Patterns of sinus arrhythmia in patients with lesions of the central nervous system

Heart Rate Variability in Children with Moderate and Severe Traumatic Brain Injury: A Prospective Observational Study

The aim of this study was to assess the feasibility of continuous monitoring of heart rate variability (HRV) in children with traumatic brain injury (TBI) hospitalized in a pediatric intensive care unit (PICU) and collect preliminary data on the association between HRV, neurological outcome, and complications. This is a prospective observational cohort study in a tertiary academic PICU. Children admitted to the PICU ≤24 hours after moderate or severe TBI were included in the study. Children suspected of being brain dead at PICU entry or with a pacemaker were excluded. Children underwent continuous monitoring of electrocardiographic (ECG) waveforms over 7 days post-TBI. HRV analysis was performed retrospectively, using a standardized, validated HRV analysis software (CIMVA). The occurrence of medical complications (“event”: intracranial hypertension, cerebral hypoperfusion, seizure, and cardiac arrest) was prospectively documented. Outcome of children 6 months post-TBI was assessed using the Glasgow Outcome Scale – Extended Pediatric (GOS-E Peds). Fifteen patients were included over a 20-month period. Thirteen patients had ECG recordings available and 4 had >20% of missing ECG data. When ECG was available, HRV calculation was feasible (average 88%; range 70–97%). Significant decrease in overall HRV coefficient of variation and Poincaré SD2 (p < 0.05) at 6 hours post–PICU admission was associated with an unfavorable outcome (defined as GOS-E Peds ≥ 3, or a deterioration of ≥2 points over baseline score). Several HRV metrics exhibited significant and nonsignificant variation in HRV during event. This study demonstrates that it is feasible to monitor HRV in the PICU provided ECG data are available; however, missing ECG data are not uncommon. These preliminary data suggest that altered HRV is associated with unfavorable neurological outcome and in-hospital medical complications. Larger prospective studies are needed to confirm these findings and to explore if HRV offers reliable and clinically useful prediction data that may help clinical decision making.

Respiratory-cardiovascular interactions

Much of biology is rhythmical and comprises oscillators that can couple. These have optimized energy efficiency and have been preserved during evolution. The respiratory and cardiovascular systems contain numerous oscillators, and importantly, they couple. This coupling is dynamic but essential for an efficient transmission of neural information critical for the precise linking of breathing and oxygen delivery while permitting adaptive responses to changes in state. The respiratory pattern generator and the neural network responsible for sympathetic and cardiovagal (parasympathetic) tone generation interact at many levels ensuring that cardiac output and regional blood flow match oxygen delivery to the lungs and tissues efficiently. The most classic manifestations of these interactions are respiratory sinus arrhythmia and the respiratory modulation of sympathetic nerve activity. These interactions derive from shared somatic and cardiopulmonary afferent inputs, reciprocal interactions between brainstem networks and inputs from supra-pontine regions. Disrupted respiratory-cardiovascular coupling can result in disease, where it may further the pathophysiological sequelae and be a harbinger of poor outcomes. This has been well documented by diminished respiratory sinus arrhythmia and altered respiratory sympathetic coupling in animal models and/or patients with myocardial infarction, heart failure, diabetes mellitus, and neurological disorders as stroke, brain trauma, Parkinson disease, or epilepsy. Future research needs to assess the therapeutic potential for ameliorating respiratory-cardiovascular coupling in disease.

Hemodynamic Changes in Response to Hyperacute Spinal Trauma in a Swine Model

Acute spinal cord injury (ASCI) is a devastating event that can have severe hemodynamic consequences, depending on location and severity of the lesion. Knowledge of hyperacute hemodynamic changes is important for researchers using porcine models of thoracic ASCI. The goal of this study was to determine the hyperacute hemodynamic changes observed after ASCI when using pigs as their own controls. Five Yucatan gilts were anesthetized, and a dorsal laminectomy performed at T10-T12. Standardized blunt trauma was applied for 5 consecutive min, and hemodynamic variables were collected 5 min before ASCI, and at 2, 4, 6, 8, 10, 20, 30, 60, 80 and 120 min after ASCI. Arterial blood gas samples were collected at 60 min and 10 min before, and at 30 min and between 120 and 240 min after ASCI. Parametric data were analyzed using a mixed effects model with time point as the fixed factor and subject as the random factor. We found no effect on heart rate, pulse pressure, SpO2, EtCO2, and respiratory rate between baseline and timepoints after ASCI. Diastolic arterial pressure, mean arterial pressure, and systolic arterial pressure fell significantly by 18%, 16%, and 15%, respectively, at 2 min after ASCI. However, none of the decrements in arterial pressures resulted in hypotension at any time point. Heart rate did not change significantly after ASCI. Blood glucose progressively increased to 50% above baseline between 120 and 240 minutes after ASCI. Low-thoracic ASCI caused a consistent and statistically significant but clinically minor hyperacute decrease in arterial pressures (-15%) that did not produce hypotension or metabolic changes suggestive of tissue hypoperfusion. Our findings using this model suggest that mean arterial pressures should be maintained above 85 mm Hg prior to spinal trauma in order to avoid hypotensive states after ASCI.

