Heart rate variability: why do spectral analysis?
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Electrocardiography↗
Biomedical subjects
J M Karemaker
Publications and source records attributed to J M Karemaker.
Clinical approach to cardiovascular reflex testing.
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Aging↗
Blood pressure and heart rate responses to sudden changes of gravity during exercise.
Heart rate (HR) and blood pressure responses to sudden changes of gravity during 80- to 100-W leg exercise were studied. One group was exposed to sudden changes between 1.0 and 0 g in the head-to-foot direction (Gz+), starting upright and with repeated 30-s tilts to the supine position. Another group was exposed to sudden Gz+ changes between 1.8 and 0 g in an aircraft performing parabolic flight. Arterial blood pressure at the level of the carotid (carotid distending pressure, CDP) showed a large transient increase by 27-47 mmHg when Gz+ was suddenly decreased and a similar drop when Gz+ was suddenly increased. HR displayed a reverse pattern with larger transients (-22 to -26 min-1) in response to Gz+ decreases and more sluggish changes of lower amplitude in the other direction. Central blood volume, as estimated from the inverse of transthoracic impedance (1/TTI), varied in concert with Gz+. A model is proposed in which HR responses are described as a function of CDP and 1/TTI after a time delay of 2.3-3.0 s and including a low-pass filter function with time constants of 0.34-0.35 s for decreasing HR and time constants of 2.9-4.6 s for increasing HR. The sensitivity of the carotid component was around -0.8 to -1.0 min-1 . mmHg-1 (4-7 ms/mmHg). The cardiopulmonary baroreceptor component was an additive input but was of modest relative importance during the initial HR responses. For steady-state HR responses, however, our model suggests that inputs from carotid and cardiopulmonary receptors are of equal importance.
Adult↗
On the quantification of heart rate changes in autonomic function tests: relations between measures in beats per minute, seconds and dimensionless ratios.
1. Disorders of the autonomic nervous system are frequently diagnosed by measuring heart rate changes in response to deep-breathing and lying-to-standing manoeuvres. The heart rate changes in these manoeuvres are quantified in measures using various units, like beats per minute, seconds and dimensionless ratios. 2. In the present study we mathematically derived relationships between the measures which quantify heart rate changes in beats per minute, seconds and dimensionless ratios. The theoretical outcomes were experimentally confirmed by the results of the deep-breathing and the lying-to-standing test in 525 healthy and diabetic subjects. The measures were found to be non-equivalent, because the mean RR interval duration influenced the measures in different ways. 3. It is argued that measures in seconds are preferable to measures in beats per minute or ratios, because the physiological interpretation of this measure is easier, and the sensitivity of measures in seconds is expected to be greater. 4. Finally, we recommend that measures of heart rate variation in the deep-breathing and lying-to-standing manoeuvre are accompanied by information on the mean RR interval duration or mean heart rate to allow correct interpretation of the measures.
Autonomic Nervous System↗
A chemoreflex model of relation between blood pressure and heart rate in sleep apnea syndrome.
In obstructive sleep apnea syndrome (OSAS), pronounced low-frequency (LF) oscillations of blood pressure and interbeat interval (I) occur during recurrent apneas. We investigated the time relations between LF oscillations of diastolic pressure (D) and I in 12 patients with OSAS by means of spectral analysis. A high coherency between I and D was found, allowing a description of the relation by gain and phase. Oscillations in I and D were almost in counterphase in the LF range. Simple physiological models were implemented to interpret the observed features of LF oscillations. Model 1 describes the vagal and sympathetic influence by the carotid body chemoreflex on the circulation. From derivation of the frequency response of this model, gain and phase relations were obtained as would be expected from the action of the chemoreflex. We found that a range of phase relations can be induced by this reflex, depending on the relative vagal and sympathetic efferent influence on the circulation. This range of phase relations was indeed observed in 10 patients. Extended models that also included the orienting reflex (model 1a) or the baroreflex and a mechanical influence of breathing on the circulation (model 2) could not fit the data without a major contribution of the chemoreflex. We conclude that the relation between LF oscillations in I and D in OSAS can be explained by assuming that stimulation of the carotid body chemoreflex is the main source of these oscillations.
Adult↗
Doppler evaluation of cardiac filling and ejection properties in humans during parabolic flight.
