Sensing Properties of Pacemaker Leads

It is already general practice to attribute sensing properties to geometry and surface structure of pacemaker leads. We have to analyze critically whether claims of having found leads with high sensitivity are in accordance with experimental and theoretical findings. From a model can be derived what kind of typical signal structure will originate from an electrode when an excitation wave crosses it, and what of this signal is influenced by electrode parameters. With decreasing surface area, the frequency content of the signal, the impedance, and, theoretically, the amplitude, increases. If the pacemaker characteristics are not matched to the lead properties, this inverse relationship becomes a direct one: If the input impedance is too low or the upper cut-off frequency of the bandpass is not high enough, the effective heart signal seems to be diminished with decreasing size. This, however, is more a pulse generator than a lead problem. If all pacers would possess an input impedance of ≥100 Kω and an upper cut-off frequency of ≥350 Hz, an attenuation of the heart signal would be ≤10% and thus, the results with different leads would be very similar and of equally high sensitivity.     

Hyperlactataemia, hyperkalaemia and heart block in acute iron overload: the fatal role of the hepatic iron-incorporation rate in rats on ferric citrate infusions

An animal model is presented that provides constant and controllable conditions for approaching gradually, and within reasonable time, different stages of iron overload and, probably, an iron-induced mitochondrial disorder. Thirty-five rats were infused with ferric citrate, sodium citrate and saline at constant rates for 6-24 h. In the 200-3200 micrograms Fe h-1 loading range, the iron-incorporation capacity of the liver was not saturable and the fractional iron uptake by the liver remained at approximately 30% even at a loading rate of 3200 micrograms Fe h-1. Up to a loading rate of 200 micrograms Fe h-1, iron storage was not associated with toxic effects. Beyond this loading rate, however, the liver was no longer able to prevent a massive plasma iron increase on one side and hyperlactataemia on the other. These signs most probably represent hepatocellular decompensation with respect to a critical iron-storage rate. The product of plasma iron x exposition time was significantly correlated with increased plasma lactate levels (r = 0.89, P less than 0.005), whereas increased plasma iron levels per se were not. Hyperlactataemia was associated with hyperkalaemia and progressive cardiac conduction defects leading to cardiac arrest at lactate concentration of 9.1 +/- 4.3 mmol l-1. The hypothesis is discussed that toxicity in acute iron overload may entirely be due to hepatocellular (mitochondrial) damage, and not to multiple organ iron overload.     

Influence of Different Conditioning Methods on Force and Fatigue Resistance in Chronically Stimulated Skeletal Muscles

A study was undertaken to compare different conditioning methods for the transformation of latissimus dorsi muscle into a fatigue resistant one for application in circulatory assist. In ten sheep four electrodes were sutured to the epineurium of the left thoracodorsal nerve for indirect electrical stimulation of the latissimus dorsi muscle. In six sheep a "carousel stimulation, " a special multichannel stimulation, in combination with a recently developed conditioning protocol was used for muscle conditioning ( multichannel method ). In four sheep, a conventional stimulation protocol using single channel stimulation was applied for transformation of the muscle (single channel method). The final experiments were carried out when fatigue resistance was obtained. The maximum tetanic forces at different preloads were determined and fatigue resistance was tested during 20 minutes of continuous stimulation. Both conditioning patterns led to fatigue-free chronic stimulation. Muscles conditioned by multichannel stimulation exhibited between 20% and 33% less force than the contralateral unconditioned muscles, whereas in the single channel group this loss was between 32% and 43%. Thus, the multichannel method revealed relatively superior in preserving muscle force for chronic stimulation.     

Exercise Induced Sympathetic Influences Do Not Change Interatrial Conduction Times in VDD and DDD Pacing

Using telemetry, right atrial electrogram (RA), and marker channel of atrial sense events (M_A) in combination with the left atrial electrogram (LA), recorded by a filtered bipolar esophageal lead, interatrial conduction during submaximal exercise and at rest was examined in 46 DDD pacemaker patients. The RA-LA and M_A-LA conduction times measured in the presence of atrial sensing (VDD) as well as the conduction time S_A-LA from atrial stimulus (S_A) to LA, determined during atrial pacing (DDD) were found to be individual constants independent of exercise induced sympathetic influences. Thus, having determined an optima! mechanical interval (LA-LV)_mech/opt from left atrium to ventricle by other methods, the optimal AV delay for DDD as well as for VDD operation can be calculated by the sum of the appropriate interatrial conduction time (S_A-LA, respectively M_A-LA) and the (LA-LV)_mech/opt interval. Due to the constant S_A-LA and M_A-LA, the difference between these two values (AV delay correction interval) is a constant as well, which remains unchanged during exercise. Therefore, in selecting the rate responsive AV delay, only hemodynamic and not electrophysiologica] measurements need to be considered.     

