Effect of aortic valvular regurgitation upon the impedance cardiogram

Development of a miniature and ASIC based impedance cardiograph

In view of cardiac output monitoring application of impedance cardiography a miniature impedance cardiograph has been developed, which is based on an application specific integrated circuit (analog front end chip AFE4300). Carrier current capability of this chip has been enhanced four times (up to 3.2 milliamperes) for cardiology applications with the help of an external resistance, not provisioned in the original design. Also the sensed signal is externally amplified before feeding to AFE chip to increase resolution in the impedance range of 10-50 Ohms.The AFE chip is interfaced to a Personal Computer with the help of a microcontroller board through a USB cable. Application software programs the AFE chip for sending a carrier current of 3.2 milliamperes (peak to peak) to the subject's body segment through current output ports and sensing the chest impedance through voltage input ports. The chest impedance is read in detector mode and communicated to Laptop through serial peripheral interface of the chip. Chest impedance, change in impedance as a function of time and rate of change of impedance are displayed on the graphic user interface. Fifteen minute data from volunteers have shown consistent recordings, as described in this paper.

Comparison of bioreactance non-invasive cardiac output measurements with cardiac magnetic resonance imaging

Impedance cardiography measurement of cardiac output gained wide interest due to its ease of use and non-invasiveness. However, validation studies of different algorithms yielded diverging results. Bioreactance (BR) as a recent adaption differs fundamentally as the flow signal is derived from phase shifts. Our aim was to assess the accuracy and reproducibility of BR, as compared to the non-invasive gold standard--cardiac magnetic resonance imaging (CMR). We prospectively included 32 stable patients. BR was performed twice in the supine position and averaged over 30 seconds. Mean bias was 0.2 ± 1.8 l/minute (1 ± 28%, percentage error 55%) with limits of agreement ranging from  -3.4 to 3.7 l/minute. Reproducibility was acceptable with a mean bias of 0.1 ± 0.9 l/minute (1 ± 14%, 27%). Low cardiac output was significantly overestimated (-1.1 ± 1.5 l/minute), while high cardiac output was underestimated (1.5 ± 1.7 l/minute), (P=0.001), although reproducibility was unaffected. Bias and weight were moderately correlated in men (r = 0.50, P=0.02). No differences for accuracy were found in nine patients who had an arrhythmia (0.3 ± 1.4 versus 0.1 ± 2.0 l/minute, P=0.76), while clinically relevant differences were found in patients with mild aortic valve disease (1.9 ± 2.2 versus -0.3 ± 1.7 l/minute, P=0.02). Overall, BR showed insufficient agreement with CMR, overestimating low and underestimating high cardiac output states. Reproducibility was acceptable and not negatively affected by the circulatory condition. Consequently, absolute values acquired with BR should be interpreted with caution and must not be used interchangeably in clinical practice.

Noninvasive measurement of cardiac output: two methods compared in patients with mitral regurgitation

In search for the origin of the less reliable cardiac output (CO) estimations by means of electrical impedance cardiography (EIC), the authors hypothesized that cardiac valve pathology might be one of the reasons. Twenty-six patients were examined by means of echo Doppler (ED) and EIC. The cardiac valve status was obtained by means of echocardiography and color Doppler flow, while CO was obtained by means of both methods. Seventeen patients had no valve pathology (nVP) while nine patients had mild to moderate mitral regurgitation (MVR). The overall correlation between the calculation of CO by means of the two methods was good (r = 0.85, p < 0.001, mean difference and standard deviation: 0.20+/-0.74 L/min), while there was no significant difference between the paired values. After division into an nVP and an MVR population, the results showed an even closer agreement between the CO values in the nVP population (r = 0.88, p < 0.001, mean difference and standard deviation: 0.15+/-0.68 L/min). Furthermore, significant differences were found in the first derivative of the impedance (dZ/dt) signals of these groups. Although the agreement between ED and EIC was slightly lower in the MVR population, EIC reliably estimated CO, even in case of MVR. The impedance signal itself gave an indication for the existence of MVR.

