Defibrillation Efficacy Using High-Frequency Switching to Proportion Current Among Simultaneous Shock Pathways

High-Frequency Current-Proportioned Defibrillation. Introduction: Multiple-pathway electrode configurations generally allow improved current distribution over the heart and lower defibrillation thresholds than single-pathway systems. However, current distributions using multiple pathways are largely determined by electrode type and location. We hypothesized that switching the current among multiple pathways at high frequency (HF) could allow the switching duty cycle to control the proportion of time-averaged current flowing in each pathway, thus permitting altered (possibly improved) defibrillation efficacy using the same electrodes and shock waveform. Methods and Results: In dogs, we measured the current (I₅₀) for 50% defibrillation success using catheter electrodes in the right ventricular apex (cathode) and superior vena cava (A-pathway anode) and a subcutaneous patch on the left chest wall (B-pathway anode). In group 1 (N = 7), we measured I₅₀s for shocks that used HF to proportion 10% to 90% of the current to the A-pathway. Shocks with 10% to 30% of the current in the A-pathway had significantly lower I₅₀s than nonproportioned shocks using all three electrodes. However, the resistance differed among single and simultaneous pathways so energy did not necessarily parallel these changes. In group 2 (N = 6), we measured I₅₀s for shocks to the B-pathway alone, for nonproportioned shocks to A and B, and for shocks that proportioned 80% of the current to the B-pathway using either HF, sequential, or amplitude proportioning methods. All proportioning methods had similar ISUs that were significantly lower than the I₅₀ for nonproportioned shocks to A and B and that were comparable to shocks to the B-pathway alone. Conclusions: Shocks with most current proportioned to the B-pathway had lower defibrillation currents than nonproportioned shocks using both pathways. Thus, defibrillation efficacy was changed by HF proportioning without changing the electrodes or shock waveform. These findings suggest that HF proportioning may be a method to improve defibrillation.     

Residents' perceptions of an environmental enhancement project in Australia

This paper describes a study which aimed to explore the impactof an environmental enhancement project comprising the creationof a Community Park in Cringila, NSW. The study, conducted overa number of weeks, used a focus group technique involving anumber of local men, women, community leaders and school children.Qualitative data analysis revealed a number of recurrent themesemerging from the people's responses. The themes suggested thatthe creation of a community park had provided a venue for physicalactivity, been a source of aesthetic and environmental benefit,enhanced communal congeniality and provided a sense of civicpride for local residents. Such findings are interpreted tobe meaningful experiences for the local residents. The paperdiscusses these results and the implications of such findingsfor the development of policies to improve the environment ofour cities and to enhance the health and wellbeing of communities.     

RR Variability and Baroreflex Sensitivity in Patients with Ventricular Tachycardia Associated with Normal Heart and Patients with Ischemic Heart Disease

Recent studies have suggested that disordered autonomic function, particularly the loss of protective vagal reflexes are associated with an increased incidence of arrhythmic deaths following myocardial infarction (MI). Heart rate variability (HRV) and baroreflex sensitivity (BRS) are measures of myocardial autonomic function and predict arrhythmic deaths post-Mi. Patients with ventricular tachycardia associated with a “normal heart” frequently have exercise-induced arrhythmia suggesting that the autonomic nervous system is important in the genesis of this form of ventricular tachycardia (VT). This study examines HRV and BRS in patients with VT associated with a “normal heart” and compares these values to patients post-Mi with and without evidence of arrhythmia. Twenty patients with VT associated with a “normal heart,” 16 patients with MI but without arrhythmia on follow-up, and 11 patients with MI and VT on follow-up were studied. HRV was measured from 24-hour Holter recordings and BRS was measured from plots of change in systolic blood pressure versus change in heart rate following an intravenous injection of 0.4–0.6 mg phenylephrine. HRV was significantly higher in the patients with VT associated with a normal heart (34.2 ± 10.8 msec) compared to the patients post-Mi, without (23.7 ± 6.7 msec) and with (14.8 ± 3.8 msec) arrhythmia (F = 9.2, P < 0.001) and these differences were unaffected by adjustment for age. Baroreflex sensitivity was also higher in patients with VT associated with a “normal heart” (10.1 ± 6.8 msec/mmHg) compared to patients post-Mi, without (6.1 ± 3.2 msec/mmHg) and with 3.2 ± 3.1 msec/mmHg) arrhythmia, (F = 7.2, P < 0.02), though statistical significance was lost after adjustment for age (F = 1.2, P = 0.3). We conclude that patients with VT associated with “normal hearts” have HRV and BRS that is higher than in patients post-MI. Alterations of autonomic tone are, therefore, unlikely to be important in VT associated with a “normal heart,” whereas these appear to be important in patients with arrhythmic events post-MI.     