Visuo-motor and interoceptive influences on peripersonal space representation following spinal cord injury

Peripersonal space (PPS) representation is modulated by information coming from the body. In paraplegic individuals, whose lower limb sensory-motor functions are impaired or completely lost, the representation of PPS around the feet is reduced. However, passive motion can have short-term restorative effects. What remains unclear is the mechanisms underlying this recovery, in particular with regard to the contribution of visual and motor feedback and of interoception. Using virtual reality technology, we dissociated the motor and visual feedback during passive motion in paraplegics with complete and incomplete lesions and in healthy controls. The results show that in the case of paraplegics, the presence of motor feedback was necessary for the recovery of PPS representation, both when the motor feedback was congruent and when it was incongruent with the visual feedback. In contrast, visuo-motor incongruence led to an inhibition of PPS representation in the control group. There were no differences in sympathetic responses between the three groups. Nevertheless, in individuals with incomplete lesions, greater interoceptive sensitivity was associated with a better representation of PPS around the feet in the visuo-motor incongruent conditions. These results shed new light on the modulation of PPS representation, and demonstrate the importance of residual motor feedback and its integration with other bodily information in maintaining space representation.

Autonomic Dysfunction and Associations with Functional and Neurophysiological Outcome in Moderate/Severe Traumatic Brain Injury: A Scoping Review

The quantification and objective documentation of autonomic dysfunction in traumatic brain injury (TBI) is neither well studied nor extensively validated. Most of the descriptions of autonomic dysfunction in the literature are in the form of vague non-specific clinical manifestations. Few studies propose the use of objective measures of assessing the extent of autonomic dysfunction to link them to the outcome of TBI. Our goal was to perform a scoping systematic review of the literature on the objective documentation of autonomic dysfunction in terms of functional and physiological variables to be linked to outcome of TBI. PubMed/MEDLINE^®, BIOSIS, Scopus, Embase, Cochrane Libraries, and Global Health databases were searched. Two reviewers independently screened the results. Full texts for citations passing this initial screen were obtained. Inclusion and exclusion criteria were applied to each article to obtain final articles for review. The initial search yielded 2619 citations. Of 69 articles selected for final review, 14 were chosen based on the inclusion and exclusion criteria and are included in the results of this article. 9 of these articles assessed autonomic dysfunction using functional variables and 7 assessed autonomic dysfunction using physiological variables. Some studies included both functional and physiological variables. Of the nine studies linking autonomic dysfunction to functional variables, nine included heart rate variability (HRV), three included baroreflex sensitivity (BRS), and two included blood pressure variability (BPV). A total of 2714 adult patients were studied. Although the nature of association between autonomic dysfunction and outcome is unclear, the objective quantification of autonomic dysfunction seems to be associated with global patient outcome and other neurophysiological measures. Further studies are needed to validate its use and explore the underlying molecular mechanisms of the described associations.

Hidden Signals-The History and Methods of Heart Rate Variability

The understanding of heart rate variability (HRV) has increased parallel with the development of modern physiology. Discovered probably first in 1847 by Ludwig, clinical applications evolved in the second part of the twentieth century. Today HRV is mostly used in cardiology and research settings. In general, HRV can be measured over shorter (e.g., 5-10 min) or longer (12 or 24 h) periods. Since 1996, most measurements and calculations are made according to the standard of the Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. As the first step, the series of times between successive R-peaks in the ECG are in milliseconds. It is crucial, however, to identify and remove extrasystoles and artifacts according to standard protocols. The series of QRS distances between successive heartbeats can be analyzed with simple or more sophisticated algorithms, beginning with standard deviation (SDNN) or by the square root of the mean of the sum of squares of differences between adjacent normal RR (rMSSD). Short-term HRV is frequently analyzed with the help of a non-parametric fast Fourier transformation quantifying the different frequency bands during the measurement period. In the last decades, various non-linear algorithms have been presented, such as different entropy and fractal measures or wavelet analysis. Although most of them have a strong theoretical foundation, their clinical relevance is still debated.

Heart Rate Variability in Intensive Care

Clinicians have long been aware that the normal oscillations in a heart beat are lost during fetal distress, during the early stages of heart failure, with advanced aging, and with critical illness and injury. However, these oscillations, or variability in heart rate and other cardiovascular signals, have largely been ignored or discounted as variances from the mean or average values. It is becoming increasingly clear that these oscillations reflect the dynamic interactions of many physiologic processes, including neuroautonomic regulation of heart rate and blood pressure. We present a synthesis and review of the current literature concerning heart rate variability with special reference to intensive care. This article describes the background of time series analysis of heart rate variability including time and frequency domain and nonlinear measurements. The implications and potential for time series analysis of variability in cardiovascular signals in clinical diagnosis and management of critically ill and injured patients are discussed.

Heart rate variability in neonates with hypoxic ischemic encephalopathy

OBJECTIVE

To evaluate the changes in heart rate variability (HRV) in newborns with hypoxic-ischemic encephalopathy (HIE).

METHODS

Twenty-two newborns (14 boys, 8 girls) with moderate or severe HIE and 24 term neonates with similar gestational and postnatal age for control were included in this study. Normalized low and high frequency components of HRV and their ratio were evaluated for 24-h in newborns with HIE and control subjects.

