Voltage gradients in transvenous and subcutaneous defibrillation and their risk of myocardial damage.

INTRODUCTION: Transvenous implantable cardioverter-defibrillator (ICD) shocks have been associated with cardiac biomarker elevations and are thought in some cases to contribute to adverse clinical outcomes and mortality, possibly from myocardium exposed to excessive shock voltage gradients. Currently, there are only limited data for comparison with subcutaneous ICDs. We sought to compare ventricular myocardium voltage gradients resulting from transvenous (TV) and subcutaneous defibrillator (S-ICD) shocks to assess their risk of myocardial damage. METHODS: A finite element model was derived from thoracic magnetic resonance imaging (MRI). Voltage gradients were modeled for an S-ICD with a left-sided parasternal coil and a left-sided TV-ICD with a mid-cavity, a septal right ventricle (RV) coil, or a dual coil lead (TV mid, TV septal, TV septal + superior vena cava [SVC]). High gradients were defined as > 100 V/cm. RESULTS: The volumes of ventricular myocardium with high gradients > 100 V/cm were 0.02, 2.4, 7.7, and 0 cc for TV mid, TV septal, TV septal + SVC, and S-ICD, respectively. CONCLUSION: Our models suggest that S-ICD shocks produce more uniform gradients in the myocardium, with less exposure to potentially damaging electrical fields, compared to TV-ICDs. Dual coil TV leads yield higher gradients, as does closer proximity of the shock coil to the myocardium.

Safety and efficacy aspects of pulsed field ablation catheters as a function of electrode proximity to blood and energy delivery method.

BACKGROUND: Pulsed field ablation (PFA) is a promising technology based on electroporation. It is unclear if different catheter designs imply efficacy and safety differences. OBJECTIVE: To vary geometry, blood exposure, and energy delivery methods among 3 representative catheter designs, and then compare lesion transmurality, extra-atrial safety, and embolic risk. METHODS: A computed tomography-derived computer model was used. Balloon, flexible-circuit splined, and circular catheters were placed near the left pulmonary veins. Four energy delivery methods were tested: multi-unipolar, sequential unipolar, interlaced, and wide interlaced. A posterior wall target was defined. Efficacy was defined as percent target with >600 V/cm. Safety aspects included aortic/esophageal electroporation damage and a bubble-generation surrogate (electrode current density), with 90% transmurality requirement. RESULTS: Balloon catheters had highest efficacy, followed by flexible polymer splined and circular catheters. On energy delivery methods, the multi-unipolar one was most efficacious, followed by interlaced bipolar and sequential-unipolar ones. Electroporation risks to aorta and esophagus were highest with multi-unipolar energy delivery. Bubble risk was lowest with balloon catheters. CONCLUSION: Computer models show that catheters with electrodes on a balloon surface or on flexible circuit splines are about 4 times more efficacious than circular catheters with electrodes exposed to atrial blood. Multi-unipolar energy delivery methods have a higher risk of electroporating aortic and esophageal tissue, when compared to bipolar interlaced methods. Considering embolic risks, circular catheters had the highest bubble-generating potential. A balloon or flexible circuit splined system with a wide interlaced delivery method showed the best balance in efficacy and safety.

Determinants of Subcutaneous Implantable Cardioverter-Defibrillator Efficacy: A Computer Modeling Study.

OBJECTIVES: This study determined the impact of subcutaneous implantable cardioverter-defibrillator (S-ICD) coil and generator position on defibrillation threshold (DFT). BACKGROUND: S-ICD implantation can occasionally result in unacceptably high DFT. Implant position characteristics associated with high DFTs in S-ICD patients have not been fully elucidated. METHODS: A 3.8-million-element computer model built from magnetic resonance images was used to simulate the electric fields that occur during defibrillation. Generator positions were tested from posterior to anterior in 4-cm increments. The left parasternal coil was tested with 0, 5, and 10 mm of underlying subcutaneous fat and the generator with 20 mm of underlying fat. The estimated DFT for the S-ICD was defined as the energy delivered when producing an electric field of 4 volts/cm in at least 95% of the ventricular myocardium. RESULTS: Estimated DFTs were 22, 29, 64, and 135 joules for posterior, standard (lateral), mid-anterior, and anterior generator locations, respectively. Defibrillation thresholds were 29, 58, and 95 joules with 0, 5, and 10 mm subcoil fat, respectively, and 45 joules with 20 mm subgenerator fat. Combining anterior generator position with subcoil fat resulted in a very high DFT (379 joules). Shock impedance increased with both subcoil and subgenerator fat but was minimally affected by anterior/posterior generator position. CONCLUSIONS: The model suggests that an S-ICD implantation strategy involving posterior generator location and coil and generator directly over the fascia without underlying fat is likely to markedly lower DFTs with the S-ICD and assist in troubleshooting of patients with unacceptably high DFTs.

