Dan E. Gutfinger

Usefulness of monitoring congestive heart failure by multiple impedance vectors

Introduction: We investigated trends in intrathoracic impedance measured between multiple implanted electrodes for monitoring pulmonary edema secondary to congestive heart failure (CHF) in an experimental model. Methods: Biventricular ICDs were implanted in 16 dogs and 5 sheep. Continuous RV pacing (230–250 bpm) was applied over several weeks. Meanwhile, impedance was measured every hour along 4 intrathoracic and 2 intracardiac vectors. Four cardiogenic impedance vectors were also monitored. Cardiac function was assessed biweekly by catheterization and echocardiography. Left atrial (LA) pressure was measured daily by an implanted LA pressure sensor. Results: All animals developed CHF after 2–4 weeks of pacing as evidenced by changes in function (EF, 52 vs. 34%; LV end-diastolic volume, 65 vs. 97 ml; LV end-diastolic pressure, 7 vs. 16 mmHg; LA volume, 17 vs. 33 ml; LA pressure, 7 vs. 26 mmHg), clinical symptoms, or autopsy. Steady state impedance decreased during CHF: LV-Can, 17±9%; LV-RV, 15±8%; LV-RA, 13±6%; RV-Can, 13±8%; RVcoil-Can, 8±6%; RA-Can, 6±6%. Change in LV-Can impedance was greater than that of RA-Can, RV-Can, and RVcoil-Can (P<0.05). LV-Can impedance correlated well with LA pressure (r2=0.73), while RV-Can and RVcoil-Can were weakly correlated (r2=0.43 and r2=0.52, respectively). Changes in LV-RV and LV-RA impedances were also larger than those of RVcoil-Can and RA-Can (P<0.05). Meanwhile, all impedances were associated with circadian variability at baseline (5±2%) which diminished during CHF (2±1%); P=0.02. Furthermore, significant variations were observed in cardiogenic impedances during progression into CHF as evidenced by reduced peak-to-peak amplitude and increased fractionation of the signals. Conclusions: All impedance vectors decreased during CHF. Impedance measurement employing left heart sensors correlated well with LA pressure, and may improve detection of CHF onset compared to sensing by RA or RV leads alone. This approach has important clinical implications for managing heart failure patients in the ambulatory setting.

Monitoring of heart failure: comparison of left atrial pressure with intrathoracic impedance and natriuretic peptide measurements in an experimental model of ovine heart failure

Monitoring of HF (heart failure) with intracardiac pressure, intrathoracic impedance and/or natriuretic peptide levels has been advocated. We aimed to investigate possible differences in the response patterns of each of these monitoring modalities during HF decompensation that may have an impact on the potential for early therapeutic intervention. Six sheep were implanted with a LAP (left atrial pressure) sensor and a CRT-D (cardiac resynchronization therapy defibrillator) capable of monitoring impedance along six lead configuration vectors. An estimate of ALAP (LAP from admittance) was determined by linear regression. HF was induced by rapid ventricular pacing at 180 and 220 bpm (beats/min) for a week each, followed by a third week with daily pacing suspensions for increasing durations (1–5 h). Incremental pacing induced progressively severe HF reflected in increases in LAP (5.9 ± 0.4 to 24.5 ± 1.6 mmHg) and plasma atrial (20 ± 3 to 197 ± 36 pmol/l) and B-type natriuretic peptide (3.7 ± 0.7 to 32.7 ± 5.4 pmol/l) (all P<0.001) levels. All impedance vectors decreased in proportion to HF severity (all P<0.001), with the LVring (left ventricular)-case vector correlating best with LAP (r2=0.63, P<0.001). Natriuretic peptides closely paralleled rapid acute changes in LAP during alterations in pacing (P<0.001), whereas impedance changes were delayed relative to LAP. ALAP exhibited good agreement with LAP. In summary, impedance measured with an LV lead correlates significantly with changes in LAP, but exhibits a delayed response to acute alterations. Natriuretic peptides respond rapidly to acute LAP changes. Direct LAP, impedance and natriuretic peptide measurements all show promise as early indicators of worsening HF. ALAP provides an estimate of LAP that may be clinically useful.

