LF HF Research Articles

Age-related changes in cerebral hemodynamics and their correlations with cardiac autonomic functions

Objectives: This study was designed to evaluate the correlations between cerebral hemodynamics and cardiac autonomic functions in aging process using transfer function techniques.Methods: Arterial blood pressure (ABP), middle cerebral artery flow velocity (MCAFV) detected by transcranial Doppler sonography and electrocardiogram were recorded simultaneously in 20 young (27.5 ± 0.9 years) and 20 middle-old-aged (54.3 ± 1.5 years) healthy volunteers. Variability of ABP, MCAFV and heart rate (HR) were diffracted into very low (VLF, 0.016–0.04 Hz), low (LF, 0.04–0.15 Hz) and high frequency (HF, 0.15–0.4 Hz) components. Cerebral vasomotor reserve was quantified by specific ABP–MCAFV transfer function measurements.Results: The young group had significantly higher LF transfer phase and HF transfer magnitudes of ABP–MCAFV transfer function; higher HF powers of HR variability; and higher baroreflex sensitivity than those of the middle-old-aged group. LF phase and HF magnitude of ABP–MCAFV transfer function were positively correlated with HF powers of HR variability.Discussion: Although aging process is not obviously associated with changes in the static values of ABP, MCAFV and HR in this study, it is accompanied by significant declines in cerebral vasomotor reserve, cardiac vagal activity and baroreflex sensitivity. Besides, age-related changes in cerebral vasomotor reserve are positively related to those in cardiac vagal activity.

Antihypertensive Agents Have Different Ability to Modulate Arterial Pressure and Heart Rate Variability in 2K1C Rats

We examined the effect of chronic (15 days) administration of antihypertensive agents, from different pharmacologic classes, on arterial pressure (AP) and heart rate variability in two-kidney, one-clip hypertensive (2K1C) rats. The 2K1C rats received by gavage one of the following: water, ramipril, losartan, atenolol, amlodipine, or hydrochlorothiazide. Sham-operated normotensive rats received water. After 15 days of treatment AP was continuously sampled from an indwelling catheter in awake rats during a 2-h period and systolic AP and pulse interval (PI) were submitted to autoregressive spectral analysis with oscillatory components quantified in low (LF: 0.25 to 0.75 Hz) and high (HF: 0.75 to 3.0 Hz) frequency bands. The AP measured by tail-cuff was 170 +/- 2 mm Hg in 2K1C and 131 +/- 3 mm Hg in normotensive rats. Pooled data indicated that all antihypertensive agents reduced the AP of 2K1C rats to 127 +/- 2 mm Hg, whereas 2K1C rats treated with water remained hypertensive (206 +/- 11 mm Hg). Variance of systolic AP was found increased in 2K1C rats treated with water (34 +/- 2 mm Hg2), whereas 2K1C rats treated with ramipril, atenolol, amlodipine, or hydrochlorothiazide presented AP variance similar to normotensive rats (16 +/- 2 mm Hg2). Losartan normalized AP of 2K1C rats but variance of systolic AP remained increased (34 +/- 7 mm Hg2). The 2K1C rats treated with water had increased LF of systolic AP, whereas 2K1C rats treated with losartan showed higher LF of systolic AP and PI. Atenolol presented lower LF and higher HF of PI. In conclusion, losartan normalized AP but did not reduce AP variability, suggesting an autonomic imbalance characterized by higher sympathetic modulation of the cardiovascular system.

