ArticlePDF Available

The Effects of Emotions on Short-Term Power Spectrum Analysis of Heart Rate Variability

  • HearthMath Institute


In summary, this work extends previous findings by demonstrating that anger produces a sympathetically dominated power spectrum, whereas appreciation produces a power spectral shift toward MF and HF activity. Results suggest that positive emotions lead to alterations in HRV, which may be beneficial in the treatment of hypertension and in reducing the likelihood of sudden death in patients with congestive heart failure and coronary artery disease.
The Effects of Emotions on Short-Term Power
Spectrum Analysis of Heart Rate Variability
Rollin McCraty
, Mike Atkinson
, William Tiller
, Glen Rein
, and Alan D. Watkins
VOL. 76 • NO. 14 • NOVEMBER 15, 1995 • PAGES 1089-1093
This study utilizes heart rate variability analysis to examine a new method of intentionally
shifting emotional states, and demonstrates that positive emotions lead to alterations in
sympathovagal balance that may be beneficial in the treatment of hypertension. Anger, on the
other hand, was shown to significantly increase sympathetic activation.
From the Institute of HeartMath, Research Division, Electrophysiology Lab. Please direct correspondence to:
(1) Rollin McCraty, IHM, 14700 West Park Avenue, Boulder Creek, CA, 95006, 408-338-8700, fax 408-338-1182,
(2) Stanford University, Stanford, CA 94305-2205
(3) Dept. University Medicine, Southhampton General Hospital, Southampton, England
... We also found that although the respiratory, beat-to-beat blood pressure and heart rhythms remained frequency entrained, the phase relationships among them were different when one intentionally changed their breathing rhythm vs normal physiological functioning. We also found that the natural entrainment frequency could and did vary depending on the positive emotion they were experiencing (McCraty et al., 1995a(McCraty et al., , 1995b. We published a study in the American Journal of Cardiology showing how states of anger and appreciation are reflected in the HRV patterns and power spectra (McCraty et al., 1995b). ...
... We also found that the natural entrainment frequency could and did vary depending on the positive emotion they were experiencing (McCraty et al., 1995a(McCraty et al., , 1995b. We published a study in the American Journal of Cardiology showing how states of anger and appreciation are reflected in the HRV patterns and power spectra (McCraty et al., 1995b). It is important to note that in this study (and several others), (McCraty et al., 1995a(McCraty et al., , 1995bTiller et al., 1996) participants were not instructed to alter their breathing rhythm, rather, they were instructed to selfinduced various emotions. ...
... We published a study in the American Journal of Cardiology showing how states of anger and appreciation are reflected in the HRV patterns and power spectra (McCraty et al., 1995b). It is important to note that in this study (and several others), (McCraty et al., 1995a(McCraty et al., , 1995bTiller et al., 1996) participants were not instructed to alter their breathing rhythm, rather, they were instructed to selfinduced various emotions. We found that participant's respiration rhythms were changing in response to the emotions they were experiencing rather than consciously altering their breathing. ...
Full-text available
This paper outlines the early history and contributions our laboratory, along with our close advisors and collaborators, has made to the field of heart rate variability and heart rate variability coherence biofeedback. In addition to the many health and wellness benefits of HRV feedback for facilitating skill acquisition of self-regulation techniques for stress reduction and performance enhancement, its applications for increasing social coherence and physiological synchronization among groups is also discussed. Future research directions and applications are also suggested.
... Because either an increase in sympathetic cardiac control, a decrease in parasympathetic control, or both (60) tends to be associated with stressors, HRV analysis with ECG data is often used to identify psychological, emotional, and mental activities (52,61,50,57,62,63,64). We employ the standard Fast Fourier transformation (FFT) to calculate the ratio of activity in the low frequency band to activity in the high frequency band (i.e., the LF/HF ratio). ...
Full-text available
Self-reported subjective judgments about well-being, mood, or mental state are at the core of analytical and empirical tools in many social sciences. However, technological advances (particularly in neuroscience) are opening new ways of monitoring physiological processes through non-intrusive means. Such dense continuous data provide new and fruitful avenues for complementing self-reported data with a better understanding of human dynamics and human interactions. Thus, in this study, we follow more than 300 individuals over a period of 24 hours, mapping their core activities (5,000 recorded activities in total) with measurements of their heart rate variability (HRV) and their assessment of subjective hedonic well-being (positive affect). Our results indicate a robust correlation between physiological measurements and self-reported affect. A lower HRV is positively correlated with self-reported positive affect. We also find that physical activities as well as relaxation are important for well-being, no matter what sort of actions follow, increasing positive feelings and inducing positive spillover effects on later endeavours. Individuals also report higher positive feelings prior to performing physical activities, providing new insights on how to harness activity data to extrapolate to the future.
