Temporal stability of ambulatory stroke volume and cardiac output measured by impedance cardiography

Recently, devices have become available that allow non-invasive measurement of stroke volume and cardiac output through ambulatory thorax impedance recording. If such recordings have adequate temporal stability, they offer great potential to further our understanding of how repeated or chronic cardiovascular activation in response to naturalistic events may contribute to cardiovascular disease. In this study, 24 h ambulatory impedance-derived systolic time intervals, stroke volume and cardiac output were measured in 65 healthy subjects across an average time span of 3 years and 4 months. Stability was computed separately for sleep and daytime recordings. To avoid confounding by differences in posture and physical activity across measurement days, temporal stability was computed using sitting activities only. During the day intraclass correlations were moderate for stroke volume (.29-.46) and cardiac output (.33-.46) and good for systolic time intervals (.55-.81). When test-retest comparison was limited to two comparable days (two work days or two leisure days), correlations for both SV (.42-.46) and CO (.43-.50) improved. CONCLUSION: Moderate long-term temporal stability is found for individual differences in ambulatory stroke volume and cardiac output measured by impedance cardiography.     

Electric properties of flowing blood and impedance cardiography

An effective resistivity is defined for axisymmetric flow through a circular tube with a uniform electric field in the longitudinal direction. The resistivity of flowing blood is found to be a function of the shear rate profile. Under axisymmetric conditions shear rate profiles are a function of a single parameter: the reduced average velocity, which is the average velocity divided by the radius of the tube. The resistivity of human blood was investigated while the blood was in laminar flow in a circular tube with different constant flow rates. The relative change in resistivity in % is given by: −0.45·H·{1-exp[−0.26·(〈v〉/R)^0.39]}; where H is the packed cell volume in % and 〈v〉/R is the reduced average velocity in s⁻¹. In accelerating flow the resistivity change is synchronous with the change in flow rate, but in decelerating flow there is an exponential decay characterized by a relaxation time τ. For packed cell volumes of 36.4% and 47.5% τ was estimated to be 0.21 s, for a packed cell volume of 53.7% τ was estimated to be 0.29 s. The resistivity changes in elastic tubes are influenced by both velocity changes and changes in diameter, but in opposite directions.     

Simultaneous measurement of stroke volume by impedance cardiography and nuclear ventriculography: Comparisons at rest and exercise

We have conducted four major impedance cardiography (ZCG) studies to provide data on validity, reproducibility, and sensitivity of response to interventions. The reference technique was quantitative nuclear ventriculography (NVG). Subjects were healthy young men in situations where minimally invasive and unobtrusive techniques were preferred. Interventions used included caffeine ingestion and exercise. Validity of ZCG estimates of stroke volume index (SVI, ml/m²) was tested in 35 men at rest. SVI was 49±9 by ZCG and 46±7 by NVG (r=0.82). Measurements of SVI during bicycle exercise showed no overall difference by the two methods (F=0.26, P=NS). Vascular resistance index (mean BP/CI) increased comparably by both techniques (+9.6% by ZCG and +9.7% by NVG) following caffeine (3.3 mg/kg). The reproducibility of ZCG was demonstrated in the day-to-day consistency of caffeine's effect on vascular resistance in 3 other studies (+11.9%, 12%, and 8.9%). Reliability across and within days was shown by repeated tests in the same subjects (SVI, r's=.96, .92). Conclusion: Impedance cardiography was shown to be a useful noninvasive technique for evaluation of cardiodynamics in biomedical research since it was highly reproducible and yielded equivalent results of relative changes produced by pharmacological and physical challenge. In addition, there was good agreement with NVG in absolute values for grouped data. Supported by funds from the National Heart, Lung, and Blood Institute, HL32050; and the Medical Research Service, Department of the Veterans Affairs.     

