Smart Drain for Post-Cardiac Surgery Left Ventricular Volumes Evaluated in Large Animal Models.

BACKGROUND: Open heart surgeries for coronary arterial bypass graft and valve replacements are performed on 400,000 Americans each year. Unexplained hypotension during recovery causes morbidity and mortality through cerebral, kidney, and coronary hypoperfusion. An early detection method that distinguishes between hypovolemia and decreased myocardial function before onset of hypotension is desirable. We hypothesized that admittance measured from a modified pericardial drain can detect changes in left ventricular end-systolic, end-diastolic, and stroke volumes. METHODS: Admittance was measured from 2 modified pericardial drains placed in 7 adult female dogs using an open chest preparation, each with 8 electrodes. The resistive and capacitive components of the measured admittance signal were used to distinguish blood and muscle components. Admittance measurements were taken from 12 electrode configurations in each experiment. Left ventricular preload was reduced by inferior vena cava occlusion. Physiologic response to vena cava occlusion was measured by aortic pressure, aortic flow, left ventricle diameter, left ventricular wall thickness, and electrocardiogram. RESULTS: Admittance successfully detected a drop in left ventricular end-diastolic volume (P < .001), end-systolic volume (P < .001), and stroke volume (P < .001). Measured left ventricular muscle resistance correlated with crystal-derived left ventricular wall thickness (R2 = 0.96), validating the method's ability to distinguish blood from muscle components. CONCLUSIONS: Admittance measured from chest tubes can detect changes in left ventricular end-systolic, end-diastolic, and stroke volumes and may therefore have diagnostic value for unexplained hypotension.

Why low-voltage shock impedance measurements fail to reliably detect insulation breaches in transvenous defibrillation leads.

BACKGROUND: Implantable cardioverter-defibrillators (ICDs) use low-voltage measures of shock impedance (LVSZ) to monitor integrity of leads. OBJECTIVE: To determine the separation distance between conductors required for LVSZ to detect insulation breaches that produce short circuits during shocks, causing failed defibrillation. METHODS: We simulated in-pocket insulation breaches between the ICD generator (CAN) and cables to the distal coil of 10 leads from 2 manufacturers. The ICD and lead were placed in an electrolyte bath. Polystyrene sheets were used to control the breach-CAN separation. We determined both the maximum lead-CAN separation for shorts during 800 V shocks and the shock strength at which shorts occurred for a fixed separation. We also calculated breach impedance and measured it using a low-voltage instrument. RESULTS: The maximum breach-CAN separation for shorting was 350-500 µm for all leads. The minimum shock strength to short varied from 650 to 771 V (24-32 J). LVSZ never triggered a warning, even with no separation between the cable's inner insulation and the CAN. Using low-voltage pulses, breach impedance was measured at approximately 500-1000 Ω. CONCLUSION: LVSZ is insensitive to insulation breaches that cause life-threatening, shorted shocks. The explanation likely relates to impedance differences between ionic conduction during LVSZ measurements and free-electron conduction in plasma discharges.

Admittance to detect alterations in left ventricular stroke volume.

BACKGROUND: Implantable cardioverter-defibrillators monitor intracardiac electrograms (EGMs) to discriminate between ventricular and supraventricular tachycardias. The incidence of inappropriate shocks remains high because of misclassification of the tachycardia in an otherwise hemodynamically stable individual. Coupling EGMs with an assessment of left ventricular (LV) stroke volume (SV) could help in gauging hemodynamics during an arrhythmia and reducing inappropriate shocks. OBJECTIVE: The purpose of this study was to use the admittance method to accurately derive LV SV. METHODS: Ultrasonic flow probe and LV endocardial crystals were used in canines (n = 12) as the standard for LV SV. Biventricular pacing leads were inserted to obtain admittance measurements. A tetrapolar, complex impedance measurement was made between the Bi-V leads. The real and imaginary components of impedance were used to discard the myocardial component from the blood component to derive instantaneous blood conductance (Gb). Alterations in SV were measured during right ventricular pacing, dopamine infusion, and inferior vena cava occlusion. RESULTS: Gb tracks steady-state changes in SV more accurately than traditional magnitude (ie, |Y|, without removal of the muscle signal) during right ventricular pacing and dopamine infusion (P = .004). Instantaneous LV volume also was tracked more accurately by Gb than ∣Y∣ in the subset of subjects that underwent inferior vena cava occlusions (n = 5, P = .025). Finite element modeling demonstrates that admittance shifts more sensitivity of the measurement to the LV blood chamber as the mechanism for improvement (see Online Appendix). CONCLUSION: Monitoring LV SV is possible using the admittance method with biventricular pacing leads. The technique could be piggybacked to complement EGMs to determine if arrhythmias are hemodynamically unstable.