RESULTS

The newborns with hypoxic-ischemic encephalopathy had significantly lower normalized low frequency (LFn) and low frequency (LF) / high frequency (HF) values and higher normalized high frequency (HFn) values when compared with the control babies. In addition, when the cases with severe HIE are compared with those of moderate HIE, decreased LFn, LF/HF values and also increased HFn values were present in the severe cases.

CONCLUSIONS

HIE is associated with reduced sympathetic nervous system activity, and increased parasympathetic nervous system activity and these activities also correlate with the severity of the disease.

Aminophylline for the treatment of symptomatic bradycardia and asystole secondary to cervical spine injury

INTRODUCTION

Bradycardia is a common complication of cervical spine damage in the weeks following injury, occurring in up to 100% of patients in some studies. Cardiac arrest and asystole have been reported in as many as 15% of these patients and cardiac events are the main cause of death within the first year. We describe the case of a 25-year-old African-American male involved in a motor vehicle collision who suffered C6-C7 subluxation.

METHODS

Following cervical discectomy and spinal fusion the patient began to develop progressive bradycardia culminating on hospital day 20 with two asystolic events requiring atropine administration. In an attempt to prevent further events and generate hemodynamic stability, aminophylline therapy was initiated.

RESULTS

Following day two of therapy, the patient's bradycardia resolved, and no further asystolic events occurred.

CONCLUSION

There is limited evidence for the use of methylxanthines in the treatment of bradycardia associated with spinal cord injury. In patients with recurrent asystolic events or symptomatic bradycardia the use of these agents should be considered.

Use of Aminophylline in the Treatment of Severe Symptomatic Bradycardia Resistant to Atropine

Bradycardia and cardiac arrest are known complications of acute spinal cord injuries and are usually temporary. If the general measures of correcting hypoxia and using atropine fail, placement of a temporary followed by a permanent pacemaker is typically considered. We describe 2 very interesting cases of severe symptomatic bradycardia resistant to atropine, where we were able to obviate the use of pacemaker placement by the simple use of intravenous aminophylline. Aminophylline had been used in the past for treating resistant bradycardia in settings such as acute inferior wall myocardial infarction, cardiac transplantation, and so on, but has never been used in the setting of acute spinal cord injuries. Aminophylline probably works in this setting by increasing cyclic adenosine monophosphate (cAMP) and activating the sympathoadrenal system.

Heart rate variability after acute traumatic brain injury in children

OBJECTIVE

To evaluate heart rate variability (HRV) by power spectral analysis of heart rate and its relationship to intracranial pressure (ICP), cerebral perfusion pressure (CPP), and outcomes in children with acute traumatic head injury.

DESIGN

Prospective, case series.

SETTING

Pediatric intensive care unit in a level II trauma center/children's hospital.

SUBJECTS

Fifteen critically ill children with documented acute traumatic brain injury and four control subjects.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

The normalized total power from 0.04 to 0.15 Hz was used to quantify low-frequency HRV and from 0.15 to 0.40 Hz to quantify high-frequency HRV. The ratio of low- to high-frequency (LF/HF) power was used as a measure of sympathetic modulation of heart rate. The power spectral data from the 5-min samples were averaged over each hour of data collection, and an hourly LF/HF ratio was obtained based on a 60-min electrocardiogram collection (twelve 5-min segments). The daily mean LF/HF ratio was calculated from the hourly LF/HF measurements. We found no linear correlation between the LF/HF ratio and either ICP or CPP (p = NS). There was a significant decrease in the LF/HF ratio when the intracranial pressure was >30 mm Hg (p < .001) or the cerebral perfusion pressure was <40 mm Hg (p < .001). Children with a Glasgow Coma Scale score of 3-4 had a lower LF/HF ratio compared with those who had a Glasgow Coma Scale score of 5-8 (p < .005). Patients who progressed to brain death had a markedly lower LF/HF ratio (p < .001), with a significant decrease after the first 4 hrs of hospitalization. Patients with more favorable outcomes had significantly higher LF/HF ratios.

CONCLUSIONS

Our findings suggest that an ICP of >30 mm Hg or a CPP of <40 mm Hg may be associated with marked autonomic dysfunction and poor outcome. We speculate that HRV power spectral analysis may be a useful adjunct in determining the severity of neurologic insult and the prognosis for recovery in children. The LF/HF ratio may be helpful not only in identifying those patients who will progress to brain death but also in predicting which patients will have favorable outcomes.

Preeclamptic pregnancy is associated with increased sympathetic and decreased parasympathetic control of HR

Previous work from our laboratory using heart rate variability (HRV) has demonstrated that women before menopause have a more dominant parasympathetic and less effective sympathetic regulations of heart rate compared with men. Because it is still not clear whether normal or preeclamptic pregnancy coincides with alternations in the autonomic functions, we evaluated the changes of HRV in 17 nonpregnant, 17 normotensive pregnant, and 11 preeclamptic women who were clinically diagnosed without history of diabetic neuropathy, cardiac arrhythmia, and other cardiovascular diseases. Frequency-domain analysis of short-term, stationary R-R intervals was performed to evaluate the total variance, low-frequency power (LF; 0.04-0.15 Hz), high-frequency power (HF; 0.15-0.40 Hz), ratio of LF to HF (LF/HF), and LF in normalized units (LF%). Natural logarithm transformation was applied to variance, LF, HF, and LF/HF for the adjustment of the skewness of distribution. We found that the normal pregnant group had a lower R-R value and HF but had a higher LF/HF and LF% compared with the nonpregnant group. The preeclamptic group had lower HF but higher LF/HF compared with either the normal pregnant or nonpregnant group. Our results suggest that normal pregnancy is associated with a facilitation of sympathetic regulation and an attenuation of parasympathetic influence of heart rate, and such alterations are enhanced in preeclamptic pregnancy.

