Heart rate
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The heart rate is the number of contractions of the heart in one minute. It is measured in 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.
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.
Systole of the heart is induced as the sarcolemma of the myocardial cells of the sinoatrial node slowly depolarises beyond the threshold. At this point, the voltage-gated calcium channels open and allow calcium ions to pass through, into the sarcoplasm of the muscle cell. Some calcium ions bind to the receptors on the sarcoplasmic reticulum causing their intrinsic calcium channels to open and an influx of calcium ions into the sarcoplasm results. The calcium ions bind to the troponin, causing a conformation change, breaking the bond between tropomyosin, to which the troponin is attached, and the myosin binding sites. This allows the myosin heads to bind to the myosin binding sites on the actin filament and contraction results as the myosin heads draw the actin filaments along, are bound by ATP, causing them to release the actin, and return to their original position, breaking down the ATP into ADP and a phosphate group. The action potential spreads via the passage of sodium ions through the gap junctions that connect the sarcoplasm of adjacent myocardial cells.
Noradrenaline is released by the terminal boutons of depolarised sympathetic fibres, at the sinoatrial and atrioventricular nodes. Noradrenaline diffuses across the synaptic cleft binds to the β1-adrenoreceptors – G-protein linked receptors, consisting of seven transmembrane domains – shifting their equilibrium towards the active state. The receptor changes its conformation and mechanically activates the G-protein which is released. The G-protein is involved in the production of cyclic adenyl monophosphate (cAMP) from adenyl triphosphate (ATP) and this in turn activates the protein kinase (β-adrenoreceptor kinase). β-adrenoreceptor kinase phosphorylates the calcium ion channels in the sarcolemma, so that calcium ion influx is increased when they are activated by the appropriate transmembrane voltage. This will of course, cause more of the calcium receptors in the sarcoplasmic reticulum to be activated, creating a larger flow of calcium ions into the sarcoplasm. More troponin will be bound and more myosin binding sites cleared [of tropomyosin] so that more myosin heads can be recruited for the contraction and a greater force and speed of contraction results. [Phosphodiesterase catalyses the decomposition of cAMP to AMP so that it is no longer able to activate the protein kinase. AMP will of course, go on to be phosphorylated to ATP and may be recycled.] Noradrenaline also affects the atrioventricular node, reducing the delay before continuing conduction of the action potential via the bundle of His.
Measuring heart rate
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- 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 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).
Heart rate variability
Heart rate variability (HRV) is the variation of beat-to-beat intervals. A healthy heart in resting state has a large HRV, while decreased or absent variability may indicate cardiac disease. HRV also decreases with exercise-induced tachycardia.
Resting Heart Rate (RHR)
This is a person's heart rate when they are not doing any activities. It is usually taken first thing in the morning, before even getting out of bed. Measuring the RHR 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)
This is the maximum heart rate that a person is capable of. It is linked to age, current fitness level, and previous years of training.
Measuring MHR
The only accurate way of measuring MHR is for a person to exercise and increase the intensity until they can no longer continue - they will hit their MHR just before exhaustion. A less painful way is to use the formula:
- MHR = 220 - Age
The relationship is somewhat more complex, and the following linear scales have been developed:
Londeree and Moeschberger from the University of Missouri-Columbia:
- MHR = 206.3 - (0.711 * Age)
Miller et al from Indiana University:
- MHR = 217 - (0.85 * Age)
However, these formulae are only heuristic guidelines. They may often be incorrect, sometimes by up to 10 beats.
Exercise Type
The MHR an individual can achieve is based on the type of exercise being undertaken. Generally exercise where the whole body weight is used (e.g. running) have higher MHR than when the body is completely or partially supported (e.g. cycling or swimming).
Relative to treadmill running:
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 physcial 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.
Standard Method
The most common method for calculating THR. This is calculated by multiplying the Maximum Heart Rate (MHR) (see above) times the % intensity.
THR = MHR x %Intensity
Example for someone with a MHR 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 = ((MHR - RHR) x %Intensity) + RHR
Example for someone with a MHR of 180 and a RHR of 70:
((180 - 70) x 0.50) + 70 = 125 bpm
((180 - 70) x 0.85) + 70 = 163.5 bpm
Heart rate abnormalities
Tachycardia
Main article: Tachycardia
A tachycardia is a 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.