Development of a wearable multi-frequency impedance cardiography device

Abstract

Cardiovascular diseases as well as pulmonary oedema can be early diagnosed using vital signs and thoracic bio-impedance. By recording the electrocardiogram (ECG) and the impedance cardiogram (ICG), vital parameters are captured continuously. The aim of this study is the continuous monitoring of ECG and multi-frequency ICG by a mobile system. A mobile measuring system, based on ‘low-power’ ECG, ICG and an included radio transmission is described. Due to the high component integration, a board size of only 6.5?cm?×?5?cm could be realized. The measured data can be transmitted via Bluetooth and visualized on a portable monitor. By using energy-efficient hardware, the system can operate for up to 18?hs with a 3?V battery, continuously sending data via Bluetooth. Longer operating times can be realized by decreased transfer rates. The relative error of the impedance measurement was less than 1%. The ECG and ICG measurements allow an approximate calculation of the heart stroke volume. The ECG and the measured impedance showed a high correlation to commercial devices (r?=?0.83, p?<?0.05). In addition to commercial devices, the developed system allows a multi-frequency measurement of the thoracic impedance between 5–150?kHz.     

The concurrent recording of electroencephalography and impedance cardiography: Effects on EEG

Three experiments were performed testing the effects of a variety of impedance cardiograph electrode types and recording arrangements on recorded electroencephalography (EEG) using either a monopolar single-ear reference or a physically linked ears reference. EEG was recorded either alone or concurrently with an impedance cardiograph. When the cardiograph was recorded using a spot electrode for the top current-inducing electrode, there was an overall decrease in power density of the EEG, and this effect was dependent on the location of the recording electrode. This effect was diminished when the top cardiograph spot electrode was replaced by a mylar-coated neck band electrode and EEG was recorded using a monopolar, single-ear reference. However, there tended to be an overall increase in log power density of the EEG in each frequency band below 60 Hz when less technologically advanced EEG amplifiers were used. This effect was diminished if the EEG was recorded using a physically linked ears reference. Recommendations for the concurrent recording of EEG and impedance cardiography are discussed.     

Pediatric impedance cardiography: temporal stability and intertask consistency

The pathogenic processes responsible for cardiovascular disease have their origins in childhood. Although children's measures of heart rate and blood pressure have been found to be reliable, the reliability of impedance cardiography derived measures have not been evaluated. Thirty-three children, ages 8-11 participated in two sessions. Stressors included serial subtraction, isometric handgrip, and mirror-image tracing. Results indicated the impedance measures showed moderately high temporal stability (average scores r(avg) = .74; difference scores r(avg) = .53) and intertask consistency (average scores r(avg)= .78; difference scores r(avg)= .53). Blood pressure demonstrated the lowest reliability; Heather index, preejection period, and stroke volume demonstrated the highest. These findings suggest children's cardiovascular reactivity to laboratory stressors can be reliably and consistently assessed using impedance cardiography.     

An impedance cardiography system: A new design

An IBM compatible impedance cardiac output monitoring prototype system has been developed for use at the bedside on patients in the ICU, CCU, ER, Cath. Lab, and OR, etc. This impedance cardiography (ICG) system, whose operation is completely technician-free, provides a continuous display with digital results and four channel color waveforms on an Enhanced Graphics Display screen. The software is written in C language with several special segments in assembly code where speed is essential. In this prototype system, a real-time algorithm was introduced to modify the ensemble averaging technique so that it averages nonperiodic signals such as: ECG, dZ/dT, ΔZ, etc. Also, a real-time algorithm was developed to adaptively detect R spikes from conventional ECG signals. A signal preprocessor was developed to process signals digitally before any further work is done. This procedure reduces muscle noise, 60 Hz interference, and ventilatory movement. A special digital filter was designed to cope with the cases in which pacemakers are used. A special algorithm was also developed to further reduce the ventilation artifacts so that a period of apnea is unnecessary during the performance of the measurements. An anatomically specified electrode configuration has been defined enabling precise and reproducible positioning of the electrodes—hopefully leading to electrode standardization. At the present time, this prototype system has been compared with standard hand calculation and correlated with the clinical “gold standard,” the Swan-Ganz thermodilution cardiac output. Using 144 sets of data from 10 healthy volunteers, 4 critically ill patients, and 8 healthy exercising volunteers, calculations of cardiac output were made using our system and the standard hand calculation of stroke volume, based upon Kubicek's equation; there was a relatively high and stable correlation: r=0.93, p<0.005 (healthy); r=0.94, p<0.002 (ill); r=0.95, p<0.002 (exercise). From 20 patients at two different hospitals all with Swan-Ganz catheters in their hearts 65 correlation studies between our system and the standard thermodilution technique were performed; the results were encouraging in terms of accuracy and consistency (r₁=0.84, p<0.01, n=10 CCU patients), and (r₂=0.93, p<0.01, n=ICU, patients). These results along with a growing body of data from other investigators indicate that this noninvasive and technician-free system for measuring cardiac output could have a significant role in patient care.     