Analysis of heart-rate variability: a noninvasive predictor of death and poor outcome in patients with severe head injury

BACKGROUND

Analysis of heart-rate variability (HRV) is a promising new technique for noninvasive quantification of autonomic function. We measured HRV in patients with severe head injury to assess its potential as a monitoring tool.

METHODS

Analysis of HRV was prospectively done on all intensive care unit patients. Concurrent data on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) were collected. Registry data were reviewed to identify patients with severe head injury, defined as Head/Neck Abbreviated Injury Scale score > or = 4. Mortality, likelihood of discharge to home, ICP, and CPP were compared between patients with abnormal HRV and those without.

RESULTS

Low HRV was associated with increased mortality and decreased rate of discharge to home. Abnormal HRV was associated with episodes of increased ICP and decreased CPP.

CONCLUSION

Assessment of HRV is a noninvasive method that can be widely used. Abnormal HRV was associated with poor outcome and altered cerebral perfusion. Monitoring of HRV may improve outcome by allowing earlier detection and treatment of intracranial pathology.

Blood pressure and heart rate variability in autonomic disorders: a critical review

Spectral analysis (SA) of blood pressure (BP) and heart rate (HR) fluctuations has been proposed as a unique approach to obtain a deeper insight into cardiovascular regulatory mechanisms in health and disease. A number of studies performed over the last 15 years have shown that autonomic influences are involved in the modulation of fast BP and HR fluctuations (with a period <1 min), particularly at frequencies between 0.2 and 0.4 Hz [high frequency (HF) region or respiratory frequency] and around 0.1 Hz [mid frequency (MF) region]. In patients with secondary or primary autonomic dysfunction, SA of BP and HR signals recorded at rest or during orthostatic challenge in a laboratory environment have shown the occurrence of a reduction in the power of MF and/or HF, BP and HR components. Such a reduction is associated or may even precede the clinical manifestation of autonomic neuropathy. However, the above results collected in standardized laboratory conditions cannot reflect the features of neural cardiovascular control during daily life in ambulant individuals with autonomic failure. To investigate this issue, SA techniques have been applied to 24 h beat-to-beat intra-arterial and non-invasive finger BP recordings obtained in elderly subjects and in pure autonomic failure patients, respectively. In these conditions, HR powers displayed a reduction over a wide range of frequencies (from 0.5 to below 0.01 Hz). Conversely, BP powers underwent a complex rearrangement characterized by a reduction in the power around 0.1 Hz and by an increase in the powers at the respiratory frequency and at frequencies below 0.01 Hz. Dynamic quantification of the sensitivity of the baroreceptor-heart rate reflex by combined analysis of systolic BP and pulse interval (i.e. the interval between consecutive systolic peaks) powers around 0.1 Hz (alpha technique) has shown that in elderly subjects, and even more so in pure autonomic failure patients, baroreflex sensitivity is markedly reduced over the 24 h, and is no longer characterized by its physiological day-night modulation. In conclusion, although in some instances SA of cardiovascular signals may fail to fully reflect the features of autonomic cardiovascular control, the evidence discussed clearly demonstrates that this approach represents a promising tool for a dynamic assessment of the early impairment of neural circulatory control in autonomic failure. This is particularly the case when these analyses are performed on 24 h continuous BP and HR recordings in ambulant subjects.

Application of chaos theory to a model biological system: evidence of self-organization in the intrinsic cardiac nervous system

The neutral organization that determines the specific beat-to-beat pattern of cardiac behavior is expected to be demonstrated in the independent regulation of the RR intervals (chronotropy) and the corresponding QT subintervals (inotropy), as the former defines the rate of contraction and the latter has a linear negative correlation with the peak pressure inside the contracting ventricular muscles. The neurons of the isolated cardiac nervous system, many of which are located in the fat-pads of the heart, exhibit the same types of mechanical and chemical receptors and the same types of cholinergic and noradrenergic effectors as those found in the neural superstructure. In the surgically isolated and perfused rabbit heart we studied the responses of the QT and RR intervals evoked by block of coronary blood flow. We found that if we separated each RR cycle into QT and RR-QT components, then the dynamics of variation for each subinterval series often had the same fractional number of degrees of freedom (i.e., chaotic dimensions), a finding which suggests they are both regulated by the same underlying system. The ischemia/anoxia evoked transient dimensional increases and separations between the two subinterval series that, after the temporary divergence, reconverged to having the same lower value. The dimensional fluctuations occurred repeatedly and preceded or coincided with alterations in the magnitude and sign of the slope of QT vs RR-QT. We interpret the dimensional fluctuations of the two subinterval series as correlates of adaptation-dependent self-organization and reorganization in the underlying intrinsic cardiac nervous system during accumulating ischemia/anoxia. Such attempts at functional reorganization in this simple neurocardiac system may explain the transient dimensional changes in the RR intervals that precedes by 24 hrs the occurrences of fatal ventricular fibrillation in high-risk cardiac patients.