Concepts in the application of pneumatic ventricular assist devices for ischemic myocardial injury

&NA; The clinical results of “bridge‐to‐recovery” from ischemic myocardial injury using pneumatic ventricular assist devices (VADs) have been disappointing, because no significant improvement in the hospital discharge rate (25%) has been observed during the past 10 years. Interestingly, similar results have been reported using the less sophisticated and more widely available centrifugal pumps. It is well recognized that appropriate patient selection and early device implantation are important determinants of patient survival; however, it is less clear why there is a lack of difference in the results between pneumatic VADs and the centrifugal pumps. The reasons for the lack of difference in the results between pneumatic VADs and centrifugal pumps are multifactorial, and to some extent may be due to a conservative approach in the application of the more capable pneumatic VADs. In an effort to provide a more effective approach to the clinical application of pneumatic VADs for bridge‐to‐recovery, two pneumatic devices (the Jarvik 7‐70 total artificial heart and the Symbion acute VAD) were functionally characterized using an in vitro mock circulatory system. The performance under pneumatic VAD asynchronous pumping compared to electrocardiogram synchronous counterpulsation was also evaluated. Based upon the results obtained, a two phase approach was developed. In the initial phase (i.e., the first 12‐48 hrs), device output is maximized through asynchronous pumping to rapidly reverse the effects of cardiogenic shock. During the second phase (i.e., after hemodynamic stabilization and early evidence of end‐organ recovery), electrocardiogram synchronous counterpulsation is used to focus more specifically on recovery of the heart. Unlike previous approaches, this two phase approach provides a more physiologic setting to enhance recovery by effectively reducing myocardial oxygen consumption through asynchronous pumping (Phase 1) and maximally increasing myocardial oxygen delivery through electrocardiogram synchronous counterpulsation (Phase 2). This approach, coupled with appropriate patient selection and early device institution, may improve recovery in the clinical setting of reversible myocardial injury. ASAIO Journal 1995; 41:162‐168.

Integrated microscopy techniques for comprehensive pathology evaluation of an implantable left atrial pressure sensor

Abstract The safety and efficacy of an implantable left atrial pressure (LAP) monitoring system is being evaluated in a clinical trial setting. Because the number of available specimens from the clinical trial for histopathology analysis is limited, it is beneficial to maximize the usage of each available specimen by relying on integrated microscopy techniques. The aim of this study is to demonstrate how a comprehensive pathology analysis of a single specimen may be reliably achieved using integrated microscopy techniques. Integrated microscopy techniques consisting of high-resolution gross digital photography followed by micro-computed tomography (micro-CT) scanning, low-vacuum scanning electron microscopy (LVSEM), and microground histology with special stains were applied to the same specimen. Integrated microscopy techniques were applied to eight human specimens. Micro-CT evaluation was beneficial for pinpointing the location and position of the device within the tissue, and for identifying any areas of interest or structural flaws that required additional examination. Usage of LVSEM was reliable in analyzing surface topography and cell type without destroying the integrity of the specimen. Following LVSEM, the specimen remained suitable for embedding in plastic and sectioning for light microscopy, using the positional data gathered from the micro-CT to intersect areas of interest in the slide. Finally, hematoxylin and eosin (H&E) and methylene blue staining was deployed on the slides with high-resolution results. The integration of multiple techniques on a single specimen maximized the usage of the limited number of available specimens from the clinical trial setting. Additionally, this integrated microscopic evaluation approach was found to have the added benefit of providing greater assurance of the derived conclusions because it was possible to cross-validate the results from multiple tests on the same specimen.

Comparative pathology of an implantable left atrial pressure sensor.