... The activity and relative level of these two cardiac neural systems cause the main dynamical changes of heart rate (HR) in response to external stimulus, and on a shorter time scale, the fluctuations of the heart R-wave to R-wave (RR) time interval known as heart rate variability (HRV). HRV during rest has been shown to be influenced by psychological [3] as well as physiological factors such as age, body mass index, diseases [4], heart functions and heart diseases [5,6], body position [7] and physical fitness [8,9]. During physical exercise, HRV dynamics is drastically modified due to the break of the balance between both branches of the autonomic nervous system. ...
Full-text available
The present study proposes to measure and quantify the heart rate variability (HRV) changes during effort as a function of the heart rate and to test the capacity of the produced indices to predict cardiorespiratory fitness measures. Therefore, the beat-to-beat cardiac time interval series of 18 adolescent athletes (15.2 ± 2.0 years) measured during maximal graded effort test were detrended using a dynamical first-order differential equation model. HRV was then calculated as the standard deviation of the detrended RR intervals (SDRR) within successive windows of one minute. The variation of this measure of HRV during exercise is properly fitted by an exponential decrease of the heart rate: the SDRR is divided by 2 every increase of heart rate of 20 beats/min. The HR increase necessary to divide by 2 the HRV is linearly inversely correlated with the maximum oxygen consumption (r = -0.60, p = 0.006), the maximal aerobic power (r = -0.62, p = 0.006), and, to a lesser extent, to the power at the ventilatory thresholds (r = -0.53, p = 0.02 and r = -0.47, p = 0.05 for the first and second threshold). It indicates that the decrease of the HRV when the heart rate increases is faster among athletes with better fitness. This analysis, based only on cardiac measurements, provides a promising tool for the study of cardiac measurements generated by portable devices.
... Beat-to-beat spontaneous fluctuations in heart rate (HR) or oscillations in consecutive R-R intervals (heart rate variability -HRV) depend on autonomic nervous system regulation of the cardiac function and pacemaker properties of myocardium [1]. The analysis of HRV is recognized as a powerful non-invasive assessment that reflects brain -heart interaction [2] and in particular sympathetic and parasympathetic nervous control of the myocardium at rest [3] and in different physiopathological conditions, such as physical activity [4,5], mental stress [6] and emotional stress [7]. ...
Conference Paper
Full-text available
Abstract. Dehydration is a stress condition that induces an increase in sympathetic outflow. The aim of this study was to determine weather mild dehydration affects HRV in resting subjects. Sixteen young subjects rested supine for one hour, before and after a 12.5 km walk. During the walk, 8 subjects drank water ad libitum (group A) and the other 8 were restricted from drinking (group B). During the last 30 min of rest, both before and after the walk, heart rate (HR) was recorded continuously for 15 min. From the tachograms of 8 subjects (4 subjects of group A and 4 subjects of group B) FFT and autoregressive spectra, as well as Poincaré plot analysis were done. During the walk, subjects lost on average 0.8% of body mass in group A and 2.2% of body mass in group B. Despite mass loss, no significant changes were observed in either HR, power spectra or Poincaré plot parameters in any of the two groups. We conclude that mild dehydration associated with up to 2.2 % mass loss, did not alter the autonomic nervous regulation of the cardiac function in healthy supine subjects.
... These changes in the successive heartbeats are an integral component of physiological research and are becoming more standardized in the field (Laborde et al., 2017;Berntson et al., 1997;Malik, 1996) to provide diagnostic information in certain medical conditions (Yeh et al., 2009;Gronwald et al., 2019) and insights into the parasympathetic nervous system (Malik, 1996). Past studies have shown that shifts in HRV can be induced by a variety of different types of affective stimuli such as watching positive and negative videos (Barquero-Pérez et al., 2020;Ghiasi et al., 2020), the Cold Pressor Stress Test (Ghiasi et al., 2018;Ghiasi et al., 2020), and imagining scenarios which evoke an emotional response (McCraty et al., 1995). Using these different affective induction techniques, researchers have proposed various models to measure changes in the autonomic nervous system in reaction to an affective stimulus. ...