Autonomic control of heart rate after brain injury in children

OBJECTIVES

To study sequential changes in heart rate, respiratory rate, blood pressure, heart rate power spectra, and plasma catecholamine concentrations in patients with acute brain injury and correlate these variables with the severity of neurologic dysfunction and patient outcome.

DESIGN

Prospective, clinical study.

SETTING

Pediatric intensive care unit.

PATIENTS

Thirty-seven pediatric patients with acute brain injury caused by trauma, anoxia/ischemia, hemorrhage, or infection.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

We found significant associations between low-frequency (0.01 to 0.15 Hz) heart rate power and severity of neurologic dysfunction (as assessed by the admission Glasgow Coma Scale) (p < .001) and patient outcome (as assessed by the Glasgow Outcome Scale) (p = .05). The admission (p = .05) and maximum (p < .001) values for low-frequency heart rate power and the minimum value for high-frequency (0.15 to 0.50 Hz) heart rate power obtained during hospitalization (p = .001) predicted an increased likelihood of survival. Ten brain-dead patients had significantly decreased low-frequency heart rate power (p = .008) and plasma norepinephrine (p = .015), epinephrine (p = .03), and dopamine (p = .04) concentrations when compared with six non-brain-dead patients with a Glasgow Coma Scale score of 3.

CONCLUSIONS

Our results imply that autonomic nervous system control of heart rate is disrupted in proportion to the degree of neurologic insult in children after acute brain injury. Thus, heart rate power spectral analysis and plasma catecholamine concentrations may prove to be useful adjuncts in determining severity of neurologic injury and prognosis for recovery in children suffering from brain injury. In addition, these techniques may aid in the determination of brain death.

Oscillatory functions affecting outcome of coronary heart disease: the hazard of too much or too little stability

The objective was to identify physiological and behavioral indicators predictive of sudden arrhythmic death in patients who had experienced myocardial infarction in the past. In a 10-year prospective study of 79 patients, 59 men and 20 women aged 36 to 76 who had suffered a well-documented myocardial infarction (MI) at some time in the past (6 weeks-5 years) were individually matched with healthy controls of age, sex, race, height, weight, educational background and type of job. Both patients and controls were reexamined and retested at intervals of 6-8 weeks throughout the first 7 years of the study. Fifty-three patients died, 7 of noncardiac disorders, 2 died of suicide. Forty-four, 31 men and 13 women died suddenly of apparent MI. Thirty-one, or 70% of them were autopsied. Eleven were found to have experienced a recent MI and 20 had only an old scar. Age was not a determinant of cardiac mortality among the patients. The average age of those who died was 56 and of survivors 53. Neither was the level of serum cholesterol concentration, the LDL/HDL ratio, or the treadmill test a significant determinant of death. The measured physiological data that did significantly correlate to the cardiac deaths at the 0.01 level of confidence were low RR variability or wide mean RR variation month to month, prolonged QT interval or wide mean QT variability month to month. The findings support the view that proper physiological balance requires some degree of instability, but not too little or too much. Walter Cannon in his first paper on homeostasis (Cannon, 1926), credited Charles Richet with this perception "The living being is stable... In a sense it is stable because it is modifiable--the slight instability is the necessary condition for the true stability of the organism" (Richet, 1990).

A minimal single-channel model for the regularity of beating in the sinoatrial node

It has been suggested that the normal irregular beating of the heart is a manifestation of deterministically chaotic dynamics. Evidence proffered in support of this hypothesis includes a 1/f-like power spectrum, a small noninteger correlation dimension, and self-similarity of the time series. The major cause of the normal fluctuations in heart rate is the impingement of several neural and hormonal control systems upon the sinoatrial node, the natural pacemaker of the heart. However, intrinsic fluctuations of beat rate can be seen in the isolated node, devoid of all neural and hormonal inputs, and even in a single cell isolated from the node. The electrical activity in such a single cell is generated by ions flowing through discrete channels in the cell membrane.We decided to test the hypothesis that the fluctuations in beat rate in a single cell might be due to the fluctuations in the activity of this population of single channels. We thus assemble a model consisting of 6000 channels and probe its dynamics. Each channel has one or more gates, all of which must be open to allow current to flow through the channel. Since these gates are thought to open and close in a random manner, we model each gate by a Markov process, assigning a pseudorandom number to each gate every time that it changes state from open to closed or vice versa. This number, in conjunction with the classical voltage-dependent Hodgkin-Huxley-like rate constants that control the speed with which a gate will open or close, then determines when that gate will next change state. We also employ a second method that is much more efficient computationally, in which one computes the lifetime of the ensemble of 6000 channels. We show that the Monte Carlo model has behavior consistent with the hypothesis that the irregular beating seen experimentally in single nodal cells is due to the (pseudo)random opening and closing of single channels. However, since the pseudorandom number generator used in the simulations is deterministic, one cannot state that the activity in the model is random (or stochastic). Thus, it would be premature to claim that the irregularity of beating in a single nodal cell is accounted for by the stochastic behavior of a population of a few thousand single channels lying in the membrane of the cell. Finally, we consider some implications of our work for the naturally occurring in situ fluctuations in heart rate ("heart rate variability"). (c) 1995 American Institute of Physics.