Comparative measures of systolic ejection during treadmill exercise by impedance cardiography and Doppler echocardiography

Measurements of systolic ejection dynamics by impedance cardiography were compared with simultaneous Doppler echocardiography in normal subjects and coronary artery disease patients. Patients with chest pain admitted for elective coronary angiography were monitored by simultaneous impedance cardiography and Doppler echocardiography before, during, and after treadmill exercise. Ensemble-averaged ECG, impedance cardiogram (ICG), the first derivative of ICG (dZ/dt), and Doppler waveforms were analyzed to identify systolic ejection variables. The timing of aortic valve opening was well correlated (r = 0.78) the timing of peak ejection velocity was very well correlated (r = 0.86), and the timing of aortic valve closure was moderately correlated (r = 0.69 and r = 0.73) in these subjects. The thoracic electrical impedance acceleration and normalized impedance acceleration indices were moderately correlated with Doppler model acceleration (r = 0.74, r = 0.79). The impedance cardiogram waveforms are of complex origin and are related to both aortic blood velocity and aortic blood acceleration. Users of dZ/dt timing features for determining aortic valvular events might consider alternative impedance features to improve ejection time accuracy.

A practical, cost-effective, noninvasive system for cardiac output and hemodynamic analysis

Impedance cardiography is a relatively inexpensive, noninvasive technique for measuring cardiac output on the basis of resistive changes in the thorax to electrical current flow. In conjunction with blood pressure monitoring and physiologic maneuvers, the technique may be used to monitor thoracic and total body fluid volume and express a variety of contractility indexes as well as relative and absolute measurements of stroke volume. We have tested hemodynamics in our laboratory by using a cost-effective, powerful microcomputer-based portable noninvasive technique, which makes possible the ensemble averaging of impedance cardiographic waveforms. In conjunction with physiologic maneuvers, the technique has been implemented at our institution and has provided helpful information in our experience in evaluating volume overload, hypertension, hypotension, shock, and heart failure. It is hoped that this noninvasive, relatively cost-effective approach will be more widely appreciated in the future, given the economic realities of medicine today.

Noninvasive evaluation of aortic stenosis severity utilizing Doppler ultrasound and electrical bioimpedance

Aortic valve area was calculated noninvasively in 30 patients with aortic stenosis undergoing cardiac catheterization. Continuous wave Doppler ultrasound was employed to estimate the mean transvalvular pressure gradient. The mean left ventricular outflow tract flow velocity and cross-sectional area were determined from pulsed Doppler and two-dimensional ultrasound recordings. Electrical transthoracic bioimpedance cardiography performed simultaneously with the ultrasonic study and repeated at the time of catheterization measured heart rate, systolic ejection period and cardiac output. These noninvasive data permitted calculation of aortic valve area using the Gorlin equation (range 0.21 to 1.75 cm2) and the continuity equation (range 0.25 to 1.9 cm2). Subsequent cardiac catheterization showed valve area to range from 0.21 to 1.75 cm2. The mean Doppler pressure gradient estimate was highly predictive of the gradient measured at catheterization (r = +0.92, SEE = 10). Bioimpedance cardiac output measurements agreed with the average of Fick and indicator dye estimates (r = +0.90, SEE = 0.52). Valve area estimates utilizing continuous wave Doppler ultrasound and electrical bioimpedance were superior (r = +0.91, SEE = 0.12) to estimates obtained utilizing the continuity equation (r = +0.76, SEE = 0.29) and were more reliable in the detection of patients with severe aortic stenosis (9 of 11 versus 6 of 11). These data show that 1) electrical bioimpedance methods accurately estimate cardiac output in the presence of aortic stenosis; 2) the hybridized bioimpedance-Doppler ultrasound method yields accurate estimates of aortic stenosis area; and 3) the speed, accuracy and cost-effectiveness of aortic stenosis evaluation may be improved by this hybridized approach.

