event occurred, but the actors present onstage at the time of the stressor reacted in the following 5 minute period (demonstrated by their increasingly elevated heart rate). this trend regarding stress and heart rate is supported by previous studies; negative emotion/stimulus has a prolonged effect on heart rate in individuals who are directly impacted. in regard to the characters present onstage, a reduced startle response has been associated with a passive defense, and the diminished initial heart rate response has been predicted to have a greater tendency to dissociation. current evidence suggests that heart rate variability can be used as an accurate measure of psychological stress and may be used for an objective measurement of psychological stress. factors decreasing heart rate the heart rate can be slowed by altered sodium and potassium levels, hypoxia, acidosis, alkalosis, and hypothermia. the relationship between electrolytes and hr is complex, but maintaining electrolyte balance is critical to the normal wave of depolarization. of the two ions, potassium has the greater clinical significance. initially, both hyponatremia (low sodium levels) and hypernatremia (high sodium levels) may lead to tachycardia. severely high hypernatremia may lead to fibrillation, which may cause cardiac output to cease. severe hyponatremia leads to both bradycardia and other arrhythmias. hypokalemia (low potassium levels) also leads to arrhythmias, whereas hyperkalemia (high potassium levels) causes the heart to become weak and flaccid, and ultimately to fail. heart muscle relies exclusively on aerobic metabolism for energy. severe myocardial infarction (commonly called a heart attack) can lead to a decreasing heart rate, since metabolic reactions fueling heart contraction are restricted. acidosis is a condition in which excess hydrogen ions are present, and the patient's blood expresses a low ph value. alkalosis is a condition in which there are too few hydrogen ions, and the patient's blood has an elevated ph. normal blood ph falls in the range of 7.35–7.45, so a number lower than this range represents acidosis and a higher number represents alkalosis. enzymes, being the regulators or catalysts of virtually all biochemical reactions – are sensitive to ph and will change shape slightly with values outside their normal range. these variations in ph and accompanying slight physical changes to the active site on the enzyme decrease the rate of formation of the enzyme-substrate complex, subsequently decreasing the rate of many enzymatic reactions, which can have complex effects on hr. severe changes in ph will lead to denaturation of the enzyme. the last variable is body temperature. elevated body temperature is called hyperthermia, and suppressed body temperature is called hypothermia. slight hyperthermia results in increasing hr and strength of contraction. hypothermia slows the rate and strength of heart contractions. this distinct slowing of the heart is one component of the larger diving reflex that diverts blood to essential organs while submerged. if sufficiently chilled, the heart will stop beating, a technique that may be employed during open heart surgery. in this case, the patient's blood is normally diverted to an artificial heart-lung machine to maintain the body's blood supply and gas exchange until the surgery is complete, and sinus rhythm can be restored. excessive hyperthermia and hypothermia will both result in death, as enzymes drive the body systems to cease normal function, beginning with the central nervous system. physiological control over heart rate dolphin heart rate graph a study shows that bottlenose dolphins can learn – apparently via instrumental conditioning – to rapidly and selectively slow down their heart rate during diving for conserving oxygen depending on external signals. in humans regulating heart rate by methods such as listening to music, meditation or a vagal maneuver takes longer and only lowers the rate to a much smaller extent. in different circumstances heart rate (hr) (top trace) and tidal volume (vt) (lung volume, second trace) plotted on the same chart, showing how heart rate increases with inspiration and decreases with expiration. heart rate is not a stable value and it increases or decreases in response to the body's need in a way to maintain an equilibrium (basal metabolic rate) between requirement and delivery of oxygen and nutrients. the normal sa node firing rate is affected by autonomic nervous system activity: sympathetic stimulation increases and parasympathetic stimulation decreases the firing rate. resting heart rate normal pulse rates at rest, in beats per minute (bpm): newborn(0–1 months old) infants(1 – 11 months) children(1 – 2 years old) children(3 – 4 years) children(5 – 6 years) children(7 – 9 years) children over 10 years& adults, including seniors well-trainedadult athletes 70-190 80–160 80-130 80-120 75–115 70–110 60–100 40–60 the basal or resting heart rate (hrrest) is defined as the heart rate when a person is awake, in a neutrally temperate environment, and has not been subject to any recent exertion or stimulation, such as stress or surprise. the available evidence indicates that the normal range for resting heart rate is 50-90 beats per minute. this resting heart rate is often correlated with mortality. for example, all-cause mortality is increased by 1.22 (hazard ratio) when heart rate exceeds 90 beats per minute. the mortality rate of patients with myocardial infarction increased from 15% to 41% if their admission heart rate was greater than 90 beats per minute. ecg of 46,129 individuals with low risk for cardiovascular disease revealed that 96% had resting heart rates ranging from 48 to 98 beats per minute. finally, in one study 98% of cardiologists suggested that as a desirable target range, 50 to 90 beats per minute is more appropriate than 60 to 100. the normal resting heart rate is based on the at-rest firing rate of the heart's sinoatrial node, where the faster pacemaker cells driving the