Subcutaneous bioimpedance recording: assessment of a method for hemodynamic monitoring by implanted devices.

BACKGROUND: Diagnostic evaluation of patients with suspected symptomatic arrhythmias is limited by inability to assess the hemodynamic impact of a detected rhythm. OBJECTIVE: To address this limitation, we utilized closely spaced subcutaneous electrodes, small enough to incorporate within an implantable monitor, to detect blood flow-induced pectoral muscle bioimpedance (Z) changes in a swine model of hemorrhage-induced hypotension. METHODS: In seven anesthetized and ventilated adult pigs, small ring electrodes (current electrodes 5 cm apart; voltage electrodes 3.5 cm apart) were positioned on the left pectoral muscle. Z signals (Biopac system) and invasive arterial blood pressures were recorded. Hypotension was induced by hemorrhage (50% blood volume reduction). Mean arterial pressure (MAP) and pulse pressure (PP) with corresponding pulse Z (DeltaZ) and base Z (Z(o)) were measured. A longitudinal mixed model with a first-order autoregressive error structure was used to test for associations (change in DeltaZ vs change in MAP and change in DeltaZ vs change in PP) taking into account within pig correlation. RESULTS: During bleeding-induced hypotension, Z(o) increased. Changes of DeltaZ correlated with both a change in MAP (coefficient = 1.17, P < 0.0001) and change in PP (coefficient = 0.98, P < 0.0001). A change in DeltaZ of 1-2 orders of magnitude corresponded to an approximate 40-70% drop in MAP and PP in a porcine model in which the baseline MAP was 69-70 mmHg. CONCLUSION: Our findings suggest that closely spaced subcutaneous electrodes identify changes in local tissue/vascular bioimpedance that correlate well with direct invasive measures of induced hypotension in a porcine model.

Monitoring lung edema using the pacemaker pulse and skin electrodes.

Previous clinical studies have shown that impedance measurements using right ventricular (RV) leads can monitor congestion due to heart failure. We previously reported on a three-fold advantage of bipolar left ventricular (LV) leads, which are near the lung, over RV leads in detecting pulmonary edema with impedance. A combined system of internal and external electrodes is now investigated using computer models, for use with conventional cardiac resynchronization (CRT) systems with unipolar LV leads. The system uses the normal LV pacing pulse as current source, and the resultant voltage at two skin electrodes to obtain a lung edema impedance (Z) measurement. Using gated MRIs, thoracic computer models of 3.8 million control volumes were constructed. Changes of Z with edema were simulated with a conventional totally implanted system, as well as with combined implanted-external systems. Right atrial (RA), RV, RV defibrillator coil and LV leads were used. Per cent Z responses to edema were compared. The all implanted responses were RA: 11.8%, RV: 8.6%, RVcoil: 11.3%, LV: 23.8%. The combined system responses were LV-ext: 21.45%, RA-ext: 10.13%, LV-arm leg: 26.08%. The computer models suggest that combined internal-external systems can be as sensitive as the totally implanted ones. Lung edema may be monitored at follow up or home for LV paced patients with only two external electrodes. Using very low impedance configurations optimized by computer can greatly maximize the response, with a cost of poor stability.

Determinants of radial artery pulse wave analysis in asymptomatic individuals.