31 Effects of test meal on multi‐channel Electrogastrography and autonomic nervous system function in different subtypes of patients with functional dyspepsia

Previous studies have shown that patients with functional dyspepsia (FD) had significantly lower percentage of slow wave coupling (%SWC) in fasting state. The aim of this study was to utilize MEGG and ANS test to investigate the difference of MEGG parameters and ANS parameters between different subtypes of FD patients and healthy subjects (HS).Methods: Sixty‐two patients with the diagnosis of FD were enrolled in this study (16 M, 46 F, mean age: 43 years) and divided into dysmotility‐like FD group (15 M, 35 F, mean age: 42 years) and ulcer‐like FD group (1 M, 11 F, mean age: 46 years) based on Rome II criteria. 50 HS were recruited as control (25 M, 25 F, mean age: 44 years). MEGG were recorded simultaneously with the electrocardiogram (ECG) recording for 30 min in the fasting state and 60 min after a standard 450 Kcal test meal. MEGG parameters included dominant frequency (DF), normal percentage of 2–4 cpm gastric slow waves (%N) and %SWC. ANS parameters included sympathovagal balance LF/HF Ratio, mainly sympathetic activity LF and vagal activity HF. Results: (1) Compared with HS, %SWC was significantly lower in ulcer‐like FD group in fasting state (P < 0.05) but not in fed state. No significant difference of %SWC was shown between dysmotility‐like FD group and HS either in fasting state or in fed state. There was no significant difference between the two FD groups and HS in DF and %N neither in fasting state nor in fed state. (2) Compared with HS, LF/HF ratio and LF were significantly lower and HF was significantly higher in fasting state in dysmotility‐like FD group (P < 0.05), this difference was abolished by test meal in the 1st 30 min fed state and the lower LF and higher HF were present in the 2nd 30 min fed state again in dysmotility‐like FD group. There was no significant difference between ulcer‐like FD group and HS in any of those ANS parameters either in fasting state or in fed state.Conclusions: Gastric slow wave coupling was abnormal in patients with ulcer‐like FD. Patients with dysmotility‐like FD had abnormal ANS function in fasting state and test meal could only correct the abnormal ANS for short time, which suggest that different subtypes of FD may have different patterns in ANS test.

Different effects on circulatory control during volatile induction and maintenance of anesthesia and total intravenous anesthesia: autonomic nervous activity and arterial cardiac baroreflex function evaluated by blood pressure and heart rate variability analysis

To evaluate the different effects on autonomic circulatory control during volatile induction/maintenance of anesthesia (VIMA) vs total intravenous anesthesia (TIVA). Prospective study. Operating theater of a university hospital. Twenty patients, with American Society of Anesthesiologists physical status of I or II, were randomly allocated into the VIMA group (n = 10) or the TIVA group (n = 10). In the VIMA group, anesthesia was induced with 5% sevoflurane and 60% N2O in oxygen and maintained with 2% sevoflurane and 60% N2O in oxygen. In the TIVA group, anesthesia was induced with propofol 2.0 mg/kg intravenously by bolus injection and fentanyl 2 microg/kg, and maintained with an intravenous infusion of propofol 5 mg/kg.per hour and air-oxygen mixture. Monitoring included recordings of electrocardiographic and arterial blood pressure waveforms. Autonomic nervous activity and arterial cardiac baroreflex function were evaluated by analysis of blood pressure variability, heart rate variability, and transfer function analysis between these 2 variables. In the VIMA group, the low-frequency component of blood pressure variability (LF(SBP)) and low- and high-frequency components of the R-R interval variability (LF(RR) and HF(RR)) decreased significantly during anesthesia. In the TIVA group, LF(SBP) and LF(RR) decreased significantly. The degree of reduction in LF(SBP) was greater in the VIMA group than in the TIVA group. However, changes in R-R interval variability and cardiac baroreflex indices were not significantly different between the 2 groups. Our results demonstrated that although reductions in autonomic nervous modulation to the heart might not be so different between the 2 groups, reduction in sympathetic nervous modulation to peripheral vasculature is greater in the VIMA group than in the TIVA group.