A left atrial pressure (LAP) monitoring system was developed for guiding the management of patients with heart failure. The LAP sensor is implanted into the left atrium via transseptal catheterization and affixed to the interatrial septum by nitinol anchors. The long-term safety of permanent implantation of the LAP sensor in patients was evaluated based on the comparative pathology in animals. Tissue specimens of the LAP sensor from 7 patients, 49 canines, and 14 ovine were examined for thrombosis and tissue overgrowth. Implant duration ranged from 22 to 1,686 days. Gross examination showed minimal-to-moderate tissue overgrowth with no evidence of migration, erosion, or perforation. There was no excessive host-to-device reaction or active thrombogenesis in any of the subjects that followed the antithrombotic therapy protocol. Micro-computed tomography scanning confirmed the structural integrity of the device. Low vacuum scanning electron microscopy and histology showed neoendocardial tissue overgrowth with no inflammation or fibrin. The pathology of both animal models was found to closely approximate the pathology in humans and favorably supports the long-term safety of the device.

Simplified Parsonnet Risk Scale Identifies Limits to Early Patient Discharge

Abstract Background: Fast‐track recovery after coronary artery bypass surgery has influenced patient care positively. Predicting patients who fall off track and require prolonged (≥7 days) hospitalization remains uncertain. The Parsonnet risk assessment score is effective in predicting length of stay, but is limited by inaccurate subdivision of risk categories. We simplified the Parsonnet risk scale to better identify patients eligible for fast‐track recovery. Method: The cases of 604 consecutive patients who underwent isolated coronary artery bypass grafting (CABG) using cardiopulmonary bypass (CPS) were reviewed retrospectively. A rapid recovery protocol emphasizing reduced CPB time, preoperative intra‐aortic balloon pump (IABP) criteria, and atrial fibrillation prophylaxis was applied to all patients. The five original divisions of the Parsonnet risk scale were reduced to three risk categories: Low (0–10; Group A), Intermediate (11–20; Group B), High (> 20; Group C). Comparisons of progressive risk categories were analyzed to identify predictive factors associated with fast‐track outcomes. Results: The thirty‐day operative mortality for the entire group was 3.6%. Three clinical features were identified that distinguished risk progression—female gender, reoperative CABG, and increased age. Additionally, the presence of diabetes (p < 0.05), congestive heart failure (p < 0.01), and peripheral vascular disease (p < 0.001) distinguished Groups A and B, while acute myocardial infarction (p < 0.05) influenced outcomes in Group C. Group A (48%) mean risk score 5.9 ± 3.2 was compared to Group B (34%) 14.8 ± 2.6, which was further compared to Group C (18%) 26.4 ± 2.8. The mean length of stay for Group A (5.3 ± 4.1 days) was notably less than Group B (6.1 ± 4.7 days; p < 0.05); however, both groups responded favorably to fast‐track techniques. Group C did not respond comparably (9.2 ± 9.2 vs 6.1 ± 4.7 days; p < 0.001) and experienced prolonged recovery. The simplified Parsonnet risk scale did not identify differences in operative mortality and revealed only pneumonia (p < 0.05) and atrial fibrillation (p < 0.01) to be greater in Group C. As risk increased, significantly less revascularization was performed (Group A 3.6 ± 1.2 grafts/patient vs Group B 3.3 ± 1.2 [p < 0.01]; Group B 3.3 ± 1.2 vs Group C 2.5 ± 1.0 [p < 0.001]). Conclusion: A simplified Parsonnet risk scale (three categories) is an effective tool in identifying factors limiting fast‐track recovery. Low‐ and intermediate‐risk patients represent the majority (82%) and respond well to fast‐track methods. High‐risk patients (18%) are limited by a greater percentage of female patients, reoperative CABG, and the very elderly, resulting in fast‐track failure. Strategies to improve recovery in high‐risk patients may include evolving off‐pump techniques.