Full-text available
Heart Rate Variability (HRV) can be a useful metric to capture meaningful information about heart function. One of the non-linear indices used to analyze HRV, Detrended Fluctuation Analysis (DFA), finds short and long-term correlations in RR intervals to capture quantitative information about variability. This study focuses on the impact of visual and mental stimulation on HRV as expressed via DFA within healthy adults. Visual stimulation can activate the automatic nervous system to directly impact physiological behavior such as heart rate. In this investigation of HRV, 70 participants (21 males) viewed images on a screen followed by a math and recall task. Each viewing segment lasted 2 min and 18 s. The math and memory recall task segment lasted 4 min total. This process was repeated 9 times during which the participants’ electrocardiogram was recorded. 37 participants (12 males) opted in for an additional 24-h Holter recording after the viewing and task segments of the study were complete. Participants were randomly assigned to either a pure (organized image presentation) or mixed (random image presentation) image regime for the viewing portion of the study to investigate the impact of the external environment on HRV. DFA α1 was extracted from the RR intervals. Our findings suggest that DFA α1 can differentiate between the viewing [DFA α1 range from 0.96 (SD = 0.25) to 1.08 (SD = 0.22)] and the task segments [DFA α1 range from 1.17 (SD = 0.21) to 1.26 (SD = 0.25)], p < 0.0006 for all comparisons. However, DFA α1 was not able to distinguish between the two image regimes. During the 24-hour follow up, participants had an average DFA α1 = 1.09 (SD = 0.14). In conclusion, our findings suggest a graded response in DFA during short term stimulation and a responsiveness in participants to adjust physiologically to their external environment expressed through the DFA exponent.
Purpose: To evaluate the effectiveness of heart-rate variability (HRV) biofeedback in improving autonomic function, mood, and sleep in elite bobsleigh athletes. Methods: Eight Chinese Winter Olympic bobsleigh athletes (age: 24 [2] y, body mass: 89 [15] kg, and height: 184 [5] cm) completed a randomized crossover study with and without HRV biofeedback before a single night's sleep. HRV biofeedback was provided 35 minutes prior to bedtime in the experimental condition. The assessment of HRV took place 45 and 10 minutes before bedtime. The Profile of Mood States questionnaire was completed 50 and 15 minutes prior to bedtime. Sleep duration and quality were measured through an air-mattress sleep-monitoring system. Results: Sleep efficiency (P = .020; F = 7.831; CI, 0.008 to 0.072) and the percentage of deep sleep duration increased (P = .013; F = 10.875; CI, 0.006 to 0.035), while the percentage of light sleep decreased (P = .034; F = 6.893; CI, -0.038 to -0.002). Presleep HRV biofeedback increased parasympathetic and decreased sympathetic activity. Mood states of anger (P = .006, F = 7.573), panic (P = .031, F = 4.288), tension (P = .011, F = 6.284), depression (P = .010, F = 6.016), fatigue (P = .000, F = 16.901), and total mood disturbance (P = .001, F = 11.225) were reduced before sleep. Conclusion: Presleep HRV biofeedback improved some measures of autonomic function, mood, and sleep quality in Chinese Olympic bobsleigh athletes. Presleep HRV biofeedback provides a practical strategy that may help reduce sleep disturbances during periods of training and competition.
Although numerous agencies in Singapore have recently taken active steps in promoting the importance of emotion regulation and resilience building, there is a paucity of publication on such intervention programs. The purpose of this quantitative experimental study, the first in Singapore, was to examine the effectiveness of the HeartMath System (HMS). Thirty-four healthy participants from all walks of life in Singapore participated and were randomly assigned to the intervention group and the wait-list group. All participants completed the pre- and post- assessments, and the intervention group received a 2.5-h workshop and practiced the HMS for 13 days. The findings were examined with a 2 × 2 mixed factorial MANOVA, univariate analyses, paired sample t-tests, and a Pearson correlational coefficient analysis between heart rate variability (HRV) measurements, personal and organization quality assessment (POQA-R4), sense of coherence (SOC-13), and Zimbardo time perspective inventory (ZTPI). Results showed significant multivariate interactions in HRV (p < .001), SOC-13 (p < .05), and ZTPI (p < .05) measures, all with very large effect sizes. Significant condition x time univariate interactions were observed in five 5-min resting HRV, three 3-min stress-preparation HRV, Relational Tension, Total SOC, SOC (Manageability), BTP (Balanced Time Perspective), PF (Present Fatalistic), and PN (Past Negative), all with very large effect sizes. There were noticeable directional changes and very large effect sizes observed in POQA-R4. Correlational analyses revealed that participants with higher HRV, SOC, and BTP experienced reduced stress and increased resilience. The current findings support the use of the HMS, a research-based resilience-building program consisting of simple, practical techniques that Singaporeans can use “in-the-moment” and “on-the-go,” to help them build coherence and increase emotion regulation flexibility towards resilience.
