Parallel conductance estimation by hypertonic dilution method with conductance catheter: effects of the bolus concentration and temperature

Novel method for measurement of medium size arterial lumen area with an impedance catheter: in vivo validation

There is no doubt that the transformation of a cardiac catheter into a conductance catheter that allows reliable and accurate assessment of lumen cross-sectional area (CSA) will provide a powerful diagnostic and treatment tool for the invasive cardiologist. The objective of this study was to develop a method based on the impedance catheter that allows accurate and reproducible measurements of CSA for medium size vessels (e.g., coronary, femoral, and carotid arteries). Two solutions of NaCl (0.5% and 1.5%) with known conductivities were injected directly into the lumen of the artery in eight swine. We showed that the CSA can be determined analytically from two Ohm's law-type algebraic equations that account for the parallel conductance of the current into the surrounding tissue. Excellent agreement was found between the conductance catheter with the proposed two-injection method and B-mode ultrasound (US). The root mean square error for the impedance measurements was 4.8% of the mean US diameter. The repeatability of the technique was assessed with duplicate measurements. The mean of the difference between the two measurements was nearly zero, and the repeatability coefficient was within 2.4% of the mean of the two measurements. The validated method was used to assess the degree of acute vasodilatation of the vessel in response to flow overload.

Measurement of Coronary Lumen Area Using an Impedance Catheter: Finite Element Model and in Vitro Validation

The measurement of coronary lumen cross-sectional area (CSA) is important for coronary physiology and cardiology. The general objective of this study is to develop an accurate and reproducible method to measure the lumen CSA of left anterior descending (LAD) artery using an impedance or conductance catheter. The conductance catheter technique is based on a cylindrical model of the chamber of interest. The first aim of this study was to validate the assumptions of the cylindrical model using a finite-element analysis (FEA) of the conductance catheter in the lumen of the vessel that takes into account the conductance of current through the vessel wall and surrounding tissue (parallel conductance, Gp). The FEA was used to determine the heterogeneity of potential and electrical fields and to optimize the design of the catheter relative to the diameter of the vessel. An optimum relationship between vessel and catheter diameter was obtained based on FEA. The second aim was to validate the in vitro CSA of LAD artery obtained from the conductance catheter method using A-mode ultrasound (US). The present study offers a novel approach to correct for the Gp that involves the injection of two solutions of NaCl (0.5% and 1.5%) with known conductivities directly into the lumen of the coronary artery in a porcine heart. In six hearts obtained from a slaughterhouse, we showed that the CSA and Gp can be determined analytically from two Ohm's law-type algebraic equations (cylindrical model) that account for the parallel conductance. The mean difference in diameter between the conductance catheter using the proposed two-injection method and U.S. was -0.02. The root mean square error for the impedance measurements was 2.8% of the mean US diameter. The future application of this technique to the in vivo condition is discussed.

The trans-cardiac conductance method for on-line measurement of left ventricular volume: assessment of parallel conductance offset volume

The trans-cardiac conductance (TCC) method provides on-line left ventricular (LV) volume signals by determining the electrical conductance of blood in the LV by means of central venous and epithoracic electrodes. Conductive structures outside the LV blood pool cause a "parallel conductance" offset term (Vp) that can be determined by bolus injections of hypertonic saline in the pulmonary artery (Vp(saline)), which cause a transient increase in blood conductivity. This study in anesthetized sheep evaluates the accuracy of the saline calibration method and the variabilities of Vp between animals, between hemodynamic conditions and during the cardiac cycle. The conventional intra-cardiac conductance catheter method was used to obtain independent estimates of Vp by the zero-volume method (Vp(zero volume)). Mean baseline Vp(saline) and Vp(zerovolume) were 104 +/- 6 ml and 106 +/- 6 ml, respectively. Bland-Altman analysis showed a small nonsignificant bias (-2.5 ml) and narrow limits of agreement (4.6 ml). Vp was not significantly different between hemodynamic conditions (baseline, dobutamine, volume load, propranolol), but had a substantial interanimal variability (IAV) (38%). Average variations during the cardiac cycle were < 10% of mean Vp. We conclude that the saline method can be applied to determine Vp for TCC. IAV is substantial, so that Vp must be determined in each animal, but within-animal variability is relatively small.