BACKGROUND: Noninvasive techniques to evaluate arterial stiffness include noninvasive radial artery pulse contour analysis. Diastolic pulse contour analysis provides a separate assessment of large (C1) and small artery (C2) elasticity. Analysis of the systolic pulse contour identifies two pressure peaks (P1 and P2) that relate to incident and reflected waves. This study aimed to compare indices from systolic and diastolic pulse contour analysis from the radial pressure waveform and to correlate these indices with traditional risk factors in asymptomatic individuals screened for cardiovascular disease. METHODS: In 298 consecutive subjects (206 male and 92 female healthy subjects with a mean age of 50 +/- 12 years), noninvasive radial artery pressure waveforms were acquired with a piezoelectric transducer and analyzed for 1) diastolic indices of C1 and C2 from the CR-2000 CVProfiler, and 2) systolic indices of augmentation as defined by augmentation pressure (AP), augmentation index (AIx), and systolic reflective index (SRI = P2/P1). These indices were then correlated to each other as well as to individual traditional risk factors and the Framingham Risk Score. RESULTS: Diastolic indices were significantly and inversely correlated to systolic indices with C2 showing a stronger inverse association than C1. C2 and Alx were significantly correlated with height, weight, and body mass index in men but not in women. All indices correlated better to blood pressure in women than men. In women, only systolic indices were significantly correlated to HDL cholesterol and only diastolic indices were significantly correlated to LDL cholesterol. All indices were significantly correlated to the Framingham Risk Score, which was stronger in women then men, but when adjusted for age only diastolic indices remained significant in women. CONCLUSIONS: Diastolic and systolic indices of pulse contour analysis correlate differently with traditional risk factors in men and women.

Improved lung edema monitoring with coronary vein pacing leads: a simulation study.

This computer simulation study compared the ability of left ventricular coronary vein (LV) pacemaker leads against right ventricular (RV) and right atrial (RA) leads to monitor lung edema using electrical impedance measurements. MRI images were used to construct electrical models of the thorax. Four lead configurations were tested with increases of pulmonary edema, intravascular fluids and heart dilation. The impedance changes observed at end systole with severe lung edema were 8.5%, 11.2%, 12.3% and 26.8% for the RA, RV, RV coil and LV configurations, respectively. Sensitivities in ohms per litre of lung fluid were 19.15, 19.15, 25.07 and 52.11 for the same configurations. The impedance changes for intravascular fluid overload with constant lung status were 1%, 1.3%, 9.2% and 6.4% while the sensitivities were 2, 2, 17 and 11 ohms per litre of intravascular fluid, respectively. Regional analysis of the thoracic sources of impedance revealed a high sensitivity near pacing electrodes and generator, and a low sensitivity to the right lung and all pulmonary vessels. Simulations showed that LV leads have a threefold advantage in sensitivity when monitoring lung edema in comparison to conventional RV leads. To monitor vascular and lung fluids independently, combined impedance configurations may be used. Regional sensitivities must be taken into account for proper clinical interpretation of impedance changes.

Cardiac resynchronization therapy optimization by finger plethysmography.

OBJECTIVES: We tested a simple noninvasive method for cardiac resynchronization therapy (CRT) optimization using standard finger photoplethysmography (FPPG). BACKGROUND: CRT can increase left ventricular cardiac output in patients with heart failure and ventricular conduction delay. Optimal therapy delivery depends on an appropriate AV delay. Multiple invasive and noninvasive methods have been attempted to identify patients and the best AV delay for CRT, but all suffer from a combination of high patient risk, cost, complexity, and low reproducibility. METHODS: FPPG and invasive aortic pressure data were simultaneously collected from 57 heart failure patients during intrinsic rhythm alternating with very brief periods of pacing at 4 to 5 AV delays. After correcting data for artifacts, the median percentage responses for each AV delay were classified as positive, negative, or neutral compared to baseline (Wilcoxon rank test). RESULTS: FPPG correctly identified positive aortic pulse pressure responses with 71% sensitivity (95% CI: 60-80%) and 90% specificity (95% CI: 84-94%) and negative aortic pulse pressure responses with 57% sensitivity (95% CI: 44-69%) and 96% specificity (95% CI: 91-98%). The magnitude of FPPG changes were strongly correlated with positive aortic pulse pressure changes (R(2) = 0.73, P < .0001) but less well correlated with negative aortic pulse pressure changes (R(2) = 0.43, P < .0001). FPPG selected 78% of the patients having positive aortic pulse pressure changes to CRT and identified the AV delay giving maximum aortic pulse pressure change in all selected patients. CONCLUSIONS: FPPG can provide a simple noninvasive method for identifying significant changes in aortic pulse pressure with high specificity, including identifying patients in whom aortic pulse pressure increases with CRT and the AV delay giving the maximum aortic pulse pressure.