Human Cardiovascular system - Heart-rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute (BPM), it can vary as the body!!!s need for oxygen changes, such as during exercise or sleep. The measurement of heart rate is used by medical professionals to assist in the diagnosis and tracking of medical conditions. It is also used by individuals, such as athletes, who are interested in monitoring their heart rate to gain maximum efficiency from their training. Heart rate is measured by finding the pulse of the body. This pulse rate can be measured at any point on the body where an artery!!!s pulsation is transmitted to the surface - often as it is compressed against an underlying structure like bone - by pressuring it with the index and middle finger. The thumb should not be used for measuring another person!!!s heart rate, as its strong pulse may interfere with discriminating the site of pulsation Some commonly palpated sites include:
The ventral aspect of the wrist on the side of the thumb (radial artery)
The ulnar artery
The neck (carotid artery),
The inside of the elbow, or under the biceps muscle (brachial artery)
The groin (femoral artery)
Behind the medial malleolus on the feet (posterior tibial artery)
Middle of dorsum of the foot (dorsalis pedis).
Behind the knee (popliteal artery)
Over the abdomen (abdominal aorta)
The chest (aorta), which can be felt with one!!!s hand or fingers. However, it is possible to auscultate the heart using a stethoscope.
The lateral edge of the mandible
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. 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).
The most accurate way of measuring HRmax for an individual is via a cardiac stress test. In such a test, the subject exercises while being monitored by an EKG. 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), or until certain changes in heart function are detected in the EKG, at which point the subject is directed to stop. Typical durations of such a test range from 10 to 20 minutes. Conducting a maximal exercise test can require expensive equipment. If you are just beginning an exercise regimen, you should only perform this test in the presence of medical staff due to risks associated with high heart rates. Instead, people typically use a formula to estimate their individual Maximum Heart Rate.
Formula for HRmax
Formula for HRmax
Various formulas are used to estimate individual Maximum Heart Rates, based on age, but maximum heart rates vary significantly between individuals. Even within a single elite sports team, such as Olympic rowers in their 20s, maximum heart rates can vary from 160 to 220. This variation is as large as a 60 or 90 year age gap by the linear equations given below, and indicates the extreme variation about these average figures. The most common formula encountered, with no indication of standard deviation, is: HRmax = 220 ? age This is attributed to various sources, often Fox and Haskell, and was 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 it was never supposed to be an absolute guide to rule people!!!s training. While 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 b/min). 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 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 , and prescription of exercise training HR ranges) 2) The formula deemed least objectionable was: HRmax = 205.8 ? (0.685 age) This was found to have a standard deviation that, although large (6.4 bpm), was still considered 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) HRmax = 217 ? (0.85 age) (Often attributed to Miller et al. from Indiana University) HRmax = 208 ? (0.7 age) (Another tweak to the traditional formula is known as the Tanaka method. Based on a study of literally thousands of individuals, a new formula was devised which is believed to be more accurate ) 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 formulaHRmax = 206.9 ? (0.67 age)and a nonlinear equationHRmax = 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. These figures are very much 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., etc. may each have predicted heart rates of 180 (= 220-Age), but these two people could have actual Max HR 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 Max HR 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.
Recovery heart rate
This is the heart rate measured at a fixed (or reference) period after ceasing activity; typically measured over a 1 minute period. For death, it has been hypothesized* that a delayed fall in the heart rate after exercise might be an important prognostic marker. Less than 30 bpm reduction at one minute after stopping hard exercise was a predictor of heart attack. More than 50 bpm reduction showed reduced risk of heart attack. 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. Devices with built in accelletry and heart rate sensors can automatically measure heart rate recovery. For instance the BioHarness can be used to measure heart recovery as the device also logs and transmits the vector magnitude units of the person acceleration. The acceleration number is used to determine when the person is moving and at rest and hence detect the start time for the heart rate recovery period.
Target heart rate
The 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, gender, 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. 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 above). Example for someone with a HRmax of 180 (age 40, estimating HRmax as 220 ? age): 65% intensity: (220 ? (age = 40)) * 0.65 ? 117 bpm 85% intensity: (220 ? (age = 40)) * 0.85 ? 153 bpm
The Karvonen method factors in Resting Heart Rate (HRrest) to calculate Target Heart Rate (THR), using a range of 50%85%: THR = ((HRmax ? HRrest) %Intensity) + HRrest Example for someone with a HRmax of 180 and a HRrest of 70: 50% intensity: ((180 ? 70) 0.50) + 70 = 125 bpm 85% intensity: ((180 ? 70) 0.85) + 70 = 163 bpm
An alternative to the Karvonen method is the Zoladz method, which derives exercise zones by subtracting values from HRmax. THR = HRmax 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 HRmax of 180: Zone 1 (easy exercise) : 180 ? 50 + 5 ? 135 bpm Zone 4 (tough exercise): 180 = 20 + 5 ? 165 bpm
Heart rate reserve
Heart rate reserve (HRR) is a term used to describe 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 HRR is equivalent to percentage of VO2 reserve. HRR = HRmax ? HRrest
Heart rate abnormalities
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.
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. 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 children and small adults tend to have faster heart rates than average adults. 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.
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.
Heart rate as a risk factor
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.