Forschungsfrage: Im Zentrum der Forschungsbemühungen steht die Lehrerpersönlichkeit, welche durch die Selbstwirksamkeit als Eigenschaft beschrieben werden soll. Methodik: Als Methode wird die qualitative Analyse der Hermeneutik verwendet. Dabei werden Begriffe und Modelle zur Lehrerpersönlichkeit und Selbstwirksamkeit erläutert und in ihrer Bedeutung für das pädagogische Handeln erschlossen. Ergebnis: Selbstwirksamkeit ist eine unbedingt notwendige persönliche Ressource, die regulierend auf kognitive, emotionale, motivationale und selektive Prozesse einwirkt und Entwicklungsprozesse unterstützt. Dennoch sind der Selbstwirksamkeit Grenzen gesetzt, so dass diese nicht als hinreichend für die Beschreibung der gesamten Lehrerpersönlichkeit angesehen werden kann. Wie die zunehmende Beanspruchung auf die Lehrerpersönlichkeit wirkt, wird durch die Belastungsforschung dargelegt. Abgesehen von begrifflichen Kontroversen ist die berufliche Selbstwirksamkeit entscheidend für die Bewältigung schwieriger Situationen. Auch in der Lehrerbildung wird die Bedeutung der Lehrerselbstwirksamkeit hervorgehoben. Neben Einflussmöglichkeiten im schulischen Rahmen stehen Ansatzpunkte im Mittelpunkt, die bei der Lehrerpersönlichkeit ansetzen und durch sie aktiv unterstützt werden können. Zudem wird versucht, aus dem Erkenntnisgewinn Konsequenzen für die Rekrutierung und Ausbildung der Lehramtsstudenten abzuleiten und bildungspolitische Ansprüche zu formulieren, da ein gewisses Maß an Selbstwirksamkeit zu den Mindestvoraussetzungen für den beruflichen Erfolgt gehört.
This paper investigates the correlation between linear and non-linear features of short-term heart rhythm fluctuations during internally and externally operative attention. Posner’s spatial cueing paradigm was used with internal and external peripheral cueing, which provided comprehensive heart beat intervals for analysis. Heart rhythms were acquired noninvasively in fourteen young healthy volunteers (26.4 ± 2.2 years old; 7 females) participated for a 5-min baseline session, six alternating sessions of computer-based internal and external attention task, and followed by 5-min recovery session. Linear features of heart rhythm fluctuations were time and frequency-domain parameters and, non-linear features were the Poincare-plot parameters (SD1, SD2, SD2/SD1 ratio, and ellipse area), which were analyzed for each session using non-parametric Spearman correlation coefficient. The values of SD1 (short-term fluctuations of heart rhythm) were significantly correlated with mean inter-beat intervals (r = 0.701) and mean heart rate (r = −0.701) during internally operative attention, while there was an insignificant correlation during externally operative attention. Although, the values of SD2 (long-term fluctuations of heart rhythm) were found to be significantly correlated with SDNN (r = 0.965 and 0.890), low frequency (r = 0.701 and 0.640) and total power (r = 0.543 and 0.701) of heart rate variability (HRV) during both internally and externally operative attention, respectively. Moreover, the values of the SD2/SD1 ratio were significantly correlated with mean inter-beat intervals (r = −0.618) and mean heart rate (r = 0.618) during internally operative attention, while there was an insignificant correlation during externally operative attention. The values of ellipse area were significantly correlated with mean inter-beat intervals (r = 0.6), high frequency (r = 0.670), total power (r = 0.727) of HRV and were negatively correlated with mean heart rate (r = −0.6) during externally operative attention, while there was an insignificant correlation during internally operative attention. The high frequency of HRV was significantly decreased during internally operative attention (529.39 ± 306.02) as compared to baseline (805.64 ± 477.26). However, the SD2 unfolding the long-term variations in heart rhythm was significantly reduced in both internally and externally operative attention (55.46 ± 11.83 and 58.17 ± 14.01, respectively) as compared to recovery sessions (68.43 ± 20.19). The non-linear features of Poincare plot of inter-beat intervals are significantly correlated with linear features of short-term heart rhythm fluctuations and provide a distinction between internally and externally operative attention.