The cardiac filling and ejection properties of seven normal human subjects were examined during microgravity created on a National Aeronautics and Space Administration aircraft during parabolic flight. Doppler echocardiography was used to measure intracardiac velocities in sitting and supine subjects during three phases of flight: hypergravity (phase I), early microgravity (phase III), and late microgravity (phase IV). Heart rate declined 6% (P < 0.001) and right ventricular inflow velocities rose (46%, early; 26%, mean; P < 0.01) between phase I and phases III or IV in the sitting position only. Peak left ventricular outflow velocities rose 12% and inflow velocities rose (13%, early; 20%, mean) between phases I and IV while subjects were in the supine position (P < 0.05). A 14% rise in early velocities alone was seen between phases I and IV while subjects were in the sitting position (P < 0.05). In subjects entering microgravity while sitting, right heart chambers can accept additional venous return. When microgravity was entered while subjects were supine, however, venous augmentation was not observed. Left heart filling was more prominently enhanced when microgravity was entered while subjects were supine, suggesting a shift of fluid within the pulmonary vasculature.
Adult↗
Effects of aging on blood pressure variability in resting conditions.
The objective of this study was to determine the effect of aging on beat-to-beat blood pressure and pulse interval variability in resting conditions and to determine the effect of aging on the sympathetic and vagal influence on the cardiovascular system by power spectral analysis of blood pressure and pulse interval. We studied three groups of healthy, normotensive subjects: young (10 to 15 years, n = 16), adult (20 to 40 years, n = 16), and elderly (70 to 90 years, n = 25). Beat-to-beat blood pressure was measured by Finapres during 20 minutes supine and 10 minutes standing. Overall systolic and diastolic blood pressures and pulse interval variability were determined as SD and as coefficient of variation. Also, relative powers of the mid-frequency (0.08 to 0.12 Hz) and high-frequency bands (0.15 to 0.40 Hz) were determined by spectral analysis. In these subjects no differences in blood pressure variability (either as SD or coefficient of variation) were found between age groups, except for the coefficient of variation of standing diastolic blood pressure, which decreased with aging. Pulse interval variability decreased with aging. Power of the mid-frequency band of systolic and diastolic blood pressures was markedly decreased in the elderly, especially in the standing position. Power of the high-frequency band of pulse interval was also decreased in the elderly. Baroreflex sensitivity calculated by fast Fourier transformation spectral analysis was decreased in the elderly subjects compared with the younger groups. In conclusion, we found no change in the overall variability of blood pressure with aging. Mid-frequency spectral power of blood pressure and mid- and high-frequency spectral powers of pulse interval variability were decreased in the elderly. These results suggest that aging does not merely influence the magnitude of blood pressure and pulse interval variability but causes a complex rearrangement of the variability pattern by changes in neurocardiovascular regulation.
Adolescent↗
G-suit inflation to 50 mmHg alters the cardiovascular transients when entering micro-G in parabolic flight.
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Adult↗
Effects of epidural analgesia and atropine on heart rate and blood pressure variability: implications for the interpretation of beat-to-beat fluctuations.
Cardiovascular variables such as heart rate, arterial blood pressure, stroke volume and the shape of electrocardiographic complexes all vary beat-by-beat. This variability occurs because of the dynamic response of cardiovascular regulatory systems to perturbations in cardiovascular function. We applied spectral analysis to the effects of sympathetic vasomotor blockade by epidural analgesia and parasympathetic blockade of the heart by atropine on the beat-to-beat variability of heart rate and blood pressure in humans. High-frequency fluctuations in heart rate (+/- 0.2 Hz) are caused by respiratory induced fluctuations of blood pressure, mediated by the vagus nerve. Low-frequency fluctuations (0.06-0.12 Hz) are related to sympathetic baroreflex control of vasomotor activity and heart rate. In our study, even partial parasympathetic blockade of the heart by atropine decreased the power in the high and low frequency heart rate fluctuations. There were no significant changes in blood pressure fluctuations in either frequency range. Sympathetic blockade by epidural analgesia decreased only low-frequency fluctuations of both heart rate and blood pressure. From a cardiovascular model and our experimental results we support the view that high frequency fluctuations in heart rate are due to the vagal response to blood pressure fluctuations caused by respiration and that the fluctuations around 0.1 Hz in both heart rate and blood pressure have their origin in the sympathetic baroreflex control loop of vasomotor activity.
Adolescent↗
The effect of oxprenolol dosage time on its pharmacokinetics and haemodynamic effects during exercise in man.