Relationships between thermic effect of food, insulin resistance and autonomic nervous activity

The thermic effect of food (TEF) is higher in lean than in obese human subjects. Relationships between TEF and insulin resistance during meals, from the point of view of autonomic nervous activity, were evaluated. Autonomic nervous activity was evaluated in 20 young adults using the spectral analysis of heart rate variability from one hour before to two hours after a meal. Heart rate data were analyzed based on low frequency components (LF power, 0.04-0.15 Hz), high frequency components (HF power, 0.15-0.40 Hz), and LF/HF ratios. Energy expenditure and the TEF were measured 30 min after a meal. Homeostasis model of insulin resistance index (HOMA-IR) was also measured. The LF/HF ratio was significantly increased 30 min after a meal (p<0.05). No correlation between LF power and HF power with TEF was found, but the LF/HF ratio was significantly and positively correlated with TEF (r=+0.56, p<0.05). Moreover, a significant negative correlation was found between the HOMA-IR and TEF (r=-0.601, p<0.05). The findings suggest that a reduction in insulin sensitivity induces a poor response of sympathetic nervous activity in the postprandial phase and a reduction in postprandial energy expenditure.

Effects of Different Classes of Antihypertensive Drugs on Cerebral Hemodynamics in Elderly Hypertensive Patients

This study was designed to estimate the effect of different classes of antihypertensive drugs on cerebral hemodynamics in elderly stroke-free, nondiabetic hypertensive patients by applying frequency-domain techniques. A total of 60 hypertensive patients divided into treatment groups of 15 unmedicated (HT), 15 nifedepine sustained release form (CCB), 15 atenolol (BB), and 15 valsartan (AIIA), and 15 age-matched healthy volunteers (CON) were studied prospectively. Variability of arterial blood pressure (ABP) and middle cerebral artery flow velocity (MCAFV) detected by transcranial Doppler sonography were diffracted into very-low frequency (VLF, 0.016 to 0.04 Hz), low frequency (LF, 0.04 to 0.15 Hz), and high frequency (HF, 0.15 to 0.4 Hz). Cerebral hemodynamics was quantified by the ABP-MCAFV transfer function. The ABP and MCAFV were statistically different between CON and HT groups, but not significantly different among CON, CCB, BB, and AIIA groups. The LF phase and HF magnitude in the HT, CCB, and BB groups were significantly attenuated than in those of the CON group, but not statistically different between the AIIA and CON groups. There was no statistical difference in VLF and LF transfer magnitude among the five groups. Although cerebral vasomotor reserve and cerebral blood flow are impaired in unmedicated hypertensive patients, the cerebral autoregulatory response was preserved both in controlled and uncontrolled hypertension. Nifedipine, atenolol, and valsartan are all effective as monotherapy to control ABP and restore cerebral blood flow. Valsartan has the effect to normalize the changes in cerebral vasomotor reserve to a level similar as in age-matched healthy subjects.

Power spectral analysis of heart rate variability as a predictive test in choosing the most effective length for tilt-training

In patients with refractory neurally mediated syncope, tilt training--standing motionless against a wall for increased periods of time per day over one month--can often eliminate recurrent episodes and reduce presyncopal symptoms. We designed dual retrospective and prospective studies to assess cardiovascular autonomic function in subjects with recurrent syncope and identify the most effective length of tilt training between one and three months. In the retrospective study, before tilt training, and in the prospective study, before and after training, all subjects underwent a recording for short-term spectral analysis of heart rate and systolic blood pressure variability. Before tilt-training, autonomic nervous system function differs in patients with recurrent neurally mediated syncope who respond to tilt training for one month and those who do not. "Responders", patients experiencing no episodes of syncope during the 12-month follow-up, had higher low-frequency power of RR (LF(RR)) (p < 0.05) and LF(RR) in normalized units (NU) (p < 0.001) and lower high-frequency power (HF(RR)) (p < 0.05) and HF(RR)NU (p < 0.001) than "non-responders", patients reporting at least one syncopal episode during the 12-month follow-up. In the retrospective study, no difference was found between spectral data for "non-responders" with positive responses to tilt test with and without nitro derivatives. Prolonging tilt-training to three months increased the number of responders (late-responders) by 80% (p < 0.001) and power spectral analysis of heart rate variability (HRV) before tilt training can identify late-responders by their low LF(RR)NUs (<40) and high HF(RR)Nus (>60). Furthermore in late-responders, tilt training brings about a change in cardiovascular autonomic function: at 3 months, LF(RR)NUs increase and HF(RR)NU diminish. Power spectral analysis of HRV seems to be a useful tool to preselect patients who are most likely to benefit from prolonged therapy, thus increasing compliance.

