Heart rate

Heart rate is the number of heartbeats per unit of time - typically expressed as beats per minute (bpm) - which can vary as the body's need to absorb oxygen and excrete carbon dioxide 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. The R wave to R wave interval (RR interval) is the inverse of the heart rate.

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.^[1]

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 heart), which can be felt with one's hand or fingers. However, it is possible to auscultate the heart using a stethoscope.
11. The temple (Superficial Temporal Artery)
12. The lateral edge of the Mandible Facial artery.

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).

Measuring HR_max

HR_max is the maximal safe heart rate for an individual. 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 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), continuing 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. 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.

Formula for HR_max

Fox and Haskell formula; widely used.

Various formulas are used to estimate individual Maximum Heart Rates, based on age, but maximum heart rates vary significantly between individuals.^[4] Even within a single elite sports team, such as Olympic rowers in their 20s, maximum heart rates can vary from 160 to 220.^[4] 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:

HR_max = 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.^[4] 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.^[5] It gained widespread use through being used by Polar Electro in its heart rate monitors,^[4] which Dr. Haskell has "laughed about",^[4] as it "was never supposed to be an absolute guide to rule people's training."^[4]

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 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.^[5]

A 2002 study^[5] 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 , and prescription of exercise training HR ranges)

2) The formula deemed least objectionable was:

HR_max = 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:

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")

HR_max = 208 − (0.7 × age)

(Another "tweak" to the traditional formula is known as the Tanaka method. Based on a study of thousands of individuals, a new formula was devised which is believed to be more accurate).^[6]

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 equations—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).^[7]

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):^[4] the range is extremely broad, and some say "The heart rate is probably the least important variable in comparing athletes."^[4]

The 2010 research conducted at Northwestern University revised maximum heart rate formula for women. According to Martha Gulati et al. it is:

HR_max = 206 − (0.88 × age)^[8][9]

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)))^[10]

and for women

HR_max = 190.2/(1 + exp (0.0453 * (Age - 107.5)))^[11