We have studied the effect of dosage time of oxprenolol (Trasicor) on its pharmacokinetics and pharmacodynamics in six healthy volunteers. The drug effects measured were heart rate and systolic blood pressure during exercise. Oxprenolol was taken orally at 08.00 h, 14.00 h, 20.00 h, and 02.00 h in randomized order, with 1 week between successive doses. There were differences in the pharmacokinetics of oxprenolol for the ratio between the apparent volume of distribution and systemic availability (P = 0.04) and for elimination half-life (P = 0.006). Both were lowest after administration at 14.00 h (163 (77) l and 1.2 (0.6) h; mean (SD)) and highest after administration at 02.00 h (229 (100) l, and 1.7 (0.6) h). The systolic blood pressure during exercise before oxprenolol did not vary with dosage time, but heart rate during exercise before intake was lowest before dosage time 08.00 h and highest before dosage time 20.00 h (P = 0.03). The time-course of heart rate during exercise after oxprenolol was described by a model that incorporated the factors drug concentration and spontaneous diurnal variation. EC50 and Emax did not vary between dosage times. The spontaneous diurnal variation in heart rate during exercise was unaffected by oxprenolol, leading to an apparently greater effect of oxprenolol during the night than during the day.
Adult↗
Differences in circulatory control in normal subjects who faint and who do not faint during orthostatic stress.
We have determined if there are differences in normal subjects who fainted and those who did not faint during prolonged standing. We studied the short-term orthostatic responses in relation to heart rate, blood pressure measured by Finapres, left ventricular stroke volume analysed by pulse contour method, cardiac output and systemic vascular resistance, and also postural blood pressure and heart rate variability as assessed by spectral analysis. Thirteen healthy males without a history of syncope were studied. Three fainted after 10-13 min standing; the ten non-fainters remained upright for 20 min. The initial (first 30 s) postural circulatory adjustment was comparable for blood pressure but the rebound bradycardia was smaller in the fainters (heart rate at 22 s amounted to +13 +/- 10 beats/min above control vs. +1 +/- 5 beats/min in the non-fainters). Upright heart rate at 2 min standing was higher in the fainters (+31 +/- 2 beats/min vs. +20 +/- 5 beats/min), and blood pressure at 7 min standing was lower (-2/+5/+8 +/- 5/5/5 mmHg vs. +11/+13/+16 +/- 10/6/5 mmHg). The responses of stroke volume and cardiac output were comparable but systemic vascular resistance gradually decreased in the fainters from 5 min standing to the onset of fainting (+4 +/- 13% vs. +33 +/- 19% at 7 min standing). In fainters, the variability in upright blood pressure around 0.1 Hz was larger (8.8 mmHg2/Hz for diastolic blood pressure vs. 5.7 +/- 1.5 mmHg2/Hz in non-fainters).(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
The validity and reproducibility of the skin vasomotor test--studies in normal subjects, after spinal anaesthesia, and in diabetes mellitus.
Skin sympathetic vasomotor control can be examined in the extremities by the skin vasomotor test. In this test the change in skin blood flow and skin temperature in the hand and foot in response to a cold stimulus is utilized as an index of distal sympathetic nerve fibre integrity. This is of importance in conditions such as diabetes mellitus as peripheral autonomic neuropathy is associated with orthostatic hypotension and diabetic foot complications. The validity and reproducibility of the test as a marker of distal sympathetic nerve function has been studied. The test was performed in nine healthy control subjects and in nine subjects (undergoing minor surgery) after a sympathetic nerve conduction block (L2-L3) was achieved in the lower extremities by spinal analgesia. Changes in skin temperature (p < 0.001) and skin blood flow (p < 0.005) in responses to cooling were significantly larger in the control group than in the group with spinal analgesia. Repeated skin temperature measurements on 42 occasions (test-retest period of 4 weeks) in eight healthy and 34 diabetic subjects indicated a reliability coefficient of 80%. We conclude, therefore, that the skin vasomotor test provides a valid and reproducible quantitative assessment of skin sympathetic nerve function in upper and lower extremities.
Adult↗
Effects of thiopentone, etomidate and propofol on beat-to-beat cardiovascular signals in man.
Analysis of beat-to-beat fluctuations of heart rate and blood pressure is a promising new approach to the clinical diagnosis and management of alterations in cardiovascular regulation. We investigated the effects of three induction agents on beat-to-beat heart rate and blood pressure fluctuations in man. Beat-to-beat fluctuations were analysed by spectral analysis. Two spectral areas are of main interest. One area is centred on the respiratory frequency and shifts with changes in respiratory rate (High Frequency area). These high frequency fluctuations in heart rate are caused by respiratory-induced blood pressure fluctuations, mediated by the vagus nerve through the baroreflex mechanism. Variability in the Low Frequency area, which occurs between 0.06 and 0.12 Hz is considered to be related to haemodynamic fluctuations due to the sympathetic baroreflex control loop of vasomotor activity and heart rate. Results from our study indicate that thiopentone, etomidate and propofol show considerable differences in their effects on beat-to-beat variability of heart rate and blood pressure. These differences can be explained by their specific effects on the cardiovascular system.
Adult↗
Noninvasive cardiac output measurement by arterial pulse analysis compared with inert gas rebreathing.
