Mapping the cardiogenic impedance signal on the thoracic surface

Method to measure autonomic control of cardiac function using time interval parameters from impedance cardiography

The time difference between the electrocardiogram and impedance cardiogram can be considered as a measure for the time delay between the electrical and mechanical activities of the heart. This time interval, characterized by the pre-ejection period (PEP), is related to the sympathetic autonomous nervous control of cardiac activity. PEP, however, is difficult to measure in practice. Therefore, a novel parameter, the initial systolic time interval (ISTI), is introduced to provide a more practical measure. The use of ISTI instead of PEP was evaluated in three groups: young healthy subjects, patients with Parkinson's disease, and a group of elderly, healthy subjects of comparable age. PEP and ISTI were studied under two conditions: at rest and after an exercise stimulus. Under both conditions, PEP and ISTI behaved largely similarly in the three groups and were significantly correlated. It is concluded that ISTI can be used as a substitute for PEP and, therefore, to evaluate autonomic neuropathy both in clinical and extramural settings. Measurement of ISTI can also be used to non-invasively monitor the electromechanical cardiac time interval, and the associated autonomic activity, under physiological circumstances.

Change in pulse transit time and pre-ejection period during head-up tilt-induced progressive central hypovolaemia

OBJECTIVE

Traditional vital signs such as heart rate (HR) and blood pressure (BP) are often regarded as insensitive markers of mild to moderate blood loss. The present study investigated the feasibility of using pulse transit time (PTT) to track variations in pre-ejection period (PEP) during progressive central hypovolaemia induced by head-up tilt and evaluated the potential of PTT as an early non-invasive indicator of blood loss.

METHODS

About 11 healthy subjects underwent graded head-up tilt from 0 to 80 degrees . PTT and PEP were computed from the simultaneous measurement of electrocardiogram (ECG), finger photoplethysmographic pulse oximetry waveform (PPG-POW) and thoracic impedance plethysmogram (IPG). The response of PTT and PEP to tilt was compared with that of interbeat heart interval (RR) and BP. Least-squares linear regression analysis was carried out on an intra-subject basis between PTT and PEP and between various physiological variables and sine of the tilt angle (which is associated with the decrease in central blood volume) and the correlation coefficients (r) were computed.

RESULTS

During graded tilt, PEP and PTT were strongly correlated in 10 out of 11 subjects (median r = 0.964) and had strong positive linear correlations with sine of the tilt angle (median r = 0.966 and 0.938 respectively). At a mild hypovolaemic state (20-30 degrees ), there was a significant increase in PTT and PEP compared with baseline (0 degrees ) but without a significant change in RR and BP. Gradient analysis showed that PTT was more responsive to central volume loss than RR during mild hypovolaemia (0-20 degrees ) but not moderate hypovolaemia (50-80 degrees ).

CONCLUSION

PTT may reflect variation in PEP and central blood volume, and is potentially useful for early detection of non-hypotensive progressive central hypovolaemia. Joint interpretation of PTT and RR trends or responses may help to characterize the extent of blood volume loss in critical care patients.

Imaging of thoracic blood volume changes during the heart cycle with electrical impedance using a linear spot-electrode array

Electrical impedance (EI) measurements conducted on the thorax contain useful information about the changes in blood volume that occur in the thorax during the heart cycle. The aim of this paper is to present a new (tomographic-like) method to obtain this relevant information with electrical impedance measurements, using a linear electrode array. This method is tested on three subjects and the results are compared with results, obtained from magnetic resonance cine-images showing the cross-sectional surface area changes of the aorta, the vena cava, the carotid arteries, and the heart. This paper shows that the different sources of the thoracic EI waveform may be separated in time and location on the thoracic surface and that aortic volume changes may be estimated accurately.

Comparing spot electrode arrangements for electric impedance cardiography

This study investigates whether an arrangement with nine spot electrodes, for thoracic bio-impedance cardiography, can be replaced by an arrangement with five spot electrodes. The study was conducted on 15 healthy subjects, six females and nine males, in supine rest. The variables obtained from the measurements were the mean of the impedance of the thorax segment between the recording electrodes, the maximum negative deflection of the first derivative of the thoracic impedance, the left ventricular ejection time and an estimate of left ventricular stroke volume. An analysis of variance for a randomized complete block design was used to determine whether significant differences exist in the group means of the observed variables between six different electrode arrangements. If no statistically significant differences were found in these group means between pairs of arrangements, Bland and Altman analyses were used to determine the differences in the observed variables between pairs of arrangements for individual subjects. This study concludes that reducing the number of spot electrodes from nine to five, does not yield significant differences in the group means of the observed variables, but it could result in large differences in the values of these variables for individual subjects.