Heart rate variability in children with neurocardiogenic syncope.

In order to characterize the autonomic profile of syncopal children, we have studied heart rate variability (HRV) of 73 children, ages 11-18, with neurocardiogenic syncope and a positive outcome of head-up tilt testing (HUT).HRV was calculated over a 24-hour period for the time-domain indices (SDNN, SDANNi, SDNN, rMSSD, pNN50), and over 5-minute segments from night and day for frequency-domain indices (LF, HF, LF/HF). The obtained results were compared to reference values calculated for Polish children. 55% of the children had mixed response to HUT, 41% vasodepressor and 4% cardioinhibitory. Patients with syncope had significantly lower values of rMSSD and pNN50 in comparison to healthy children. Moreover, in the frequency-domain analysis they exhibited significantly higher LF and lower HF values. The day-night rhythm of HRV and the age-related changes of HRV were, however, similar in syncopal and healthy children. In addition to this, we found a significantly lower SDNN value in children with cardioinhibitory response during HUT in comparison to children with mixed response. We concluded that 1) based on HRV analysis children with neurocardiogenic syncope had alterations in basal autonomic balance, which indicated an increased sympathetic modulation in these patients, 2) syncopal children had adequate circadian rhythm of autonomic activity, 3) the changes of HRV indices with age in these groups are not altered in comparison to healthy children, 4) syncopal children may exhibit differences in HRV indices values depending on the kind of vasovagal response observed during HUT.

Selective reductions of cardiac autonomic responses to light bicycle exercise with aging in healthy humans

We examined on 56 (age 38+/-2 [range: 16-60] years) healthy subjects the effects of aging on cardiovascular autonomic responses to progressive supine bicycle exercise of light intensity. Autoregressive spectral analysis of RR interval and systolic arterial pressure (SAP) variabilities provided measures of the exercise-induced changes in baroreflex gain (by index alpha) and in sympathetic and vagal modulation of the SA node (by the normalized low [LF] and high frequency [HF] component of RR interval variability, respectively), as well as of changes in sympathetic vasomotor control (LF(SAP)). For each hemodynamic and autonomic variable, the gain of the response was expressed both as individual step increments, and as the slope of the linear regression of the sequential data points from rest and during the three steps of exercise. Age resulted significantly correlated to changes in spectral derived markers of SA modulation (LF(RR), HF(RR) and index alpha). Conversely, no significant relationships were found with changes in RR interval, in SAP and indices of vascular regulation (LF(SAP)). In addition, exercise-induced changes in indices of SA node regulation were more evident in the youngest tertile (age 25+/-1 years), compared to the oldest tertile (age 52+/-1 years). In conclusion, we have observed that aging progressively and selectively reduces the cardiac autonomic excitatory response to light exercise, while hemodynamic and vascular responsiveness are maintained.