New signal processing techniques for improved precision of noninvasive impedance cardiography

Impedance cardiographic determination of clinically important cardiac parameters such as systolic time intervals, stroke volume, and related cardiovascular parameters has not yet found adequate application in clinical practice, since its theoretical basis remains controversial, and the precision of beat-to-beat parameter estimation has until recently suffered under severe shortcomings of available signal processing techniques. High levels of noise and motion artifacts deteriorate signal quality and result in poor event detection. To improve the precision of impedance cardiography, new techniques for event detection and parameter estimation have been developed. Specifically, matched filtering and various signal segmentation and decomposition techniques have been tested on impedance signals with various levels of artificially superimposed noise and on actual recordings from subjects in a laboratory study of cardiovascular response to a cognitive challenge. Substantial improvement in the precision of impedance cardiography was obtained using the newly developed signal processing techniques. In addition, some preliminary evidence from comparisons of the impedance cardiogram with invasive aortic electromagnetic flow measurement in anesthetized rabbits is presented to address questions relating to the origin of the impedance signal. This research was supported by program project research grant HL36588, by research grant HL41335, and by research training grant HL07426 from the National Heart, Lung and Blood Institute of NIH.     

The current status and future directions of impedance cardiography in ICU

Impedance cardiography has not achieved popularity in the Intensive Care Unit (ICU) to date probably because of the limitations in technique and interpretation associated with the altered physiology of critically ill patients, and also because of interference from other machinery in the ICU. The current climate of questioning the existing technology for bedside cardiovascular assessment however spurs the need to evaluate impedance cardiography as a noninvasive alternative. Validation in noncritically ill patients is good when compared to other technologies (e.g., thermodilution, Fick, dye dilution (r>0.9)). Reliability is good with a coefficient of variation in an ICU population of 8.9%, (compared to 18.6% for thermodilution). It has also shown promise in detecting the clinically significant changes of central intravascular volume. Impedance cardiography appears to be useful for measurement of stroke volume (SV) and ejection fraction (EF). From these, left ventricular end-diastolic volume (V_ED) can be calculated, probably a more reliable estimate of left ventricular filling than pulmonary capillary wedge pressure (P_W), measured by pulmonary artery (PA) catheter. In addition, V_ES can be calculated and with the knowledge of left ventricular end-systolic pressure (P_ES) (from invasive arterial monitoring), an end-systolic pressure-volume (ES-PV) (relationship can be derived. This is thought to be a measure of contractility that is independent of preload and afterload. The ultimate test in the ICU for impedance cardiography is whether clinical outcome of critically ill patients is altered by the use of this technology. Such outcome testing is essential before the true value of impedance cardiography in the management of critically ill patients can be determined.     