Mitral regurgitation and characteristic changes in impedance cardiogram

We have noted that the impedance cardiographic waveform of patients with mitral regurgitation may show typical abnormalities not found in other forms of cardiac disease, either valvular or congenital. In order to investigate this we reviewed the impedance cardiograms of all our patients who had also undergone cardiac catheterisation, and selected two groups both of which were inclusive and complete: (1) 22 patients with the diagnosis of normal heart, and (2) 36 patients with the diagnosis of isolated mitral regurgitation. An index was calculated algebraically from the change of impedance (delta L) tracing by adding together the height of the C wave and the height of the nadir of the X descent and subtracting the height of the V wave, that is delta Z index (in units of ohms) = C + X - V. The mean delta Z index for the normal group was 1.64 and for the mitral regurgitation group 0.96 ohms. Similarly, an index was calculated from the first time derivative of the change of impedance (dZ/dt) tracing, that is dZ/dt index (in units of ohms/s) = C' + X' - V'. The mean dZ/dt index for the normal group was 1.32 and for the mitral regurgitation group 0.48 ohms/s. Though there was some overlap of individual points between the two groups, the mean values for both the delta Z index and the dZ/dt index separated the group with mitral regurgitation from the normal group with a high level of statistical significance. We concluded that mitral regurgitation might be associated with a characteristic abnormality of the impedance cardiographic waveform. In addition, an index can easily be calculated from the tracings which may be useful in identifying patients with mitral regurgitation.

Effect of mitral valvular regurgitation on transthoracic impedance cardiogram

Mitral valvular regurgitation consistently modified the wave form of the first derivative of the transthoracic impedance cardiogram. The transthoracic impedance cardiogram was recorded in 23 control subjects (group 1), and 23 patients with isolated mitral regurgitation (group 2). Simultaneous transthoracic impedance cardiogram, electrocardiogram, and mitral valve echocardiograms in group 1 showed that the primary diastolic wave ("O") of the transthoracic impedance cardiogram occurred synchronously with the maximal opening of the mitral valve. In group 2, the primary systolic wave (dZ/dt max) was diminished, and the "O" of the transthoracic impedance cardiogram was raised. The area under the systolic wave of the transthoracic impedance cardiogram (S) and the area under the diastolic opening of the transthoracic impedance cardiogram (D) were measured and the ratio D/(D + S) calculated. This ratio, called the mitral regurgitation fraction was (0.50 +/- 0.14) in group 2 which was higher than that found in group 1 control subjects (0.11 +/- 0.08). The mitral regurgitation fraction (15 to 77%) determined by the impedance method was closely correlated with the mitral regurgitation fraction (20 to 74%) obtained during cardiac catheterisation; it also increased during isometric handgrip and decreased during amyl nitrite inhalation. In three mitral regurgitation patients the transthoracic impedance cardiogram returned to normal configuration after surgical implantation of a prosthetic mitral valve. These data suggest that the transthoracic impedance cardiogram is quantitatively altered in patients with mitral regurgitation.

The effect of acute aortic regurgitation on the transthoracic impedance cardiogram

In nine anesthetized dogs, recordings of the first derivative of the transthoracic impedance cardiogram (ICG) were made during varying grades of acute aortic regurgitation. Acute aortic regurgitation was induced using a specially designed umbrella catheter, passed retrograde across the aortic valve into the left ventricle. The RFA (representing the fraction of the aortic reverse flow to the aortic forward flow) was computed using an electromagnetic flow probe implanted around the ascending aorta. Both the peak of the scalar ICG, dz/dtmax, which occurs at peak systolic ejection, and the nadir of the scalar ICG, X, which marks the closing of the aortic valve, increased with aortic regurgitation. The planimetered areas of the ICG during systole (S), and in early-diastole (X) increased during aortic regurgitation. These areas, S and X, correlated with the electromagnetic normalized aortic stroke volume (r = 0.90) and the regurgitant volume (r = 0.78), respectively. The ICG ratio X/S was correlated directly with the electromagnetic aortic regurgitant fraction (r = 0.86). This study demonstrates that the ICG waveform is consistently modified by experimental aortic regurgitation. Furthermore, these changes can be quantitatively related to the degree of aortic regurgitation.