EDROPHONIUM ANTAGONISM OF ATRACURIUM DURING ENFLURANE ANAESTHESIA

To determine the influence of enflurane on the ability of edrophoniumto antagonize atracurium block, dose—response curves wereconstructed for edrophonium in the presence of 0%, 1% and 2%enflurane, and for 2% enflurane discontinued at the time ofadministration of edrophonium. One hundred ASA Physical StatusI or II patients (four groups of 25), selected randomly andundergoing elective surgery, received atracurium 0.5 mg kg^–1,with thio-pentone, nitrous oxide and enflurane. Supplementarydoses of fentanyl were given if needed. Train-of-four (TOF)stimulation was applied every 12 s, and the force of contractionof the adductor pollicis muscle was recorded. When first twitchheight (T1) had recovered spontaneously to 10 % of initial value,edrophonium 0.1, 0.2, 0.4 or 1 mg kg^–1 was administeredby random allocation. Enflurane concentrations remained constant,except that enflurane was discontinued in 50% of the patientswho had received 2% enflurane. Monitoring was continued forat least 10 min, at which time T1 and TOF ratio (T4/T1) weremeasured. The ED₈₀ for T1 recovery depended on the dose of enflurane:0.08 (SEM 0.03), 0.21 (0.06) and 0.42 (0.18) mg kg^–1 for0%, 1% and 2% enflurane, respectively (P < 0.005). With enflurane2% discontinued, the ED₈₀ was 0.095 (0.050) mg kg^–1 (P< 0.02 compared with 2% enflurane). The ED₅₀ for TOF responseswere 0.13 (0.05), 0.46 (0.10) and 1.04 (0.38) mg kg^–1for 0%, 1% and 2% enflurane, respectively (P < 0.001). With2% enflurane discontinued, the ED₅₀ for TOF was 0.17 (0.12)mg kg^–1 (P < 0.05 compared with 2% enflurane). It isconcluded that, even when given at the same degree of spontaneousrecovery, the effect of edrophonium is markedly attenuated byenflurane.     

Predicting post-operative delirium in elderly patients undergoing surgery for hip fracture

Background: Delirium in elderly patients with hip fracture has a significant negative influence on the disease course. Awareness of risk factors for postoperative delirium (POD) may lead to the development of effective preventive strategies. The aims of this study were: to find patients’ features that are predictors of POD, and; to develop a model predicting the risk for POD. Patients and methods: Seventy‐seven elderly patients (81.9 years of age, SD 7.5 years) were non‐delirious prior to surgery and enrolled in the study. Delirium was diagnosed by Confusion Assessment Method and Algorrhithm. Patients’ characteristics as potential predictors of POD were analyzed by logistic regression analysis on SAS software. Results: Postoperative delirium was diagnosed in 37 patients. Use of multiple (>3) medications, lower scores on cognitive tests (<20 on Set Test and <24 on Mini‐mental Status Exam), albumin level less than 3.5 g/dL, hematocrit level less than 33% and age over 81 years were predictors of POD. A logistic regression formula including these predictors weighed by their parameter estimates can be used to calculate the probability of POD. The model had a good fit and a good predictive power. A Delirium Predicting Scale was derived based on parameter estimates of these predictors. Patients can be classified as low‐, intermediate‐ or high‐risk for POD. Conclusions: A logistic regression model, which includes patients’ age, medication history, cognitive performance measured by Set Test and Mini‐Mental Status Exam, albumin and hematocrit levels, can be used to predict risk for POD after surgical repair of fractured hip in elderly patients.     

RETROPERITONEOSCOPIC SURGERY IS NOT ASSOCIATED WITH INCREASED CARBON DIOXIDE ABSORPTION

PURPOSE: Previous studies have suggested that retroperitoneal laparoscopy is associated with greater carbon dioxide absorption and related postoperative morbidity, such as subcutaneous emphysema and pneumothorax. We prospectively compared the effects of carbon dioxide absorption during transperitoneal and retroperitoneal laparoscopic renal and adrenal surgery at our institution. MATERIALS AND METHODS: Data were collected prospectively on 51 patients who underwent laparoscopic renal (26) or adrenal (25) surgery via the transperitoneal (18) or retroperitoneal (33) approach from September 1997 to February 1998. RESULTS: There was no significant difference in carbon dioxide elimination in patients who underwent transperitoneal laparoscopy compared to retroperitoneoscopy at any interval. Subcutaneous emphysema occurred in 12.5% of the transperitoneal and 45% of the retroperitoneal group (p = 0.09). Patients with subcutaneous emphysema had greater carbon dioxide elimination during the first 2.5 hours of insufflation compared to those without subcutaneous emphysema and, thereafter, carbon dioxide elimination decreased to baseline. CONCLUSIONS: In contrast to previous reports our prospective nonrandomized study suggests that retroperitoneoscopy is not associated with greater carbon dioxide absorption compared to transperitoneal laparoscopy. Patients with subcutaneous emphysema exhibited only transient increases in carbon dioxide absorption above control levels.     

The Defibrillation Threshold: A Comparison of Anesthetics and Measurement Methods

In 18 open-chested mongrel dogs (18.0 ± 1.7 kg) we compared three anesthetics and three methods for measuring the defibrillation threshold. Six animals were anesthetized with pentobarbital (30 mg/kg) and maintained with a pentobarbital infusion (4 mg/kg per hour). All other animals were anesthetized with sodium brevital (10 mg/kg) and maintained with either halothane gas (1.5%, N = 6) or isoflurane gas (1.8%, N = 6). In each dog. we measured the energy required for 50% successful defibrillation (E50) with: (A) a 3 reversal—up/down method; (B) a 15 shock—up/down method; and (C) a percent success method. Anesthetics and methods were selected in a balanced random order. Ventricular fibrillation was induced with 50 Hz electrical pacing. After 15 seconds, monophasic truncated exponential shocks were delivered by way of a spring-patch electrode configuration. The animal was rescued (if needed) and fibrillation/defibrillation episodes were repeated at 3 minute intervals. After each determination of the E50, the E50 was delivered in ten successive defibrillation trials to determine its actual success rate. We found no significant difference in E50 among anesthetics; a significant difference (P < 0.05) in E50 between method C (9.7 ± 2.6 joules) and method B (8.2 ± 1.6 joules); no significant difference among anesthetics or methods for the actual success rate of the E50 (45 ± 21% successful); and method A required significantly fewer fibrillation episodes and number of shocks and less cumulative energy than the other methods. We concluded that the anesthetics tested had little effect on E50 but that the method used to determine E50 could have an effect. Also, the E50 estimated by all methods consistently produced an actual success rate lower than 50%.