Stroke volume and systolic time intervals: Beat-to-beat comparison between echocardiography and ambulatory impedance cardiography in supine and tilted positions

The aim of the study was to compare stroke volume (SV), ejection time (ET) and pre-ejection period (PEP) measurements obtained using a central haemodynamics ambulatory monitoring device based on impedance cardiography (ICG), in supine and tilted positions (60°), with pulsed Doppler echocardiography as a noninvasive reference method. The Holter-type ICG device was used for off-line, beat-to-beat, automatic determination of SV, ET and PEP. ICG data were compared with those obtained simultaneously using pulsed Doppler echocardiography in the ascending aorta from a suprasternal projection, 1 min before and 10 min after tilting. The tests were performed in 13 young, healthy subjects (six men and seven women, aged 23–33 years). Linear regression between the measured values obtained for all subjects was described by the following formulas: SV_icg=13.9+0.813*SV_echo (r=0.857 SEE=9.03, n=496), ET_icg=16.8+0.987*ET_echo (r=0.841, SEE=21.3, n=496), PEP_icg=22.8+0/890*PEP_echo (r=0.727, SEE=14.6, n=496). The data showed that ambulatory impedance cardiography gives useful absolute values of SV and systolic time intervals measured in supine and tilted positions.     

Comparison of a three‐quarter electrode band configuration with a full electrode band configuration for impedance cardiography

Impedance cardiography is a technique commonly used in psychophysiological studies. However, concerns about the utility of full circumferential band electrodes (FB) have been raised. The current study was designed to compare FB with a three‐quarter circumferential band configuration (PB). A total of 47 participants (66% female, mean [SD] age=20.4 [3.0] years) underwent 2 testing sessions, once using FB and once using PB. Session order was randomized and balanced. Each session consisted of 5 min of rest, math task, recovery, and cold pressor test. Average baseline and task pre‐ejection period (PEP), stroke volume (SV), cardiac output (CO), heart rate (HR), blood pressure (BP), and total peripheral resistance (TPR) was calculated from impedance cardiography and blood pressure monitoring. Participants were are asked to rate measures of comfort after each session. There were no significant difference between the mean levels of PEP, SV, CO, HR, and TPR for the PB versus the FB configurations. However, both systolic BP and diastolic BP were higher during the FB session. Intraclass correlations were high (r_icc=.63–.93) between PB and FB. Bland‐Altman analyses revealed a low level of bias (≤5%) between the configurations. Based on limits of agreement between ±30%, there was equivalence in PEP between the 2 configurations, and SV, CO, and TPR were close to reaching equivalence. Participants clearly indicated greater comfort with the PB configuration compared to the FB. The current study provides incremental evidence that suggests a three‐quarter PB configuration may be utilized for standard psychophysiological testing instead of the standard FB configuration. However, further studies are needed to validate the PB configuration against other techniques.     

Stroke volume measurement during supine and upright cycle exercise by impedance cardiography

This study evaluated impedance cardiography (ZCG) estimates of stroke volume (SV) during exercise. Seven subjects were studied at rest and during progressive cycle exercise in supine and upright positions. SV was determined by ZCG (SVzcg) during exercise and for the first 5 cardiac cycles following exercise. SVzcg was compared with separate measurements of SV by CO₂ rebreathing (SVco ₂). Static blood resistivity (ρ) was measured at each level of exercise. No significant differences were found between supine exercise and immediate post-exercise values for the peak of the first derivative of the impedance change (dZ/dt _max), left ventricular ejection time (LVET), or SVzcg. Small differences indZ/dt _max and SVzcg, but not LVET, were found in exercise to post-exercise cycling in the upright position. Intra-individual SVzcg and SVco ₂ were moderately correlated (upright meanr=0.64, supiner=0.42) from rest to 70% of peak . Similar correlations were found between Pulse-O₂ ( /heart rate, used as an index to SV) and both SVzcg (uprightr=0.73, supiner=0.57) and SVco ₂ (uprightr=0.8, supiner=0.65). The ZCG parametersdZ/dt _max and LVET correlated better with Pulse-O₂ (dZ/dt _max: uprightr=0.92, supiner=0.73; LVET: uprightr=−0.9, supiner=−0.9). SVzcg calculated with the Kubicek equation performed as well as SVco ₂. ZCG might be a superior method if the inversely correlated parameters,dZ/dt _max and LVET, were not expressed as a product to calculate SV.     