Cardiovascular autonomic reflexes in brain infarction

BACKGROUND AND PURPOSE

Increased sympathetic activity is associated with cardiovascular complications in stroke, but the role of the parasympathetic nervous system has not been carefully outlined. In the present study our purpose was to assess quantitatively autonomic cardiovascular disturbances in brain infarction by measuring cardiovascular autonomic reflexes.

METHODS

We studied the autonomic regulation of cardiovascular functions prospectively in 40 patients with brain infarction (acute phase, 1 month, and 6 months) and in 55 healthy control subjects by recording heart rate and blood pressure responses to normal and deep breathing, the Valsalva maneuver, tilting, and isometric work.

RESULTS

In the acute phase, heart rate responses to normal breathing, deep breathing, the Valsalva maneuver, and tilting were significantly (P < .05) impaired in both hemispheric and brain stem infarctions, thus indicating hypofunction of the parasympathetic nervous system. At 1 month heart rate responses to normal breathing (brain stem, P < .05), the Valsalva maneuver (brain stem, P < .01), and tilting (hemispheric, P < .05) were still significantly lower than those of the control subjects, but at 6 months significant suppression of the response was found only in tilting (hemispheric, P < .05).

CONCLUSIONS

These findings suggest that in addition to the previously well-established sympathetic hyperfunction, brain infarction also seems to cause parasympathetic hypofunction, brain which may be involved in cardiovascular and other known manifestations of autonomic failure associated with stroke.

Circadian rhythm of heart rate variability in survivors of cardiac arrest

Reduced heart rate (HR) variability is associated with increased risk of cardiac arrest in patients with coronary artery disease. In this study, the power spectral components of HR variability and their circadian pattern in 22 survivors of out-of-hospital cardiac arrest not associated with acute myocardial infarction were compared with those of 22 control patients matched with respect to age, sex, previous myocardial infarction, ejection fraction and number of diseased coronary arteries. Survivors of cardiac arrest had significantly lower 24-hour average standard deviation of RR intervals than control patients (29 +/- 10 vs 51 +/- 15 ms, p less than 0.001), and the 24-hour mean high frequency spectral area was also lower in survivors of cardiac arrest than in control patients (13 +/- 7 ms2 x 10 vs 28 +/- 14 ms2 x 10, p less than 0.01). In a single cosinor analysis, a significant circadian rhythm of HR variability was observed in both groups with the acrophase of standard deviation of RR intervals and high-frequency spectral area occurring between 3 and 6 A.M. which was followed by an abrupt decrease in HR variability after arousal. The amplitude of the circadian rhythm of HR variability did not differ between the groups. Thus, HR variability is reduced in survivors of cardiac arrest but its circadian rhythm is maintained so that a very low HR variability is observed in the morning after awakening, corresponding to the time period at which the incidence of sudden cardiac death is highest.

Microcomputer collection and analysis of RR-interval data in the BB-rat

An analog to digital converter and microcomputer system for the collection of real-time RR-interval data in the BB-rat is described. Calculation of the statistic R is discussed and a commented program listing in Microsoft basic, for performing this transformation, is included as an appendix.

Diminished respiratory sinus arrhythmia in asphyxiated term infants

Spectral analysis techniques were used to quantitate the association between respiration and heart rate variability in eight healthy and eight asphyxiated infants born at term gestation. Respiratory sinus arrhythmia was demonstrated in all healthy infants. This arrhythmia was significantly diminished in asphyxiated newborn infants. We conclude that newborn infants with low Apgar scores have a reduced respiratory sinus arrhythmia and that this reduction could account for the loss of short-term heart rate variability commonly associated with asphyxia.

R-R intervals of ECG in depression

ECG measurements of consecutive 100 R-R intervals were done on depressive patients. The patient group presented lower mean values for coefficients of variation (CV) at each generation than the control group (normal subjects); the differences were significant especially for the subjects 40 to 49 years of age (P less than 0.05) and for those 60 to 69 years of age (P less than 0.01). These results can probably be regarded as a significant and objective index of autonomic disturbances in depression.

Autonomic nervous dysfunction in severe pre-eclampsia

We evaluated autonomic nervous function in 14 subjects with severe pre-eclampsia and 11 subjects with normal third-trimester pregnancies using standard cardiovascular tests, i.e. heart rate responses to deep breathing and standing up, and blood pressure response to standing up. Both of the heart rate responses were lower in subjects with severe pre-eclampsia than in the control group (P less than 0.01 and P less than 0.001, respectively). Four subjects with severe pre-eclampsia had postural hypotension, defined as a systolic blood pressure fall of at least 30 mmHg on standing. Our results show that severe pre-eclampsia may be associated with autonomic nervous dysfunction. The inability to regulate heart rate and blood pressure in response to postural alterations may compromise the placental and renal blood flow in certain conditions, emphasising the importance of bed rest in severe pre-eclampsia.

Respiratory sinus arrhythmia in the human fetus

This study explores the relationship between fetal heart rate variability and fetal breathing movements in the term fetus. Fourier analysis was applied to fetal heart rate variability during fetal breathing and nonbreathing episodes. A distinct component in the order of 0.7 to 0.95 Hz was demonstrated in fetal heart rate variability during breathing episodes. These frequencies have the same distribution as the corresponding rate of breathing and therefore indicate that respiratory sinus arrhythmia exists in the term fetus.