Validation and utility of novel volume reduction technique for determination of parallel conductance

The parallel conductance volume, created by the conductivity of structures surrounding the ventricular blood pool, can be estimated by using a saline dilution technique. This paper examines the use of a novel volume reduction method, during a standard vena caval preload reduction maneuver, as an alternative to the routinely used saline dilution method to calibrate conductance catheter measurements in the left (LV) and right ventricle (RV) of animals and humans. The serial reproducibility of both methods was examined by measurement of percent difference, and by assessing the coefficient of repeatability 1) between two measurements within the same subject, 2) between the two techniques, and 3) interobserver variability. The effect of ventricular size and contractile state on the volume reduction technique was also observed. It was essential to ensure the technique was not affected by inotropic state. The volume reduction technique and saline dilution method were repeated at three different loading states (baseline, 5, and 10 microg x kg(-1) x min(-1) of dobutamine). The coefficient of repeatability between serial measurements was similar for both the volume reduction and saline dilution methods, and good interobserver variability was demonstrated. The volume reduction technique was compared with the saline dilution technique over a large range of ventricular sizes. No significant difference was observed in the RV or LV of adult humans or in the LV of neonatal pigs and children. There was no significant effect on either the saline dilution or the volume reduction technique as the inotropic state increased. In conclusion, the volume reduction technique is neither affected by ventricular size nor contractile state, is repeatable between different observers, and can be used to substitute the saline dilution method when preload reduction of the ventricle is being employed.

The measurement of impedance to assess myocardial contractility and rhythm stability

By introducing an alternating current through the ventricular cavity via a multipolar catheter the potential difference measured between adjacent electrodes can be used to assess ventricular volume. This is possible by the measurement of intracardiac impedance. The data obtained can be used to construct pressure-volume loops allowing the continuous real-time assessment of myocardial contractility. Clinical applications of this research tool are now becoming apparent. Further derivations of impedance measurement have been incorporated into commercially available permanent pacing systems, allowing effective heart rate modulation.

Estimation of parallel conductance by dual-frequency conductance catheter in mice

The conductance catheter method has substantially enhanced the characterization of in vivo cardiovascular function in mice. Absolute volume determination requires assessment of parallel conductance (V(p)) offset because of conductivity of structures external to the blood pool. Although such a determination is achievable by hypertonic saline bolus injection, this method poses potential risks to mice because of volume loading and/or contractility changes. We tested another method based on differences between blood and muscle conductances at various catheter excitation frequencies (20 vs. 2 kHz) in 33 open-chest mice. The ratio of mean frequency-dependent signal difference to V(p) derived by hypertonic saline injection was consistent [0.095 +/- 0.01 (SD), n = 11], and both methods were strongly correlated (r(2) = 0.97, P < 0.0001). This correlation persisted when the ratio was prospectively applied to a separate group of animals (n = 12), with a combined regression relation of V(p(DF)) = 1.1 * V(p(Sal)) - 2.5 [where V(p(DF)) is V(p) derived by the dual-frequency method and V(p(Sal)) is V(p) derived by hypertonic saline bolus injection], r(2) = 0.95, standard error of the estimate = 1.1 microl, and mean difference = 0.6 +/- 1.4 microl. Varying V(p(Sal)) in a given animal resulted in parallel changes in V(p(DF)) (multiple regression r(2) = 0.92, P < 0.00001). The dominant source of V(p) in mice was found to be the left ventricular wall itself, since surrounding the heart in the chest with physiological saline or markedly varying right ventricular volumes had a minimal effect on the left ventricular volume signal. On the basis of V(p) and flow probe-derived cardiac output, end-diastolic volume and ejection fraction in normal mice were 28 +/- 3 microl and 81 +/- 6%, respectively, at a heart rate of 622 +/- 28 min(-1). Thus the dual-frequency method and independent flow signal can be used to provide absolute volumes in mice.