Autonomic cardiac control. II. Noninvasive indices and basal response as revealed by autonomic blockades

Heart period, systolic time intervals, low and high frequency heart period variability, blood pressure, and respiration were measured in female subjects under three drug conditions (saline, atropine sulfate, metoprolol) while sitting and standing on three consecutive days. Following preinfusion baseline recordings, saline, metoprolol (14 mg), or atropine sulfate (2 mg) was infused for 15 min (by using a double-blind procedure). Recordings were taken during a postinfusion baseline and in response to an orthostatic stressor (standing versus sitting postures). At the end of the metoprolol session, atropine sulfate was infused and responses were monitored during the post-infusion (i.e., double blockade) baseline and during orthostatic stressor. Analyses of the blockade data revealed that the preejection period (PEP) reflected sympathetic but not vagal influences on the heart, and high frequency (HF, 0.12–0.40 Hz) heart rate variability (respiratory sinus arrhythmia) reflected vagal but not sympathetic influences on the heart. No other measure provided a specific index of the tonic sympathetic or vagal activation of the heart. Postinfusion PEP under saline predicted individual differences in postinfusion cardiac sympathetic activation, whereas postinfusion heart period (but not HF variability) under saline predicted individual differences in postinfusion cardiac vagal activation.     

Fractal images induce fractal pupil dilations and constrictions

Fractals are self-similar structures or patterns that repeat at increasingly fine magnifications. Research has revealed fractal patterns in many natural and physiological processes. This article investigates pupillary size over time to determine if their oscillations demonstrate a fractal pattern. We predict that pupil size over time will fluctuate in a fractal manner and this may be due to either the fractal neuronal structure or fractal properties of the image viewed. We present evidence that low complexity fractal patterns underlie pupillary oscillations as subjects view spatial fractal patterns. We also present evidence implicating the autonomic nervous system's importance in these patterns. Using the variational method of the box-counting procedure we demonstrate that low complexity fractal patterns are found in changes within pupil size over time in millimeters (mm) and our data suggest that these pupillary oscillation patterns do not depend on the fractal properties of the image viewed.     

Association of Type D personality with the autonomic and hemodynamic response to the cold pressor test

Mechanisms relating Type D personality to poor health are largely unknown, with autonomic nervous system function being a candidate. This study examined the physiologic response to cold stress. Undergraduates (N = 101, 84% female) underwent a cold pressor test. An electrocardiogram, impedance cardiogram, and blood pressure were recorded. Type D personality was assessed by self‐report questionnaire. Type D was associated with increased systolic and diastolic blood pressure reactivity. Exploratory analyses showed Type D men to respond with increased respiratory sinus arrhythmia (i.e., higher parasympathetic activity), and decreased pre‐ejection period (i.e., larger sympathetic activity), while Type D women showed a reciprocal response pattern. In conclusion, Type D personality was associated with an exaggerated hemodynamic response to cold stress, which may contribute to an increased risk of hypertension in Type D individuals.     

Sources of the thoracic cardiogenic electrical impedance signal as determined by a model

A three-dimensional electrical model was developed to study the origin of ΔZ due to cardiac activity recorded from band electrodes around the neck and lower thorax. Volume changes were simulated with resistivity changes in the lungs, large arteries, large veins and atria, ventricles, small arteries and veins and the skeletal muscle for a typical 80 ml ventricular stroke volume. The results showed the contributions to ΔZ to be 61 per cent from the lungs, 23 per cent from the large arteries and 13 per cent from the skeletal muscle. The ΔZ signal was most sensitive to skeletal muscle volume change The results indicate that the ΔZ signal has many origins which could cause significant error in calculated cardiac function it all the regions do not change in the normal related pattern.