Cardiovascular instability following acute cervical spinal cord trauma

Irregularities in vital sign (pulse, blood pressure, cardiac rhythm) recordings are commonly observed following acute spinal cord injury. These abnormalities have been generally attributed to autonomic instability. However, there have been no clinical reports that evaluate these problems in a large group of acutely injured patients. Therefore, this study was performed on 45 patients with acute cervical spinal cord injuries to evaluate the incidence, severity, and risk factors for cardiovascular instability. This investigation revealed that there is a direct correlation between the severity of the cord injury and the incidence and severity of cardiovascular problems. Endotracheal suctioning with or without documented hypoxia are major causes of severe bradycardia and cardiac arrest within the first 2 weeks after trauma. Careful monitoring of severely injured patients and attention to the warning signs of cardiovascular instability can reduce the risk of life-threatening emergencies.

Cardiovascular and haemodynamic responses to tilting and to standing in tetraplegic patients: a review

This paper has reviewed the acute and long-term responses to changes in vertical posture in normal and tetraplegic subjects. It has discussed physiological mechanisms causing orthostatic hypotension in acute cervical spinal cord injured patients, and subsequent factors contributing to its amelioration over time. The long-term adaptive mechanisms are still controversial, probably involving multiple neurological, endocrine, renal, cardiovascular and haemodynamic factors. These factors include inhibition of vagal tone, plasma catecholamine levels, sensitivity of vascular beds to catecholamines, stretch reflexes in blood vessels, spinal BP reflexes, renin-angiotensin system, aldosterone and plasma volume changes. Individual differences may also interact with these various mechanisms, further complicating the issues. Although the fact that most tetraplegics do improve their orthostatic tolerance over time with repeated tilting is manifest, the precise mechanisms allowing this improvement are not. Research is needed to clarify these adaptive mechanisms, as well as to investigate the physiological effects of long-term therapeutic standing in devices such as standing frames.

Cluster headache: a computerized analysis of 24 h Holter ECG recordings and description of ECG rhythm disturbances

Continuous ECG recording has been carried out for at least 24 h in 27 patients suffering from cluster headache. During the study a total of 84 attacks occurred in 25 of the 27 patients who took part. A computerized analysis of the heart rate changes accompanying attacks showed the following: (i) an increase in heart rate at the onset of attacks; the degree of this increase being dependent on the heart rate before attacks, (ii) a relative decrease in heart rate and increased variations in heart rate during attacks, (iii) a relative increase in heart rate at the end of attacks, and (iv) a relative decrease in heart rate after attacks. Five patients (18.5%) showed ECG rhythm disturbances: two frequent premature ventricular beats, one transient attacks of atrial fibrillation, one first degree atrio-ventricular block, and one patient sinoatrial block.

Diabetic autonomic neuropathy. The diagnostic value of heart rate monitoring

The use of heart rate monitoring in the diagnosis of diabetic autonomic neuropathy, and its value in observing the natural history of this disorder, has been assessed. Two tests were used: measurement of heart rate variation during deep breathing and of heart rate change on standing up. Two hundred and eighty seven diabetics aged between 20 and 49 years were studied, and 21 of them were observed repeatedly over 3 to 5 years. Heart rate variation (HRV) on deep breathing proved to be the more sensitive diagnostic index of autonomic neuropathy and was abnormal or borderline in 62 of 64 patients with established autonomic symptoms. Autonomic abnormalities were also detected in some diabetics without autonomic symptoms especially in those with peripheral neuropathy, 30% of whom had abnormal HRV on deep breathing. Abnormal tests appeared to represent permanent autonomic damage and may be present for years without the development of autonomic symptoms, occasionally (7%) preceding any other manifestation of diabetic neuropathy. Serial observations of HRV on deep breathing over 3 to 5 years showed little change, although overall there was a small deterioration of autonomic function, with a decrease of HRV score of 1.0 per year. The tests used are simple, and provide quantitative bedside measurements of autonomic function. When heart rate variation is normal, autonomic neuropathy is virtually excluded.

Study of heart rate variability in sick newborn infants

Heart rate variability has been studied in a group of 66 newborn infants for periods of up to 72 hours from birth. Long term variability was reduced in infants suffering from the idiopathic respiratory distress syndrome and this was more marked with severe respiratory distress requiring mechanical ventilation. Persistent reduction in long term variability was associated with increased mortality. Reduction in heart rate variability may be due to high levels of sympathetic activity.

Diabetic autonomic neuropathy

This review attempts to outline the present understanding of diabetic autonomic neuropathy. The clinical features have been increasinly recognised but knowledge of the localization and morphology of the lesions and their pathogenesis remains fragmentary. A metabolic causation as postulated in somatic nerves accords best with clinical observations. Most bodily systems, particularly the cardiovascular, gastrointestinal and urogenital, are involved with added disturbances of thermoregulatory function and pupillary reflexes. Possible effects on neuroendocrine and peptidergic secretion and respiratory control await definition. Current interest centres around the development of a new generation of tests of autonomic nerve function that are simple, non-invasive, reproducible and allow precision in diagnosis and accurate quantitation. Most are based on cardiovascular reflexes and abnormality in them is assumed to reflect autonomic damage elsewhere. Probably no single test suffices and a battery of tests reflecting both parasympathetic and sympathetic function is preferable. Little is known of the natural history. The prevalence may be greater than previously suspected and although symptoms are mild in the majority, a few develop florid features. The relation of control and duration of diabetes to the onset and progression of autonomic neuropathy is not clearly established. Once tests of autonomic function become abnormal they usually remain abnormal. Symptomatic autonomic neuropathy carries a greatly increased mortality rate possibly due to indirect mechanisms such as renal failure and direct mechanisms such as cardio-resiratory arrest. Improved treatment of some of the more disabling symptoms has been possible in recent years.

