John J. Michele

Electrical Impedance Properties of Normal and Chronically Infarcted Left Ventricular Myocardium

Background: Previous reports have disclosed that a significant difference exists between the electrical impedance properties of healthy and chronically infarcted ventricular myocardium.Purpose: To assess the potential utility of electrical impedance as the basis for mapping in chronically infarcted left ventricular myocardium. Specifically: (1) to delineate electrical impedance properties of healthy and chronically infarcted ventricular myocardium, with special emphasis on the infarction border zone; (2) to correlate impedance properties with tissue histology; (3) to correlate impedance properties with electrogram amplitude and duration; (4) To demonstrate that endocardial impedance can be measured effectively in vivo using an electrode mounted on a catheter inserted percutaneously.Methods: An ovine model of chronic left ventricular infarction was utilized. Sites of healthy myocardium, densely infarcted myocardium and the infarction border zone were investigated. Bulk impedance was measured in vitro using capacitor cell, four-electrode and unipolar techniques. Epicardial and endocardial impedances were measured in vivo using four-electrode and unipolar techniques. Impedance was measured at multiple frequencies. Electrographic amplitude, duration and amplitude/duration ratio were measured using bipolar electrograms during sinus rhythm. Quantitation of tissue content of myocytes, collagen, elastin and neurovascular elements was performed.Results: Densely infarcted myocardial impedance was significantly lower than healthy myocardium. Impedance gradually decreased in the border zone transitioning between healthy myocardium and dense infarction. Decreasing impedance correlated with a decrease in tissue myocyte content. The magnitude of the difference in impedance between densely infarcted and healthy myocardium increased as the measurement frequency decreased. Healthy myocardium exhibited a marked frequency dependence in its impedance properties; this phenomenon was not observed in densely infarcted myocardium. There was a direct association between impedance and both electrogram amplitude and amplitude/duration ratio. There was an inverse association between impedance and electrogram duration. Endocardial impedance, measured in vivo using a electrode catheter inserted percutaneously, was demonstrated to distinguish between healthy and infarcted myocardium.Conclusions: The electrical impedance properties of healthy and infarcted left ventricular myocardium differ markedly. The properties of the infarction border zone are intermediate between healthy and infarcted myocardium. Impedance may be a useful assay of cardiac tissue content and adaptable for cardiac mapping in vivo.Condensed Abstract. To delineate the electrical impedance properties of healthy and chronically infarcted left ventricular myocardium emphasizing the infarction border zone, impedance was measured in chronically infarcted ovine hearts. Densely infarcted myocardial impedance was significantly lower than healthy myocardium. Impedance gradually decreased in the infarction border zone in transition between healthy myocardium and dense infarction. This correlated with a decreasing myocyte content. The magnitude of the difference in impedance between densely infarcted and healthy myocardium increased as measurement frequency decreased. There was a direct association between impedance and electrogram characteristics. Endocardial impedance, measured in vivo using an electrode catheter inserted percutaneously, distinguished between healthy and infarcted myocardium

Changes in local wall thickness correlate with pathologic lesion size following radiofrequency catheter ablation: an intracardiac echocardiographic imaging study.

Introduction: High‐resolution intracardiac echocardiographic (ICE) imaging can accurately assess wall thickness during radiofrequency (RF) catheter ablation procedures. This study investigated the correlation of changes in wall thickness at the ablation site with pathologic lesion size. Methods and Results: ICE image‐guided 31 RF applications (30–50 W, up to 120 sec) were performed in five anesthetized closed chest swine (n = 5, body weight 35–60 kg). Twenty‐four lesions were delivered in the right and left atria with standard RF; seven lesions were delivered in the left ventricle (LV) with irrigated (30–40 ml/min) RF. Wall thickness and tissue echo density measured by ICE imaging (preand 1‐minute post‐RF delivery) with increased focal echo density following RF deployment in the atria (4.5 ± 1.5 vs 2.3 ± 1.0 mm pre‐RF) and the LV (9.8 ± 2.3 vs 6.8 ± 2.2 mm pre‐RF; P < 0.01). The observed changes in wall thickness (ΔWT) following ablation in the LV were greater than in the atria (3.0 ± 1.4 vs 2.2 ± 1.2 mm; P < 0.05). A significant correlation between ΔWT and lesion depth (ventricular: r = 0.85, P < 0.05; atrial: r = 0.82, P < 0.01) was demonstrated at all ablation sites. Local wall thickness measured post‐RF also significantly correlated with lesion depth (r = 0.89, P < 0.01), especially with that of transmural lesions (r = 0.95, n = 23, P < 0.001) at atrial and LV sites. Conclusion: Therapeutic RF ablation results in mural swelling and increased echo density. These changes can be detected by ICE imaging and correlate with pathologic lesion size. ICE imaging may be useful in online quantification of lesion size, especially for transmural lesions during clinical catheter ablation procedures.