Long-term changes and trends in the lower ionosphere

A brief overview of long-term trends in the lower ionosphere (h<100 km, ionized component) is presented. The trends have been studied with the use of several different types of data sets: A3 radio wave absorption (LF–HF, oblique incidence on the ionosphere, continuous wave), A2 (cosmic radio noise) riometric absorption, IPHA (indirect measurements of phase reflection height), rocket measurements of electron density and ion composition. The A3 radio wave absorption in the lower ionosphere over Europe has been analyzed for trends in: (1) absolute values of absorption; (2) amplitudes of annual and semiannual variations; (3) inferred planetary wave activity. Trends in absolute values of absorption do not provide a consistent pattern, which may be due to their sensitivity to changes in the shape of electron density profile and to instrumental problems. Similarly, there is no consistent pattern in trends in relative amplitudes of annual variations. However, there appears to be a systematic tendency to the positive trend in amplitudes of semiannual variations. Some increase of the absorption-inferred planetary wave activity was found in 1970s and 1980s, whereas no change was found in 1960s and 1990s. Analyses of cosmic noise absorption measured by a chain of riometers in Finland do not reveal an evident trend in absorption. IPHA analyses in mid-latitudinal Europe point to a systematic decrease of the reflection height (level of constant electron density) near 80 km. This has been interpreted in terms of decreasing atmospheric pressure due to decreasing columnar temperature in 50–80 km. Rocket measurements of electron density display for non-winter electron density at 75 and 80 km an increase at middle latitudes (contrary to IPHA results). Rocket measurements of ion composition in the lower ionosphere are too rare, but models reveal dramatic changes with a large decrease of NO+ concentration. Since the lower ionosphere is substantially affected by the 11-year solar cycle, data sets length of more than 20 years is very desirable in long-term trend studies. Results based on various methods and various data sets do not provide sufficiently consistent observational pattern of trends in the lower ionosphere. Therefore, further investigations, tests of data quality and homogeneity, and careful application of various methods are necessary.

Statistical addition method for external noise sources affecting HF‐MF‐LF systems

The current statistical method for the addition of external component noise sources in the LF, MF, and lower HF band (100 kHz to 3 MHz) produces total median noise levels that may be less than the largest‐component median in some cases. Several case studies illustrate this anomaly. Methods used to sum the components rely on their power (decibels) distributions being represented as normal by the statistical parameters. The atmospheric noise component is not correctly represented by its decile values when it is assumed to have a normal distribution, causing anomalies in the noise summation when components are similar in magnitude. A revised component summation method is proposed, and the way it provides a more physically realistic total noise median for LF, MF, and lower HF frequencies is illustrated.

Auditory integration of bandpass filtered sentences

Auditory integration is the ability to merge auditory inputs received from different channels into a single percept. The present experiments investigated auditory integration of speech using lists of filtered CUNY sentences. Within each set, separate filtering preserved either low-frequency or high-frequency spectral information. The three sets of bandwidths were 550–750 and 1650–2250 Hz; 550–800 and 1650–2350 Hz; and 550–850 and 1650–2450 Hz. Four conditions were tested for each set of bandwidths: (1) LF only: (2) HF only; (3) LF–HF monotic (LF and HF filtered sentences presented simultaneously to one ear); and (4) LF–HF dichotic (LF sentence presented to one ear, HF sentences presented simultaneously to the other ear). Scores in the single band conditions were near floor. Scores in the monotic conditions were significantly greater than would be predicted from an additive combination of the single band conditions (i.e., superadditive effects). Scores in the LF–HF dichotic condition were significantly less than those in the LF–HF monotic condition but were greater than those for either the LF-only or HF-only conditions. Taken together, these findings suggest that listeners are able to integrate information across two auditory channels but that this integration is less than optimal. Implications of these results for theories of sensory integration are discussed.

Normalized Idioventricular QT Interval Variability in Patients with Q- and Non-Q Wave Myocardial Infarction

Background and Objectives:The aim of the study was to evaluate the difference of temporal lability in my- ocardial repolarization between acute non-Q (NQMI and Q-wave myocardial infarction (QMI, and to discern whether the locations of myocardial infarction influence such temporal lability. Subjects and Methods:Tw- elve patients with NQMI and 28 with QMI, including 16 anterior (AMI and 12 inferior MI (IMI patients were enrolled. Twenty four-hour ambulatory ECG recordings of each patient were analyzed, and the digitized data was partitioned into 30-min sections. The QT intervals were measured using a template matching strategy. We then calculated the low (LF:0.03-0.15 Hz and high frequency (HF:0.15-0.4 Hz power of the QT interval variability using an algorithm capable of removing the influence of the RR-interval on QT interval va- riability (Normalized Idioventricular QT variability Index:IV-QT. Results:For patients with QMI, the low frequency IV QT (LF IV-QT was higher than that of NQMI (1.941±0.101 and 1.556±0.114 respectively, p<0.05. No difference was seen in the high frequency IV QT (HF IV-QT of the two groups. For QMI pat- ients, both the LF IV-QT and HF IV-QT were higher in day time (6AM-6PM than in night time (6PM-6AM. Comparing the differences of these indices by the location of QMI, both the LF IV-QT and HF IV-QT of AMI were higher than those of IMI patients (2.231±0.135 vs 1.355±0.131 and 2.341±0.161 vs 1.346±0.145 respectively, p<0.0005. Both indices of each group also demonstrated a circadian change. Conclusion:In cases of QMI, the temporal lability in myocardial repolarization is larger than that seen in NQMI. Moreover, it was worse in AMI than IMI. Finally, such temporal repolarization lability tends to have a circadian pattern in QMI. (Korean Circulation J 2001;31(12 :1281-1289