Resting heartrate variability in man declines with age

The heartrate variability under resting conditions of 14 normal male subjects, age range 22--63 years, declined with increasing age. Mean heartrate did not show age-dependant changes.

Sinusoidal heart rate rhythms in severe neonatal hypoxia

Sinusoidal heart rate traces were detected in 8 patients, 6 of whom died; the 2 survivors showed evidence of gross cerebral damage. It is felt that the origin of the sinusoidal curve is probably central and reflects loss of central control of heart rate.

Propranolol in neurosurgical patients with sinus tachycardia. Cardiovascular effects and mode of use

Cardiac rate (CR) and systolic arterial blood pressure (ABP) response to the intravenous injection of one, two, or three mg propranolol were recorded in 77 patients with traumatic, neoplastic, or haemorrhagic intracranial lesions and sinus tachycardia. In most patients tachycardia occurred with no cause except the intracranial lesion; in the other patients induction of general anaesthesia or surgical procedures on the brain appeared to have an initiating role. A central imbalance, with increased sympathetic cardiotropic influences is suggested and discussed. (I) First administrations of propranolol always reduced CR but had different effects on ABP from case to case; in all the patients 2.05 +/- 0.84 mg of drug lowered CR by 28 +/- 14/min (P less than 0.01), and ABP by 4.7 +/- 11 torr. CR decrease and ABP changes were without relation to the injected dose. Previous digitalization (desacetyllanatoside C) did not modify CR response to propranolol but reduced (P less than 0.05) its arterial hypotensive action. Positive correlations were found between basal CR and CR decrease (P less than 0.01), basal CR and ABP changes (P less than 0.01), CR decrease and ABP changes (P less than 0.05). Halothane appeared to potentiate CR response (P less than 0.01). (II) CR effect was less when the same dose of propranolol was repeated within 90 min (P less than or equal to 0.05). Usefulness and the mode of use of propranolol were critically evaluated.

A simple method of fetal and neonatal heart rate beat-to-beat variability quantitation: preliminary report

Good base-line fetal and neonatal heart rate beat-to-beat variability appears to be a reassuring sign of well-being. Conversely, decreased base-line heart rate beat-to-beat variability during the latter part of the intrapartum period is often associated with neonatal acidosis and/or depression. A simple method of quantitation of the beat-to-beat neonatal heart rate is reported here. This method of variability quantitation (VQ) consists of a continuous integration and display of the baseline heart rate beat-to-beat variability on a scale of zero to four, expressed in beats per minute averaged over a one-minute period. Serial variability quantitation in 35 neonates with different clinical conditions appeared to demonstrate a good correlation between the variability quantitation and the outcome of the neonates. Further studies are planned for evaluating the method in the intrapartum period.

Heart-rate variability in brain-damaged adults

To test whether or not the characteristics of the adult heart-rate reflect the condition of the central nervous system (as they seem to do in the fetus), ten patients with neurological deficits of acute onset were studied. No patients had received drugs and none was hypoxic. The findings indicate that the normal cyclic changes in heart-rate are reduced in the presence of severe brain damage. Variability decreases rapidly if intracranial pressure rises, and the rate of return of variability reflects the subsequent state of neuronal function, even when intracranial pressure has been restored to normal. In this limited setting, then, it appears that heart-rate variability may reflect the functional state of the central nervous system.

Antepartum fetal heart rate and uterine activity studies: I. Preliminary report of accelerations and the oxytocin challenge test

A series of 344 antepartum fetal heart rate studies in 209 high-risk patients is described. The importance of evaluation of accelerations of the fetal heart rate as well as periodic decelerations associated with uterine activity is demonstrated. Our data suggest that the absence of accelerations of the fetal heart rate during the recording period may be associated with increased perinatal morbidity. Late decelerations may occur in the same recording session as accelerations of the fetal heart rate. The association of late decelerations of the fetal heart rate with no accelerations during the recording session is highly suggestive of increased perinatal morbidity. In high-risk patients, accelerations of the fetal heart rate and no late decelerations with uterine activity are a reassuring finding, with 91 per cent of patients showing no increased perinatal morbidity.

Cardiac denervation in diabetes

Evidence for vagal denervation of the heart as a feature of diabetic autonomic neuropathy has been obtained by monitoring beat-to-beat variation in heart rate. Nine diabetics with autonomic neuropathy were assessed; each showed a marked reduction or absence of beat-to-beat variation in comparison with controls. Beat-to-beat variation in normal subjects is abolished by parasympathetic blockade but unaffected by sympathetic blockade. These findings suggest that spontaneous vagal denervation of the heart was present in the cases studied. Measurement of beat-to-beat variation provides a simple test whereby cases of autonomic neuropathy can be screened for cardiac involvement.