Seasonal and Latitudinal Occurrence of Cerebral Vasospasm and Subarachnoid Hemorrhage in the Northern Hemisphere

Using patient data obtained from the International Cooperative Aneurysm Study that evaluated clinical records from 68 neurosurgical centers in 14 countries, we evaluated the data as to the monthly occurrence of cerebral vasospasm and subarachnoid hemorrhage relative to the latitude of the medical center where the data were gathered. Using the Edwards analysis for cyclic patterns, we examined peak to trough ratios and months of peak to assess the strength and nature of the cyclic variations between December 1980 and July 1983. Of the 3,521 subarachnoid hemorrhage patient records examined, 685 developed some grade of vasospasm as defined symptomatically, angiographically, and by the Fisher Grading Scale. Cyclic analysis demonstrated a strong seasonal occurrence for the incidence of subarachnoid hemorrhage, with a peak in February, in the northern hemisphere. Cerebral vasospasm incidence, after controlling for subarachnoid hemorrhage occurrence, exhibited only a small peak to trough ratio (1.15) relative to the larger ratio (1.74) seen with unadjusted vasospasm data. Age was related to both incidence and location. In populations at high risk for cerebrovascular diseases, climatic conditions may act as synchronizers of pathologic vascular events.

Failure of the somatosensory evoked potential following middle cerebral artery occlusion and high-grade ischemia in the cat--effects of hemodilution.

Acute focal ischemia was created in 10 cats by unilateral retro-orbital middle cerebral artery (MCA) occlusion. Regional cerebral blood flow (CBF) was determined utilizing the hydrogen clearance technique from electrode recordings within the gray matter and white matter of the ectosylvian gyrus of both hemispheres. The somatosensory evoked potential (SSEP) was obtained during contralateral median nerve stimulation. When the MCA was clipped the white and gray matter blood flows in the ipsilateral ectosylvian gyrus were reduced to 14.8 +/- 19.6% and 19.3 +/- 23.7% of control, and the cortical component of the SSEP was abolished. In the contralateral hemisphere an average increase of 3.5% above the control latency and a 10% mean depression in the amplitude of the cortical component of the SSEP were observed following occlusion. CBF in the contralateral hemisphere was unaffected by the MCA clip. Infusion of saline or dextran to lower the hematocrit by approximately 45% did not significantly improve blood flow or restore the SSEP in the hemisphere ipsilateral to the MCA clip. However, significant increases in the contralateral hemisphere gray matter CBF occurred following hemodilution while the latency of the cortical component of the SSEP in this same hemisphere was significantly extended. Elevations in gray and white matter blood flows were achieved in the experimental hemisphere of 3 of 10 cats suggesting a wide range of variation in the collateral circulation.

Multiplane Transesophageal and Intracardiac Echocardiography in Large Swine: Imaging Technique, Normal Values, and Research Applications.

Transthoracic echocardiographic imaging has been difficult to attain in the swine model. This study: (1) compares multiplane transesophageal echocardiography (TEE) with single plane TEE and intracardiac catheter echocardiography (ICE) for imaging of the swine cardiovascular system; and (2) defines normal values using these techniques in a closed chest large swine model (n = 24, body weight 50–114 kg). Multiplane TEE increased success rate over the single plane (the variable plane array only at 0°) TEE (P < 0.01) for imaging the left ventricular (LV) long‐axis view (100% vs 50%), LV outflow tract (100% vs 33%), right atrium and its appendage (79% vs 33%), ascending aorta (100% vs 58%), and aortic arch (100% vs 17%). TEE‐derived normal values at end‐diastole (ED) and end‐ systole (ES) were: LV internal diameter (ID) = 49 ± 3 mm (ED) and 33 ± 4 mm (ES); LV wall thickness = 7 ± 1 mm (ED); right ventricular (RV) ID = 24 ± 4 mm (ED); RV wall thickness = 4 ± 2 mm (ED); left atrial ID = 48 ± 6 mm (ES); aortic root ID = 26 ± 3 mm (ES); LV volume = 157 ± 49 ml (ED) and 57 ± 22 ml (ES). Baseline LV ejection fraction (64%± 6%), Doppler‐derived stroke volume (86 ± 14 ml), and cardiac index (107 ml/min per kg) were determined. Basal normal values, except for an elevated cardiac index in swine, are comparable to those reported for human adults. Multiplane TEE provided better overall cardiac imaging than did single plane TEE. ICE provided higher resolution imaging of individual cardiac chambers and structures when the ultrasound catheter was introduced into the right or left heart, but whole heart imaging was limited by ultrasound penetration at 12.5 MHz. Normal indices of chamber size and function provide a reference for the physiological significance of induced pathological states in this relevant animal model.