A bio-telemetric device for measurement of left ventricular pressure-volume loops using the admittance technique in conscious, ambulatory rats.

This paper presents the design, construction and testing of a device to measure pressure-volume loops in the left ventricle of conscious, ambulatory rats. Pressure is measured with a standard sensor, but volume is derived from data collected from a tetrapolar electrode catheter using a novel admittance technique. There are two main advantages of the admittance technique to measure volume. First, the contribution from the adjacent muscle can be instantaneously removed. Second, the admittance technique incorporates the nonlinear relationship between the electric field generated by the catheter and the blood volume. A low power instrument weighing 27 g was designed, which takes pressure-volume loops every 2 min and runs for 24 h. Pressure-volume data are transmitted wirelessly to a base station. The device was first validated on 13 rats with an acute preparation with 2D echocardiography used to measure true volume. From an accuracy standpoint, the admittance technique is superior to both the conductance technique calibrated with hypertonic saline injections, and calibrated with cuvettes. The device was then tested on six rats with 24 h chronic preparation. Stability of animal preparation and careful calibration are important factors affecting the success of the device.

Left ventricular epicardial admittance measurement for detection of acute LV dilation.

There are two implanted heart failure warning systems incorporated into biventricular pacemakers/automatic implantable cardiac defibrillators and tested in clinical trials: right heart pressures, and lung conductance measurements. However, both warning systems postdate measures of the earliest indicator of impending heart failure: left ventricular (LV) volume. There are currently no proposed implanted technologies that can perform LV blood volume measurements in humans. We propose to solve this problem by incorporating an admittance measurement system onto currently deployed biventricular and automatic implantable cardiac defibrillator leads. This study will demonstrate that an admittance measurement system can detect LV blood conductance from the epicardial position, despite the current generating and sensing electrodes being in constant motion with the heart, and with dynamic removal of the myocardial component of the returning voltage signal. Specifically, in 11 pigs, it will be demonstrated that 1) a physiological LV blood conductance signal can be derived; 2) LV dilation in response to dose-response intravenous neosynephrine can be detected by blood conductance in a similar fashion to the standard of endocardial crystals when admittance is used, but not when only traditional conductance is used; 3) the physiological impact of acute left anterior descending coronary artery occlusion and resultant LV dilation can be detected by blood conductance, before the anticipated secondary rise in right ventricular systolic pressure; and 4) a pleural effusion simulated by placing saline outside the pericardium does not serve as a source of artifact for blood conductance measurements.

Accuracy considerations in catheter based estimation of left ventricular volume.

Cardiac volume estimation in the Left Ventricle from impedance or admittance measurement is subject to two major sources of error: parallel current pathways in surrounding tissues and a non uniform current density field. The accuracy of volume estimation can be enhanced by incorporating the complex electrical properties of myocardium to identify the muscle component in the measurement and by including the transient nature of the field non uniformity. Cardiac muscle is unique in that the permittivity is high enough at audio frequencies to make the muscle component of the signal identifiable in the imaginary part of an admittance measurement. The muscle contribution can thus be uniquely identified and removed from the combined muscle - blood measurement. In general, both error sources are transient and are best removed in real time as data are collected. This paper reviews error correction methods and establishes that the relative magnitudes of the error concerns are different in small and large hearts.

Comparison of conductance to volume equations: the gain coefficient alpha.