self-generated rhythmic firing and responsible for the heart's autorhythmicity are located. for endurance athletes at the elite level, it is not unusual to have a resting heart rate between 33 and 50 bpm. maximum heart rate the maximum heart rate (hrmax) is the age-related highest number of beats per minute of the heart when reaching a point of exhaustion without severe problems through exercise stress. in general it is loosely estimated as 220 minus one's age. it generally decreases with age. since hrmax varies by individual, the most accurate way of measuring any single person's hrmax is via a cardiac stress test. in this test, a person is subjected to controlled physiologic stress (generally by treadmill or bicycle ergometer) while being monitored by an electrocardiogram (ecg). the intensity of exercise is periodically increased until certain changes in heart function are detected on the ecg monitor, at which point the subject is directed to stop. typical duration of the test ranges ten to twenty minutes. the theoretical maximum heart rate of a human is 300 bpm, however there have been multiple cases where this theoretical upper limit has been exceeded. the fastest human ventricular conduction rate recorded to this day is a conducted tachyarrhythmia with ventricular rate of 480 beats per minute, which is comparable to the heart rate of a mouse. adults who are beginning a new exercise regimen are often advised to perform this test only in the presence of medical staff due to risks associated with high heart rates. for general purposes, a formula is often employed to estimate a person's maximum heart rate. however, these predictive formulas have been criticized as inaccurate because they generalized population-averages and usually focus on a person's age and do not even take normal resting pulse rate into consideration. it is well-established that there is a "poor relationship between maximal heart rate and age" and large standard deviations relative to predicted heart rates. (see § limitations.) the various formulae provide slightly different numbers for the maximum heart rates by age. a number of formulas are used to estimate hrmax. haskell & fox (1970) fox and haskell formula; widely used. notwithstanding later research, the most widely cited formula for hrmax (which contains no reference to any standard deviation) is still: hrmax = 220 − age although attributed to various sources, it is widely thought to have been 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 the formula "was never supposed to be an absolute guide to rule people's training." while it is 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 bpm. consequently, the estimation calculated by hrmax = 220 − age has neither the accuracy nor the scientific merit for use in exercise physiology and related fields. yet, it is common practice to use 85% of the predicted hrmax (haskell & fox) to evaluate chronotropic response to exercise. an ongoing study has found that the 5th percentile of the study cohort maximal heart rate correlates much better with the 85% of predicted hrmax (haskell & fox) than the 5th percentile of the study cohort heart rate reserve to the 80% of the predicted heart rate reserve. tanaka, monahan, & seals (2001) from tanaka, monahan, & seals (2001): hrmax = 208 − (0.7 × age) their meta-analysis (of 351 prior studies involving 492 groups and 18,712 subjects) and laboratory study (of 514 healthy subjects) concluded that, using this equation, hrmax was very strongly correlated to age (r = −0.90). the regression equation that was obtained in the laboratory-based study (209 − 0.7 x age), was virtually identical to that of the meta-study. the results showed hrmax to be independent of gender and independent of wide variations in habitual physical activity levels. this study found a standard deviation of ~10 beats per minute for individuals of any age, meaning the hrmax formula given has an accuracy of ±20 beats per minute. robergs & landwehr (2002) a 2002 study of 43 different formulas for hrmax (including that of haskell and fox – see above) published in the journal of exercise psychology concluded that: no "acceptable" formula currently existed (they used the term "acceptable" to mean acceptable for both prediction of vo2, and prescription of exercise training hr ranges) the least objectionable formula (inbar, et al., 1994) was: hrmax = 205.8 − (0.685 × age) this had a standard deviation that, although large (6.4 bpm), was considered acceptable for prescribing exercise training hr ranges. wohlfart, b. and farazdaghi, g.r. (2003) a 2003 study from lund, sweden gives reference values (obtained during bicycle ergometry) for men: hrmax = 203.7 / ( 1 + exp( 0.033 × (age − 104.3) ) ) and for women: hrmax = 190.2 / ( 1 + exp( 0.0453 × (age − 107.5) ) ) oakland university (2007) in 2007, researchers at the oakland university analyzed maximum heart rates of 132 individuals recorded yearly over 25 years, and produced a linear equation very similar to the tanaka formula, hrmax = 207 − (0.7 × age), and a nonlinear equation, hrmax = 192 − (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 gulati (for women) (2010) research conducted at northwestern university by martha gulati, et al., in 2010 suggested a maximum heart rate formula for women: hrmax = 206 − (0.88 × age) nes, et al. (2013) based on measurements of 3320 healthy men and women aged between 19 and 89, and including the potential modifying effect of gender, body composition, and physical activity, nes et al found: hrmax = 211 − (0.64 × age) this relationship was found to hold regardless of gender, physical activity status, maximal oxygen uptake, smoking, or body mass index. however, a standard error of the estimate of 10.8 beats/min must be accounted for when applying the formula to clinical settings, and the researchers concluded that actual measurement via a maximal test may be preferable whenever possible. limitations maximum heart rates vary significantly between individuals. even within a single elite sports team,