Intracardiac echocardiographic evaluation of ventricular mural swelling from radiofrequency ablation in chronic myocardial infarction: irrigated-tip versus standard catheter.

AbstractIntroduction: The production of larger, particularly deeper lesions may improve the success rate for radiofrequency (RF) ablation of post infarction ventricular tachycardia (VT). Therapeutic RF ablation causes left ventricular (LV) mural swelling. This swelling can be detected as increased wall thickness at the ablation site by intracardiac echocardiography (ICE) and correlates with pathologic lesion size. This study compared the extent of mural swelling caused by linear ablation lesions created with irrigated tip and standard RF ablation in a porcine model of healed anterior infarction. Methods and Results: In anesthetized closed-chest swine ICE guided multiple RF applications to construct linear lesions at the border zone of the infarct region using an irrigated RF (n=6 swine) and a standard RF (n=6 swine) ablation catheter. 47 individual lesions were created with irrigated RF ablation; 57 lesions created with standard RF ablation. At all sites, wall thickness (measured at end-diastole Pre- and 1[emsp4 ]min Post-RF delivery) increased following either irrigated (p<0.0001) or standard (p<0.004) RF deployment. Irrigated RF ablation produced more mural swelling at border zone sites than standard RF ablation (wall thickness increase of 21.2 versus 15.1 %, p<0.003). This difference was more pronounced at RF sites within the infarct (40.7 versus 12.0 %, p<0.0007). Thrombus formation or intramural explosion were not observed; surface crater formation was not more frequent with irrigated compared to standard RF ablation (14/47 versus 12/57 lesions, p=NS). Conclusion: Irrigated RF ablation may produce larger lesions than standard RF ablation, particularly for ablation targets within infarcted tissue. ICE imaging provides on line data about the characteristics of the developing lesion which may prove useful in dosing irrigated-tip RF energy application.

Comparison of the somatosensory evoked potential and the direct cortical response following severe incomplete global ischemia: selective vulnerability of the white matter conduction pathways.

Eight cats were subjected to graded hemorrhagic hypotension following bilateral carotid ligation to produce incomplete global cerebral ischemia. Three additional cats served as controls. The somatosensory evoked potential (SEP) and direct cortical response (DCR) were monitored in all animals and in each case, the cortical component of the SEP was abolished during progressive ischemia while the morphology of the DCR was well-preserved but with reduced amplitude. Determinations of adenosine triphosphate (ATP), phosphocreatine (PCr), and lactate levels in cerebral cortex and white matter were made in five experimental cats and the three controls. At the time of failure of the cortical SEP, PCr was dramatically reduced and lactate moderately elevated in the white matter while ATP remained unchanged. Cortical lactate was only mildly elevated and PCr and ATP were unchanged accounting for preservation of the DCR. In this model of global ischemia, abolition of the cortical SEP is due to a block of stimulus conduction in white matter projection pathways. A hypothesis to explain the observed metabolic changes is presented and correlation is made to clinical situations.

Cerebral blood flows and tissue oxygen levels associated with maintenance of the somatosensory evoked potential and cortical neuronal activity in focal ischemia.