Influence of dietary fat and adiposity on feed intake of juvenile red sea bream Pagrus major

Abstract A two-phase experiment was conducted to examine the effects of dietary fat and adiposity on feed intake of red sea bream. In Phase 1, juveniles (9.7 g, 20 fish tank −1 , 4 replicates) were fed low fat (LF; 15.3% fat, 60.6% protein, 22.2 kJ g −1 energy, 0.61 g ml −1 density) or high fat (HF; 24.6% fat, 54.9% protein, 24.2 kJ g −1 energy, 0.68 g ml −1 density) diets for 4 weeks to produce fish which differed in adiposity. In Phase 2, all groups were fed the LF and HF diets alternately for 8 days. Fish were fed to apparent satiation by hand twice daily and daily feed consumption was recorded. Fish were weighed at the start, weekly during Phase 1 and at the end of Phase 2. Proximate composition was determined at the start of the experiment and after Phase 1 and 2. In Phase 1 the amount (18.6±0.35 vs. 17.1±0.19 g) and volume (30.6±0.58 vs. 25.2±0.28 ml) of feed consumed was significantly greater in fish fed the LF diet; energy intakes (LF, 412±7.8; HF, 413±4.7 kJ) and terminal mean fish weights (LF, 28.2±0.5; HF, 27.7±0.1 g) were similar. Fish fed the HF diet had a significantly higher proportion of visceral (37.6±1.5 vs. 28.9±1.2%) and carcass fat (9.7±0.3 vs. 8.0±0.3%). In Phase 2, fish fed the LF diet in Phase 1 consumed more of both the LF diet (2.9±0.2 vs. 2.4±0.1 g, P P

Effect of Conscious Sedation on Cardiac Autonomic Regulation during Colonoscopy

Background: Colonoscopy is associated with cardiovascular events including hypotension, hypertension, and myocardial ischemia. The pathogenetic mechanisms of these cardiovascular events are unknown, but there is evidence that the autonomic nervous system may play a role. Conscious sedation is often used to relieve the inconvenience caused by the procedure. In this study, we evaluated the effects of sedation on cardiac autonomic regulation during colonoscopy. Methods: One hundred and eighty patients undergoing elective colonoscopy were prospectively randomized into three groups: (i) sedation with intravenous midazolam (midazolam group); (ii) sedation with intravenous saline (placebo group); and (iii) no intravenous cannula (control group). Continuous electrocardiogram was recorded prior to, during, and after the colonoscopic procedure. Heart rate variability (HRV) was assessed by means of the power spectral analysis; the powers of low-frequency (LF 0.04-0.15 Hz) and high-frequency (HF 0.15-0.40 Hz) components were calculated. Results: Intubation of the colonoscope increased the LF component of HRV and decreased HF power in all study groups compared to baseline recording. Furthermore, compared to baseline, the LF/HF ratio-a marker of cardiac sympathetic regulation-increased during intubation in the midazolam (P < 0.001) and placebo (P < 0.05) groups, with no change in the control group. During intubation the midazolam group presented with higher LF and lower HF power than placebo (P < 0.001) and control groups (P < 0.01). Accordingly, the LF/HF ratio was higher in the midazolam group than in the placebo (P < 0.05) or control groups (P < 0.05). Conclusions: Midazolam potentiates the dominance of the sympathetic nervous system induced by colonoscopy. Therefore, conscious sedation with midazolam may contribute to the occurrence of cardiovascular events during colonoscopy.