Application of computer modelling and lead field theory in developing multiple aimed impedance cardiography measurements

Conventional impedance cardiography (ICG) methods estimate parameters related to the function of the heart from a single waveform that reflects an integrated combination of complex sources. We have previously developed methods and tools for calculating measurement sensitivity distributions of ICG electrode configurations. In this study, the methods were applied to investigate the prospects of recording multiple aimed ICG waveforms utilizing the 12-lead electrocardiography (ECG) electrode locations. Three anatomically realistic volume conductor models were used: one based on Visible Human Man cryosection data and two on magnetic resonance (MR) images representing end diastolic and end systolic phases of the cardiac cycle. Based on the sensitivity distributions obtained, 236 electrode configurations were selected for preliminary clinical examination on 12 healthy volunteers and 9 valvular patients. The model study suggested that a variety of configurations had clearly enhanced sensitivity to the cardiovascular structures as compared to conventional ICGs. Simulation data and clinical experiments showed logical correspondence supporting the theoretically predicted differences between the configurations. Recorded 12-lead ICG signals had characteristic waveforms and landmarks not coinciding with those of conventional ICG. Furthermore, configurations showing resemblance to invasive data and morphological variations in disease are of interest. The results indicate the applicability of the modelling approach in developing ICG measurement configurations. However, the level of clinical relevance and potential of the 12-lead method remains to be explored in studies employing dynamic modelling and acquisition of invasive reference data.

Thoracic geometry and its relation to electrical current distribution: consequences for electrode placement in electrical impedance cardiography

In thoracic impedance cardiography (TIC) measurements the neck electrodes are often positioned at the basis of the neck, close to the neck-thorax transition. Theoretically, this neck-thorax transition will cause inhomogeneities in the current density and potential distribution. This was simulated using a 3D finite element method, solely representing the geometrical neck-thorax transition. The specific conductivity was 7 10(-3) (omega cm)-1 and the injected current was 1 mA. As expected, the model generated inhomogeneities in the current distribution at the neck-thorax transition, which reached as far as 5 cm into the neck and 20 cm into the thorax. These results are supported by in vivo measurements performed in 10 young male subjects, in which the position of the neck electrodes was varied. A two-way ANOVA revealed that the stroke volume of the lowest neck position was significantly different from the other positions. Small shifts in the position of the neck electrode resulted in large changes in impedance and stroke volume (127 to 82 ml for the Kubicek equation). To standardise the electrode position, the authors strongly recommend placement of the neck electrodes at least 6 cm above the clavicula.

Impedance cardiography: the failure of sternal electrodes to predict changes in stroke volume

A study was conducted on 26 healthy male adults comparing the stroke volume (SV) and cardiac output (CO) measured with green dye and impedance cardiography in supine, sitting and during 50 W bicycle exercise. The impedance voltage pick-up electrodes were modified from the standard band location and placed at the suprasternal notch and 10 cm below. The percentage change of SV going from supine to sitting was -27.2 +/- 7.4% and -16.9 +/- 22% for the dye and impedance values respectively. The percentage change of SV going from sitting to exercise was 20.9 +/- 11.9% and 36.7 +/- 32.9% for the dye and impedance values respectively. The percentage change in SV determined by impedance shows poor correlation (supine to sitting R = 0.16 and sitting to exercise R = 0.42) with dye values. The individual impedance values for a given change in position or exercise showed considerably more variation compared with the dye. Impedance determination of SV using electrodes on the sternum does not appear to be a valid method for SV and CO measurements.

Impedance cardiography using band and regional electrodes in supine, sitting, and during exercise

The electrical impedance and its first derivative (dZ/dt) were measured at 100 kHz on 10 normal males in supine, sitting, and during upright bicycle exercise in order to compare the contribution of regional electrodes to the standard band electrode signal and to evaluate the possible use of spot electrodes for stroke volume (SV) measurements. Simultaneous measurements were made from band electrodes placed around the neck and lower thorax and from spot electrodes which recorded signals from the neck, upper thorax, and lower thorax. The results showed that approximately equal parts of the dZ/dt waveform came from the neck and upper thorax with the lower thorax contribution small but providing important features of the band signal. Changing from supine to sitting showed percentage decreases of 35% and 46% for the band and neck signals, respectively, with an increase of 19% for the upper thorax signal. The percentage increases in SV with upright exercise were 34%, 52%, and 24% for the bands, neck, and upper thorax signals, respectively. Band signal is made up of different signals from various regions of the thorax. Its ability to predict correct changes in SV may result from some "lucky" coincidences. The use of regional electrodes will probably not give the same SV information but may be important in measuring regional activities of the central circulation.