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. Ususally it is calculated as the number of contractions (heart beats) of the heart in one minute. As such heart rate is usually expressed as "beats per minute" (bpm). When resting, the adult human heart beats at about 70 bpm (males) and 75 bpm (females), but this rate varies between people. However, the reference range is taken between 60 bpm (if less termed bradycardia) and 100 bpm (if greater, termed tachycardia).
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 causes a normal person's heart rate to increase above the resting heart rate. As the physical activity becomes more vigorous, the heart rate increases more. With very vigorous exercise, a maximum heart rate can be reached.
The pulse is the most straightforward way of measuring the heart rate, but it can be deceptive when some strokes do not lead to much cardiac output. In these cases (as happens in some arrhythmias), the heart rate can be (much) higher than the pulse.
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Control of heart rate
The heart contains cardiac pacemakers that spontaneously cause the heart to beat. These can be controlled by the autonomic nervous system and circulating adrenaline. The heart beats more quickly than average in an obese person, and less quickly than average in athletes.
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).
- A electrocardiograph, or ECG,is usually the most precise method of heart rate measurement. Continuous electrocardiographic monitoring of the heart rate is routinely done in many clinical settings, especially in critical care medicine.
- Another method of measuring heart rate is using a commercially available heart rate monitor. These are specialised wearable electrocardiographic monitors 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 amplified through the use of a stethoscope.
Heart Rate Variability (HRV)
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.
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.
Resting Heart Rate (RHR, HRrest)
Resting Heart Rate is a term used to desribe a person's heart rate when they are not performing any activities. It is best measured first thing in the morning, before even getting out of bed.
A person's HRrest is likely to increase as they age. Normal rates are in the range from 60 to 80. Fit people generally have lower HRrest, so it is often used as a measure of fitness.
Measuring HRrest every day is a good way of detecting possible illness, as the RHR will be elevated by 8-10 beats if the immune system is attempting to fight something.
Maximum Heart Rate (MHR, HRmax)
Maximum Heart Rate (also called MHR, or HRmax) is the maximum heart rate that a person can achieve during maximal physical exertion. Research indicates it is most closely linked to a person's age; a person's HRmax 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 slower it decline as they age.
It is an important measure, as it used frequently in the fitness industry, specifically during the calculation of Target Heart Rate when prescribing a fitness regimen.
Measuring HRmax
The most accurate way of measuring HRmax for an individual is via a maximal exercise 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 (they will hit their HRmax just before exhaustion), 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.
As conducting an accurate maximal exercise test requires expensive equipment, and should only be performed in the presense of medical staff due to risks associated with high heart rates, people typically use predictive formulae to estimate thier individual Maximum Heart Rate.
The most common formula encountered is:
- HRmax = 220 - age
This is often attributed to various sources, including "Fox and Haskell". While the most common (and easy to remember and calculate), this particular forumla is not considered by some to be a good predictor of HRmax.
A 2003 study [1] of 43 different formulae for HRmax (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 VO2max, and prescription of exercise training HR ranges)
2) The most accurate formula of those examined was:
- HRmax = 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:
- HRmax = 206.3 - (0.711 * Age)
(Often attributed to "Londeree and Moeschberger from the University of Missouri-Columbia")
- HRmax = 217 - (0.85 * Age)
(Often attributed to "Miller et al. from Indiana University")
Target Heart Rate (THR)
Also occasionally called "Training Heart Rate", a 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 three 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.
Standard Method
The most common method for calculating THR. This is calculated by multiplying the Maximum Heart Rate (HRmax) (see above) times the % intensity.
THR = HRmax x %Intensity
Example for someone with a HRmax of 180:
50% Intensity: 180 x 0.50 = 90 bpm
85% Intensity: 180 x 0.85 = 153 bpm
Karvonen Method
The Karvonen Method is more accurate, factoring in Resting Heart Rate (RHR) (see above) into the equation. The Karvonen Method multiplies the difference between the MHR and the RHR by the Intensity, and then adds that to the RHR.
THR = ((HRmax- HRrest) x %Intensity) + HRrest
Example for someone with a HRmax of 180 and a HRrest of 70:
((180 - 70) x 0.50) + 70 = 125 bpm
((180 - 70) x 0.85) + 70 = 163.5 bpm
Zoladz Method
An alternative for the Karvonen Method is the Zoladz Method. This method only takes into account the HRmax. Exercise zones are constructed by subtracting values from HRmax.
Example for someone with a HRmax of 180:
Easy exercise zone : 180 - 55 to 180 - 45 --> 125 to 135 bpm
Tough exercise zone : 180 - 25 to 180 - 15 --> 155 to 165 bpm
Source :Heart Rate Monitor Training : The Zoladz Method
Heart Rate Reserve (HRR)
Heart Rate Reserve (HRR) is a term used to desribe 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 HRrest will drop, the Heart Rate Reserve will increase.
The formula for calculate Heart Rate Reserve:
- HRR = HRmax - HRrest
Heart rate abnormalities
Tachycardia
Main article: Tachycardia
A tachycardia is a resting heart rate more than 100 beats per minute.
Bradycardia
Main article: 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.