Effects of different insulin infusion rates on heart rate variability in lean and obese subjects

The low-frequency to high-frequency ratio (LF/HF ratio) is an index of cardiac sympathovagal balance. We hypothesized that insulin might also stimulate the LF/HF ratio. Thus, 15 lean and 15 obese subjects were studied. Each subject underwent sequential hyperinsulinemic clamps (insulin infusion rate 0.50, 1, and 2 mU/kg x min) while the heart rate was recorded by the Holter technique continuously. Indirect calorimetry allowed determination of the respiratory quotient (Rq) and substrate oxidation. The leg blood flow (LBF), leg vascular resistance (LVR), and plasma norepinephrine concentration were also measured. In seven lean subjects, hyperinsulinemic clamps were repeated along with propranolol infusion (0.1 mg x kg(-1) as an intravenous bolus dose followed by continuous intravenous infusion of 0.5 mg x kg(-1) x min(-1) throughout the study). Lean subjects had better insulin action than obese subjects. Insulin infusion was associated with an increase of the deltaLF/HF ratio in both lean (P < .001 for time-dependent changes) and obese (P < .02 for time-dependent changes) subjects; however, the extent of insulin-mediated stimulation of the LF/HF ratio was greater in lean versus obese subjects. Insulin infusion did not significantly affect HF values in both groups. Independently of gender, body fat, changes in the plasma norepinephrine concentration, LBF, and LVR, the deltaLF/HF ratio at the end of the fastest insulin infusion (0.8 +/- 0.2 v 0.3 +/- 0.2, P < .04) was still greater in lean versus obese subjects. The deltaLF/HF ratio was also more stimulated during insulin versus insulin + propranolol infusion in lean subjects. In conclusion, insulin stimulates the LF/HF ratio in both lean and obese subjects and thus produces a shift in the cardiac autonomic nervous system activity toward sympathetic predominance.

Transfer function analysis of cerebral hemodynamics in patients with carotid stenosis.

This study evaluates the validity of the transfer function analysis of spontaneous fluctuations of arterial blood pressure (ABP) and blood flow velocity of the middle cerebral artery (MCAFV) as a simple, convenient method to assess human cerebral autoregulation in patients with carotid stenosis. Eighty-three consecutive patients with various degrees of carotid stenosis and 37 healthy controls were enrolled. The carotid stenosis was graded based on the diagnostic criteria of duplex ultrasound. Instantaneous bilateral MCAFV and ABP of all participants were assessed noninvasively using transcranial Doppler sonography and the servocontrolled infrared finger plethysmography, respectively. Spectral analyses of ABP and MCAFV were performed by fast Fourier transform. The fluctuations in ABP as well as in MCAFV were diffracted into three components at specific frequency ranges designated as high-frequency (HF; 0.15 to 0.4 Hz), low-frequency (LF; 0.04 to 0.15 Hz), and very low-frequency (VLF; 0.016 to 0.04 Hz). Cross-spectral analysis was applied to quantify the coherence, transfer phase, and magnitude in individual HF, LF, and VLF components. Transcranial Doppler CO2 vasomotor reactivity was measured with 5% CO2 inhalation. The LF phase angle (r=-0.53, P<0.001); magnitude of VLF (r=-0.29, P=0.002), LF (r=-0.35, P<0.001), and HF (r=-0.47, P<0.001); and CO2 vasomotor reactivity (r=-0.66, P<0.001) were negatively correlated with the severity of stenosis. Patients with unilateral high-grade (greater than 90% stenosis) carotid stenosis demonstrated significant reduction in LF phase angle (P<0.001) and HF magnitude (P=0.018) on the ipsilateral side of the affected vessel compared with their contralateral side. The study also revealed a high sensitivity, specificity, and accuracy using LF phase angle and HF magnitude to detect a high-grade carotid stenosis. A strong correlation existed between the LF phase angle and the CO2 vasomotor reactivity test (r=0.62, P<0.001), and the correlation between the HF magnitude and the CO2 vasomotor reactivity (r=0.44, P<0.001) was statistically significant as well. We conclude that transfer function analysis of spontaneous fluctuations of MCAFV and ABP could be used to identify hemodynamically significant high-grade carotid stenosis with impaired cerebral autoregulation or vasomotor reserve.

