $V_{\mathrm {O} _{2}}$ maximum (the highest amount of oxygen one can consume during exercise) and thus an improved cardio-respiratory system, and a higher lactate tolerance ability which means your endurance will improve and you'll be able to fight fatigue better. This is a high intensity zone burning more food energy, 15% from fat.

Red Line (Maximum Effort) — 90–100% of maximum heart rate: Although this zone burns the highest amount of food energy, it is very intense. Most people can only stay in this zone for short periods. You should only train in this zone if you have a very high fitness level and have been cleared by a physician to do so.

Recovery heart rate

The recovery heart rate is one that is taken several minutes after exercise. It is taken anywhere between 2–10 minutes after exercise. It is taken for 15 seconds, and is multiplied by four in order to calculate beats per minute (bpm). The goal is to not exceed 120 bpm.

Target heart rate

Target heart rate (THR), or training heart rate, 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 on one's physical condition, age, and previous training. Below are two ways to calculate one's Target Heart Rate. In each of these methods, there is an element called "intensity" which is expressed as a percentage. THR can be calculated by using a range of 50%–85% intensity.

Karvonen method

The Karvonen method is more accurate, factoring in resting heart rate (RHR):

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 = 130; ± 5 → 125 to 135 bpm
Zone 4 (tough exercise): 180 - 20 = 160; ± 5 → 155 to 165 bpm

Heart rate reserve

Heart rate reserve (HRR) is a term used to describe the difference between a person's resting heart rate and maximum heart rate. Some methods of measurement of exercise intensity measure percentage of heart rate reserve. Additionally, as a person becomes fit, as their HR_rest will drop, the heart rate reserve will increase.

HRR = HR_max − HR_rest

Heart rate abnormalities

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 adults.

Bradycardia

Bradycardia is defined as a heart rate less than 60 beats per minute although it is seldom symptomatic until below 50 bpm. 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. Again, this number can vary as smaller people and children have faster heart rates than adults.

References

^ Erna Obenza Nishime, MD; Christopher R. Cole, MD; Eugene H. Blackstone, MD; Fredric J. Pashkow, MD; Michael S. Lauer, MD: "Heart Rate Recovery and Treadmill Exercise Score as Predictors of Mortality in Patients Referred for Exercise ECG", JAMA, 2000;284:1392-1398
^ http://www.cdc.gov/nccdphp/dnpa/physical/measuring/target_heart_rate.htm
^ http://www.sarkproducts.com/sally1.htm