The conductance catheter technique is used to measure real-time pressure and volume data in a beating heart. There are three competing equations for transforming the raw conductance signal into volume: (1) Baan's equation, (2) The cuvette equation (i.e. Relative Volume Units), and (3) Wei's equation. This paper explores the accuracy of these three equations compared to ultrasound echo in mice, and discusses the reason for discrepancy regarding both Baan's equation and the cuvette equation. We conclude that Wei's equation is the most accurate, because its nonlinear mapping yields volumes in the range of physiologic norms.

Dynamic correction for parallel conductance, GP, and gain factor, alpha, in invasive murine left ventricular volume measurements.

The conductance catheter technique could be improved by determining instantaneous parallel conductance (G(P)), which is known to be time varying, and by including a time-varying calibration factor in Baan's equation [alpha(t)]. We have recently proposed solutions to the problems of both time-varying G(P) and time-varying alpha, which we term "admittance" and "Wei's equation," respectively. We validate both our solutions in mice, compared with the currently accepted methods of hypertonic saline (HS) to determine G(P) and Baan's equation calibrated with both stroke volume (SV) and cuvette. We performed simultaneous echocardiography in closed-chest mice (n = 8) as a reference for left ventricular (LV) volume and demonstrate that an off-center position for the miniaturized pressure-volume (PV) catheter in the LV generates end-systolic and diastolic volumes calculated by admittance with less error (P < 0.03) (-2.49 +/- 15.33 microl error) compared with those same parameters calculated by SV calibrated conductance (35.89 +/- 73.22 microl error) and by cuvette calibrated conductance (-7.53 +/- 16.23 microl ES and -29.10 +/- 31.53 microl ED error). To utilize the admittance approach, myocardial permittivity (epsilon(m)) and conductivity (sigma(m)) were calculated in additional mice (n = 7), and those results are used in this calculation. In aortic banded mice (n = 6), increased myocardial permittivity was measured (11,844 +/- 2,700 control, 21,267 +/- 8,005 banded, P < 0.05), demonstrating that muscle properties vary with disease state. Volume error calculated with respect to echo did not significantly change in aortic banded mice (6.74 +/- 13.06 microl, P = not significant). Increased inotropy in response to intravenous dobutamine was detected with greater sensitivity with the admittance technique compared with traditional conductance [4.9 +/- 1.4 to 12.5 +/- 6.6 mmHg/microl Wei's equation (P < 0.05), 3.3 +/- 1.2 to 8.8 +/- 5.1 mmHg/microl using Baan's equation (P = not significant)]. New theory and method for instantaneous G(P) removal, as well as application of Wei's equation, are presented and validated in vivo in mice. We conclude that, for closed-chest mice, admittance (dynamic G(P)) and Wei's equation (dynamic alpha) provide more accurate volumes than traditional conductance, are more sensitive to inotropic changes, eliminate the need for hypertonic saline, and can be accurately extended to aortic banded mice.

Electrical conductivity and permittivity of murine myocardium.

A classic problem in traditional conductance measurement of left ventricular (LV) volume is the separation of the contributions of myocardium from blood. Measurement of both the magnitude and the phase of admittance allow estimation of the time-varying myocardial contribution, which provides a substantial improvement by eliminating the need for hypertonic saline injection. We present in vivo epicardial surface probe measurements of electrical properties in murine myocardium using two different techniques (a digital and an analog approach). These methods exploit the capacitive properties of the myocardium, and both methods yield similar results. The relative permittivity varies from approximately 100,000 at 2 kHz to approximately 5000 at 50 kHz. The electrical conductivity is approximately constant at 0.16 S/m over the same frequency range. These values can be used to estimate and eliminate the time-varying myocardial contribution from the combined signal obtained in LV conductance catheter measurements, thus yielding the blood contribution alone. To study the effects of albumin on the blood conductivity, we also present electrical conductivity estimates of murine blood with and without typical administrations of albumin during the experiment. The blood conductivity is significantly altered (p < 0.0001) by administering albumin (0.941 S/m with albumin, 0.478 S/m without albumin).