Noninvasive cardiac output (CO) measured by arterial pulse analysis was compared with that measured by inert gas rebreathing in six healthy male volunteers. Pulse contour analysis was applied to the pressure wave output of a Finapres, which noninvasively measures continuous arterial pressure in a finger. Data were collected before, during, and after a 10-day 6 degrees head-down tilt experiment. Intravenous saline loading and lower body negative pressure stimuli varied CO over 2.8-9.6 l/min, as measured by the rebreathing technique. Because pulse contour provides only relative changes in CO, to obtain absolute values it must be calibrated against another measurement. Pulse contour data were calibrated every measurement day against the mean of two to four control rebreathing CO measurements before the lower body negative pressure or intravenous saline loading stimuli. Using one averaged calibration factor per subject for a total of 27 days, we compared the results of both methods. The linear regression between pulse contour (Pc CO) and rebreathing CO (Rebr CO) was Pc CO = 0.15 + 0.98(Rebr CO) (r = 0.96). The standard deviation of the difference of the two methods was 0.5 l/min (n = 205), excluding data used for calibration. By monitoring pulse contour CO before and during rebreathing, the rebreathing maneuver itself was shown to produce a substantial increase in CO that was mainly related to an increase in heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Body position and volume status as determinants of cardiovascular responses to transition into microgravity in parabolic flight.
The condition of microgravity during spaceflight imposes a new challenge to the cardiovascular system and to its homeostatic mechanisms. Initial fluids shifts from the dependent parts to the upper parts of the body are supposed to induce a plethora of effects which eventually lead to the well-known puffy faces and chicken legs' of astronauts. At the same time some 2-3 kgs. in fluid is lost in urine and by diminished uptake. For research into these longer-term effects of spaceflight extensive physiologic experiments are required in space. In view of the high cost and the logistic problems related to space-research much work is done in simulation experiments like bedrest or head down tilt studies. For the very initial effects of micro-G parabolic flight can be used. In parabolic flights we have addressed the question of immediate cardiovascular effects of the transition into microgravity. Since a parabola will last for not more than some 25 seconds, one may expect to observe mainly changes in the outflow of the autonomic nervous system, reflecting in blood pressure and heart rate as easily measurable parameters. Such changes can be expected to be caused by the sudden disappearance of hydrostatic effects and the shifts of fluid from pools where it is kept under the influence of gravity. Hydrostatic effects will play a role in the position of the baroreceptors with respect to the heart: in the upright position the carotid sinuses are some 25 cm above heart level, consequently they observe a lower pressure than that at the heart. When this effect disappears in micro-G a suddenly increased pressure will be observed and the baroreflex is called into action. On the venous side blood will rush to the right atrium when it is no longer pulled down in the compliant vessels of the abdomen and legs. This may be expected to lead to increased pressures on the low-pressure side of the heart. Apart from changes in filling of the left heart this may lead to autonomic nervous effects on systemic blood pressure and heart rate as well.
Adult↗
Repetitive apneas induce periodic hypertension in normal subjects through hypoxia.
Periodic increases in blood pressure (BP) can occur in the sleep apnea syndrome (SAS) during recurrent apneas. To investigate the mechanisms causing this periodic hypertension, we simulated SAS by imposing a matching breathing pattern on seven healthy awake male volunteers. Continuous finger arterial BP, electrocardiogram, arterial O2 saturation (SaO2), end-tidal CO2, and tidal volume were measured. The role of hypoxia was studied by comparing apneas during depletion of O2 in the spirometer with those during 100% O2 breathing. In all subjects, BP periodically reached values greater than 150/95 mmHg in the hypoxic series. During the hyperoxic apnea series, however, BP remained stable. End-apneic mean BP was shown to be inversely correlated to SaO2 in six subjects in the SaO2 range from 60 to 100%. Although the hypoxic BP pattern closely mimicked that in SAS, the heart rate pattern in four of our subjects remained distinct from that in patients. Atropine could not prevent large BP swings in the hypoxic series. We conclude that SaO2 is a major determinant of periodic hypertension in recurrent apneas. Its effect probably results from chemoreflex modulation of peripheral resistance.
Adult↗
Head-down tilt bedrest. HDT'88--an international collaborative effort in integrated systems physiology.
An international collaborative project, initiated by the DLR-NASA Life Sciences Working Group, led to the performance of a head-down tilt bedrest (HDT) study at the DLR Institute for Aerospace Medicine. Scientific and operational questions were addressed in preparation for the D-2 Spacelab mission. Principal areas of interest were cardiovascular regulation and fluid/electrolyte metabolism. The results are detailed in a series of 13 reports to which the present paper serves as an introduction.
Adaptation, Physiological↗