While the important role of the autonomic nervous system (ANS) has been historically underappreciated, recently there has been a rapid proliferation of empirical, methodological and theoretical progress in our more detailed understanding of the ANS. Previous more simplistic models of the role of the ANS using the construct of homeostasis have been enhanced by the use of the construct of allostasis and a wide variety of technological innovations including wearable and implantable biosensors have led to improved understanding of both basic and applied knowledge. This chapter will explore in particular heart rate variability (HRV) as a rich variable which has developed an extensive literature, beginning with predicting all-cause mortality, but now encompassing a wide variety of disease and illness states; cognitive, affective and behavioral processes and performance optimization. A critical analysis of HRV from the perspective of complex adaptive systems and non-linear processes will be included and innovative future uses of HRV will be described.
Full-text available
Salivary IgA, heart rate and mood were measured in thirty individuals before and after experiencing care or anger. Two methods of inducing the emotional states were compared: self-induction and external induction via video tapes. Anger produced a significant increase in total mood disturbance and heart rate, but not in S-IgA levels. Positive emotions, on the other hand, produced a significant increase in S-IgA levels. Examining the effects over a six hour period we observed that anger, in contrast to care, produced a significant inhibition of S-IgA from one to five hours after the emotional experience. Results indicate that self-induction of positive emotional states is more effective at stimulating S-IgA levels than previously used external methods. Self-induction techniques may therefore be useful in minimizing the immunosuppressive effects of negative emotions.
Full-text available
This study examines the effects of music and positive emotional states on autonomic and immune functions in normal, healthy individuals. Autonomic activity was assessed using power spectral density analysis of heart rate variability, and salivary IgA was used as a marker of immunity. The effects of rock, new age and designer music were examined alone, and in conjunction with a self-induced positive emotional state. The results indicate that only the designer music and the self-induced state of appreciation produced a significant increase in autonomic activity and salivary IgA (S-IgA). In addition, the combination of the designer music and the self-induced appreciation produced a much greater immunoenhancement than either of these two conditions alone. We conclude that music can be designed to enhance the beneficial effects of positive emotional states on immunity, and that this effect may be mediated by the autonomic nervous system. These data raise the tantalizing possibility that music and emotional self-management may have significant health benefits in a variety of clinical situations in which there is immunosuppression and autonomic imbalance.
Full-text available
This study examined the comparative potency of several psychological stressors and exercise in eliciting myocardial ischemia as measured by left ventricular (LV) ejection fraction (EF) changes using radionuclide ventriculography. Twenty-seven subjects underwent both exercise (bicycle) and psychological stressors (mental arithmetic, recall of an incident that elicited anger, giving a short speech defending oneself against a charge of shoplifting) during which EF, blood pressure, heart rate and ST segment were measured. Eighteen subjects had 1-vessel coronary artery disease (CAD), defined by greater than 50% diameter stenosis in 1 artery as assessed by arteriography. Nine subjects served as healthy control subjects. Anger recall reduced EF more than exercise and the other psychological stressors (overall F [3.51] = 2.87, p = .05). Respective changes in EF for the CAD patients were -5% during anger recall, +2% during exercise, 0% during mental arithmetic and 0% during the speech stressor. More patients with CAD had significant reduction in EF (greater than or equal to 7%) during anger (7 of 18) than during exercise (4 of 18). The difference in EF change between patients with CAD and healthy control subjects was significant for both anger (t25 = 2.23, p = 0.04) and exercise (t25 = 2.63, p = 0.01) stressors. In this group of patients with CAD, anger appeared to be a particularly potent psychological stressor.