The middle cerebral artery was occluded in 18 cats to evaluate the physiological consequences of cerebral blood flow reductions on the somatosensory evoked potential, spontaneous neuronal activity, and oxygen availability in the ipsilateral and contralateral hemispheres. In the ipsilateral ectosylvian gyrus high-grade ischemia was produced as blood flow in the gray matter was reduced from 52.1 +/- 8.6 (mean +/- SE) to 13.3 +/- 9.0 ml/100 g/min and in the white matter from 33.8 +/- 5.6 to 6.1 +/- 6.4 ml/100 g/min. This significant reduction (p less than 0.05) was associated with abolition of the cortical component of the somatosensory evoked potentials. In all animals occlusion resulted in a predictable extended latency change and a variable amplitude response of the cortical component of the contralaterally recorded somatosensory evoked potentials. In 5 animals, oxygen availability was measured and spontaneous neuronal activity in the contralateral hemisphere was recorded. Volume expansion and hemodilution with either dextran or saline infusions elevated cerebral blood flow in the contralateral gray matter significantly (p less than 0.05) compared with the control and clip values. Ipsilateral spontaneous activity stopped within 4-12 minutes of occlusion, while contralateral spike activity persisted at rates at least equal to those recorded immediately following occlusion.

Left ventricular catheter ablation using direct, intramural Ethanol injection in swine

AbstractIntroduction: Limitations in lesion volume and particularly lesion depth may negatively effect the efficacy of catheter ablation procedures using radiofrequency energy. This study evaluated the safety and efficacy of myocardial ablation using direct intramural injection of ethanol with a novel injection catheter system. Methods: Left ventricular lesions were performed in 9 male swine (80–85 pounds); two animals were studied 6 weeks following anterior infarction produced by agarose gel embolization. An 8 Fr deflectable catheter equipped with a 27 gauge adjustable depth, retractable needle was directed to the LV using a retrograde aortic approach. Lesion deployment was guided by fluoroscopy and intracardiac echocardiography (ICE). Lesion characteristics were assessed with ICE imaging and pathologic analysis. Results: Ethanol lesions were confined to the tissue directly adjacent to the injection port. Lesions were intramural with no evidence of overlying thrombus. Lesions delivered with a single port injection needle in normal myocardium (n = 24) averaged 1910 ± 1066 mm3 with a depth of 8.9 ± 3.3 mm. Lesions directed to infarct border zones (n = 4) averaged 929 ± 882 mm3 with a depth of 4.3 ± 2.8 mm. Lesions were immediately evident on ICE imaging, and were visualized by increased echo density and tissue swelling. Pathological analysis revealed homogenous lesions with intramural hemorrhage and contraction band necrosis. Conclusions: Myocardial catheter ablation using direct ethanol injection is feasible, and relatively large and deep intramural lesions can be delivered, even in the infarct border zone. This technique may prove useful in ablation of arrhythmia substrates that are deep to the endocardial surface.

Echocardiographic transponder-guided catheter ablation feasibility and accuracy

AbstractBackground: The utility of echocardiography for catheter guidance during percutaneous endocardial ablation is increasingly apparent. However, the technique is currently imperfect due to limitations in discerning the ablation electrode from other parts of the catheter shaft. Purpose: To examine the feasibility and accuracy of echocardiography-guided ablation using commercial ablation catheters fitted with a transponder to improve localization of the ablation electrode. Methods: Fifteen healthy pigs and five pigs with chronic anterior myocardial infarction were studied. In healthy animals, echocardiographically distinct endocardial sites in right and left cardiac chambers were targeted for ablation. In infarcted animals, the left ventricular infarction border zone was targeted. Both intracardiac (ICE; 12.5[emsp4 ]megahertz and 5[emsp4 ]megahertz) and transesophageal echocardiographic (TEE) techniques were utilized. Results: In healthy animals, transponder-guided ablation was feasible with each of the echocardiographic techniques. Accuracy was 82 % (45 of 55 lesions) with ICE-12.5[emsp4 ]MHz, 87 % (27 of 31 lesions) with ICE-5[emsp4 ]MHz, and 81 % (22 of 27 lesions) with TEE. In infarcted animals, the accuracy was 38 % (3 of 8 lesions) for ICE-5[emsp4 ]MHz and 38 % (3 of 8 lesions) for TEE. Errant lesions in healthy animals were observed in earlier experiments, due to operator misinterpretation of the plane of imaging. Errant lesions in infarcted animals were observed throughout the experimental series, and may have been due to a variable relationship between echocardiographic and histologic infarction border zones. Conclusions: Echocardiographic transponder-guided catheter ablation is feasible. Accuracy for normal endocardial targets was excellent, less so for chronic infarction border.