Increased capillary filtration of albumin in diabetic patients—relation with gender, hypertension, microangiopathy, and neuropathy

The aim of this study was to investigate the factors associated with an increase in capillary filtration of albumin (CFA) in a large series of diabetic patients and its relationship with gender, hypertension, microangiopathy, and neuropathy. One hundred sixty-three unselected diabetic patients, 74 type I and 89 type II, were included. An isotopic test of CFA was performed with 99m technetium-labeled albumin injected intravenously. Radioactivity was counted externally at the forearm with a gamma camera before, during, and after venous compression. After removal of venous compression, interstitial albumin retention (AR) was calculated and the radioactivity disappearance curve was analyzed by the Fast Fourier transform, which provides an index for lymphatic uptake of interstitial albumin (low-frequency to high-frequency amplitude peak ratio [LF/HF]). An increase in AR and LF/HF was found in 65 (39.9%) and 117 (71.7%) patients, respectively. Increased AR was significantly more frequent in women than in men (P=.018) and in patients without microangiopathic complications than in those with them (P=.028). In men, it was significantly more frequent in type I versus type II diabetic patients (P=.004), and AR was significantly higher in patients with peripheral neuropathy than in those without (P=.004). The LF/HF was also significantly higher in men with peripheral neuropathy (P=.045). In women, the AR level correlated negatively with postprandial glycemia (P=.006) and was significantly higher in patients without microangiopathic complications (P=.003). These data suggest the role of hormonal factors, both sex steroids and insulin, and the major role of peripheral neuropathy in the increase in CFA. The highly prevalent increase in CFA before the onset of microangiopathic complications is consistent with the presence of a functional microcirculatory disorder that might contribute to the occurrence of microangiopathic lesions.

Autonomic changes during pregnancy: Assessment by spectral heart rate variability analysis

The aim of this study was to investigate cardiovascular autonomic balance in normal pregnancy through spectral analysis of heart rate variability. For this purpose, electrocardiograms were recorded in 8 nonpregnant women and in 32 pregnant women at 10-39 weeks of gestation. Low-/high-frequency peak (LF/HF) ratio was used as an indirect index of sympathovagal balance. In left lateral recumbency, the LF/HF ratio was lower in pregnant women than in control subjects at all gestational ages. The LF/HF ratio was not affected by turning from left lateral to supine recumbency in control subjects, whereas it significantly increased in pregnant women (eg, from a mean +/- SD of 1.05 +/- 0.81 to 2.18 +/- 1.69 after the 33rd week of gestation). Handgrip at low intensity remarkably increased the LF/HF ratio both in control subjects (from 2.21 +/- 2.76 to 2.73 +/- 1.91) and in pregnant women (from 1.28 +/- 1.17 to 2.24 +/- 0.95 at 34-39 weeks of gestation). The results suggest that a rearrangement of autonomic tone takes place in normal pregnancy, which could be interpreted either as a shift of autonomic balance toward a relative vagal predominance or as the consequence of attenuation of baroreflexes.

High altitude-related neurocardiogenic syncope

This report presents a well-documented link between vasodepressor syncope, a marked increase of LF/HF ratio response to orthostasis (sympathetic dominance), and the efficacy of beta blockers in preventing orthostatic symptoms and absence of tilt-induced syncope, with normalization of the LF/HF ratio response.