Objective. —To test the hypothesis that heightened anxiety, heightened anger intensity, and suppressed expression of anger increase the risk of hypertension, using the Framingham Heart Study. Design. —A cohort of men and women without evidence of hypertension at baseline were followed up for 18 to 20 years. Baseline measures of anxiety (tension), anger symptoms, and expression of anger (anger-in and anger-out) were taken, along with biological and behavioral predictors of hypertension (initial systolic blood pressure, heart rate, relative weight, age, hematocrit, alcohol intake, smoking, education, and glucose intolerance). Participants. —A total of 1123 initially normotensive persons (497 men, 626 women) were included. Analyses were stratified by age (45 to 59 or ≥60 years) and gender. Main Outcome Measures. —Hypertension was defined as either taking medication for hypertension or blood pressures higher than 160/95 mm Hg at a biennial examination. Results. —In univariate analyses, middle-aged men who went on to develop hypertension had greater baseline anxiety levels than men who remained normotensive (P=.04). Older hypertensive men had fewer anger symptoms at baseline (P=.04) and were less likely to hold their anger in (P=.01) than normotensives. In multivariate Cox regression analysis including biological predictors, anxiety remained an independent predictor of hypertension in middle-aged men (P=.02). Among older men, anger symptoms and anger-in did not remain significant predictors in the multivariate analysis. Further analysis showed that only middle-aged men with very high levels of anxiety were at increased risk (relative risk, 2.19; 95% confidence interval, 1.22 to 3.94). No psychological variable predicted hypertension in middle-aged or older women in either univariate or multivariate analyses. Conclusions. —The results indicate that among middle-aged men, but not women, anxiety levels are predictive of later incidence of hypertension.(JAMA. 1993;270:2439-2443)
Objective. —To test the hypothesis that heightened anxiety, heightened anger intensity, and suppressed expression of anger increase the risk of hypertension, using the Framingham Heart Study.
Recent scientific research has proven that we can, not only manage our stress, we can even prevent it. Ways to achieve stress management are presented in this book. It details a method called FREEZE-FRAME, a process in which individuals mentally stop the chaos that surrounds them and then calmly contemplate their situation. The text opens with an overview of a typical stressful day and the damage that such stress can inflict on the heart, the immune system, and other parts of the body. It then introduces the purpose of FREEZE-FRAME and how it can tame emotions that are out of control. Steps of the process are outlined along with the scientific basis of the FREEZE-FRAME method. Included in this latter section are discussions of the heart's role and its physiology, electrocardiogram analysis, and other details. Some of the personal benefits and applications of the technique are described, including business benefits, mental benefits, emotional benefits, physical benefits, and sports benefits. A separate chapter on the social benefits of FREEZE-FRAME is also detailed. Finally, a discussion of pitfalls to avoid in using this method, along with a number of tips for its successful application, are presented. (RJM)
Time and frequency domain parameters of heart rate variability (HRV) were determined in patients with severe endstage heart failure awaiting cardiac transplantation (HTX). These parameters were then correlated with mortality to investigate the performance of HRV in discriminating between groups with high and low risk of death. The standard deviation of five consecutive RR intervals (SDANN) was found to be the parameter with the greatest sensitivity (90%) and specificity (91%). Patients with SDANN values of < 55 msec had a twenty-fold increased risk of death (90% confidence limits: 4-118, P < 0.001). The results furthermore suggest that measurements of HRV are superior to other prognostic markers such as left ventricular ejection fraction, pulmonary artery wedge pressure, cardiac index, and serum sodium levels. We conclude that HRV is a powerful, noninvasive tool to assess the risk of death in candidates for HTX. HRV measurements can therefore be used as a supplement to other markers of risk to determine the optimal therapeutic strategy in patients with severe congestive heart failure.
Experience with frequency domain analysis over the past two decades strongly suggests that it represents a unique, noninvasive tool for achieving a more precise assessment of autonomic function in both the experimental and clinical settings. Available studies indicate that the significance of the HF component is far better understood than that of the lower frequency components. In general, it is considered to reflect vagal activity, and because it is readily manipulated pharmacologically, is used as an index of that activity. However, some caution is required because this parameter also is strongly influenced by the degree of coupling between respiration and heart rate, which, in turn, reflects the intensity of the respiratory effort as well as of parasympathetic activity. Respiratory pattern also can significantly influence HF power. The use of controlled breathing minimizes these problems, improves reproducibility of test findings, and also facilitates quantitative comparisons. The situation with respect to LF power is more complicated because it is modulated by both sympathetic and parasympathetic outflows (see previous discussion) as well as by other factors, including baroreceptor activity. Therefore, LF analysis per se cannot afford a precise delineation of the state of sympathetic activation. Determinations of the LF/HF ratio, an index of sympathovagal balance both under control conditions and in conjunction with interventions that maximize sympathetic and parasympathetic activity, provide additional insights, as do correlations between spectral activity and direct nerve recordings, plasma norepinephrine concentrations, and radionuclide imaging of adrenergic nerves.