Congestive heart failure (CHF) is a condition in which the heart does not adequately maintain circulation of blood. It is characterized by an increase in retained body water, especially extracellular water, often in the lungs (pulmonary edema). A decrease in extracellular fluid in CHF patients typically indicates an improvement in heart performance. Conventional methods of monitoring CHF patients either require expensive equipment and trained personnel (e.g. measuring pulmonary artery and central venous pressure with catheters, measuring blood flow through the mitral annulus and pulmonary veins with doppler echocardiography) or are not very accurate (e.g. monitoring changes in body weight, observing neck vein distension, measuring ankle dimensions). Impedance measurements of the chest, both resistive and reactive (capacitive) impedance, have been shown to correlate with total body water, extracellular body water, and the ratios of these quantities to fat free mass (U.S. Pat. No. 5,788,643). Monitoring trends in these quantities in congestive heart failure patients is a particularly useful way to determine whether medication doses need to be increased or decreased. As stated in U.S. Pat. No. 5,788,643: “Subramanyan, et al. and others have shown that both the resistive and reactive components of the body's impedance to the flow of relatively high frequency (50 kHz) electrical current is sensitive to the amount of fluid retained by a patient with CHF. As the CHF resolves, resistance and reactance both increase as does the [ratio of reactance to resistance]. See Subramanyan, et al., “Total Body Water in Congestive Heart Failure,” Jour. Asso. Phys. Ind., Vol. 28, September, 1980, pages 257–262. . . . It would be most desirable to provide a simple way of detecting increases in body water of patients with CHF before hospitalization is necessary and permitting adjustments in medication and/or diet in time to prevent an episode of acute heart failure.” The patent describes a figure of merit, calculated from impedance measurements, for deciding when medical intervention may be needed for a CHF patient.
There are several parameters that affect the impedance of the thorax. The impedance of the chest cavity is small compared to changes in the impedance of the skin, and chest cavity impedance changes substantially during the respiratory cardiac cycle, due to the changing volume of air in the lungs, and during the cardiac cycle due to the changing blood perfusion of the lungs. Various techniques are used to separate out the part of the impedance due to excess body water, and to meaningfully compare such impedance measurements taken in the same patient on different days. For example, U.S. Pat. No. 5,749,369, and Charach, G. et al., “Transthoracic Monitoring of the Impedance of the Right Lung in Patients with Cardiogenic Pulmonary Edema,” Crit. Care Med. 2001, Vol. 29, No. 6, pages 1137–1144 discuss ways to compensate for drifting skin impedance.
In addition to the techniques used in bulk measurements of impedance, impedance imaging is also useful for separating out the different contributions to the impedance. In impedance imaging, a set of many electrodes (usually 16 or 32) is placed on the body, for example encircling the chest, and the voltage is measured at each electrode, while a known current is applied between different pairs of the electrodes. The resulting data is used to produce a map of the internal impedance of the body, using various mathematical techniques, some of them similar to those used in x-ray tomography. Some image reconstruction techniques are described in a review paper by D. C. Barber, Med. Phys., (1989), Vol. 16, pages 162–169.
The finite element method, finite difference method, and boundary element method are different techniques used to solve differential equations numerically. Solving Poisson's equation to find the potential distribution in the body due to known current sources and impedance distribution, together with boundary conditions, is known as the forward problem. These numerical methods are used in the field of bio-impedance to solve the forward problem. Rosenfeld, M. et al., “Numerical Solution of the Potential Due to Dipole Sources in Volume Conductors With Arbitrary Geometry and Conductivity,” IEEE Transactions on Biomedical Engineering, July 1996, Vol. 43, No. 7, pages 679–689 use a different technique, the finite volume method, to solve the forward problem. Finding the impedance distribution with known potential distribution at the surface (measured with surface electrodes, for example), and known current sources (flowing from one surface electrode to another), is called the inverse problem. Some of the inverse problem solvers use the forward problem solver as a step in an iterative solution.
An early paper on impedance imaging by Eyuboglu, B. M. et al., “In Vivo Imaging of Cardiac Related Impedance Changes,” March 1989, IEEE Engineering in Medicine and Biology Magazine, Vol. 8, pages 39–45 discusses the use of gating and time-averaging to separate out the contributions of the respiratory and cardiac cycles to the chest impedance and impedance images, including impedance images of pulmonary embolisms. The authors state, “[T]he resistivity of most tissue changes significantly with blood perfusion into the tissue . . . . [I]t has been shown that the thoracic resistivity changes during the cardiac cycle can be imaged by ECG-gated EIT [electrical impedance tomography] . . . . The average resistivity of lung tissue increases with the amount of air inspired . . . [by] approximately 300 percent . . . from maximal expiration to maximal inspiration . . . . The resistivity of lung tissue also changes with the perfusion of blood following ventricular systole . . . . This change has been calculated as 3 percent . . . [which] may be as small as the noise level . . . . Therefore, to pick up the cardiac-related resistivity variations within the thorax during normal breathing, the respiratory component and the noise must be eliminated . . . . The respiratory component may be rejected by temporal averaging . . . . Experience has shown that averaging over at least 100 cardiac cycles is needed during shallow breathing to attenuate the respiratory component and to improve S/N ratio. Cardiac gating is required . . . . ” Brown and Barber develop numerical methods to reduce noise in U.S. Pat. No. 5,311,878, and they use differences in impedance at different electrical frequencies between 10 kHz and 600 kHz to distinguish between cardiac and respiratory effects in U.S. Pat. No. 5,746,214. Newell, J. C. et al., “Assessment of Acute Pulmonary Edema in Dogs by Eletrical Impedance Imaging,” February 1996, IEEE Transactions on Biomedical Engineering, Vol. 43, No. 2, pages 133–138 demonstrate the use of impedance imaging to detect pulmonary edemas in dogs, and discuss the variability in impedance over time and from day to day, which makes it difficult to measure long-term changes.