Evolution of Late Potentials During the First 8 Hours of Myocardial Infarction Treated with Thrombolysis

It has been demonstrated that successful thrombolytic therapy is associated with a reduction of late potentials in the signal-averaged electrocardiogram (SAECG) recorded within 48 hours after hospital admission. This study extends these observations, using for the first time a longitudinal design investigating whether ischemia and its potential reversal by thrombolytic therapy are associated with dynamic changes in SAECG recordings obtained continuously for 8 hours after the start of therapy in patients with acute myocardial infarction (MI). SAECGs were obtained from 12 patients (2 women and 10 men; ages 63 ± 13 years) with acute MI. The SAECG (X²+ Y²+ Z²)^1/2 was generated with a high pass filter of 40 Hz, noise ≤ 0.3 μV. Comparing the SAECG recordings during the first and eighth hours, there was a significant decrease in filtered QRS duration (fQRS; 119.5 ± 17.1 vs 106.3 ± 15.3 ms) and duration of the low amplitude signals ≤ 40 μV of the terminal QRS (LAS40; 48.8 ± 18 vs 34.2 ± 14.2 ms), and increase of root mean square voltage of the last 40 ms of the QRS (t-RMS; 14.8 ± 9.3 vs 37.8 ± 34.4 μV) (rank test, P ≤ 0.05). In this patient series, there was a significant improvement of fQRS, t-RMS, and LAS40 during the first 8 hours of acute MI, perhaps indicating reversal of ischemia with thrombolysis. Even during acute MI, these markers of delayed conduction allow investigation of intervention induced changes in myocardial conduction and possibly prediction of the patency of the infarct related artery using signal-averaging techniques.     

Doppler echocardiographic analysis of cardiac flow during the Mueller manoeuver

The inspiration against a closed airway, the Mueller manoeuver, leads to a negative intrathoracic pressure. It is controversially discussed whether this is causing an augmentation of right heart murmurs. There is only limited knowledge on the temporal relationship of the negative intrathoracic pressure with right and left ventricular filling and stroke volume. To investigate this relationship, the flow through the mitral, aortic, tricuspid and pulmonary valves was studied continuously by Doppler echocardiography during a standardized Mueller manoeuver in 15 healthy subjects (age 45 +/- 10 years). Five heart beats after the initiation of the manoeuver, flow through the mitral and aortic valve decreased 12.2 +/- 7.2% (P less than 0.001) and 10.1 +/- 6.6% (P less than 0.001), respectively. A transient increase of 15.1 +/- 9.2% (P less than 0.001) in tricuspid flow was followed by a 14.3 +/- 9.8% (P less than 0.005) increase of flow through the pulmonary artery. Ten heart beats after the initiation of the Mueller manoeuver, flow through the pulmonary artery again reached baseline, while tricuspid flow remained below baseline values. In contrast to previous studies, our results indicate that the Mueller manoeuver causes a small and transient increase in right ventricular stroke volume which is unlikely to cause a marked augmentation in right heart murmurs.     

Computer-aided visual analysis of long-term ECG recordings

In Sweden, long-term ECG (LECG) recordings are predominantlyused to evaluate whether symptoms such as palpitations, dizzinessand syncope are caused by cardiac arrhythmias. The analysisof LECG for this purpose differs considerably from the analysisof LECG recorded to demonstrate ventricular arrhythmias in post-infarctionpatients. A computer method was developed with the primary goalof assisting in the analysis of the former type of LECG. The computer searches the recording for episodes of arrhythmiaand presents comprehensive graphical information about, forexample, heart rate and significant arrhythmias. During analysis,the operator selects any computer-indicated episode and/or periodin which the patient experienced symptoms. The selected ECGsegment is presented instantaneously on a screen and/or a recorder.Thus the main emphasis is on the facilitation, rather than thereplacement, of visual analysis. The ECG is displayed in various modes. Continuous segments canbe inspected at 20 or 60 times real speed. Computer-indicatedepisodes of particular types of arrhythmia can be displayedepisode by episode. This allows extremely fast inspection of,for example, all episodes with abnormally long R-R intervals.A cheap, permanent record of any episode, marked with patientidentity and time, is easily produced on an ECG recorder     

Neurophysiological correlates of mental arithmetic

Thirteen subjects were extensively trained on nine multiplication problems varying in difficulty. Practice was associated with a reaction time speed up and an attenuation of the problem size effect. The introduction of previously unpracticed problems led to a performance rebound to pretraining levels, indicating practice specificity. The event-related potentials were characterized by a late positive complex, followed by a positive slow wave. Offset latency of positive slow wave and preresponse amplitude at parietal electrodes showed practice specificity effects that systematically changed with practice and problem size, indicating an association with the load imposed on working memory. The peak of the late positive complex probably reflects task learning or adaptation effects because it was attenuated by practice predominantly at frontal electrodes, showed no practice specificity, and was not affected by problem size.     

Echocardiographic Diagnosis of a Partially Thrombosed Right Coronary Artery Aneurysm in a Patient with Polycystic Disease

This report describes a patient with polycystic disease of the kidneys and the liver and an aneurysm of the proximal right coronary artery. Transthoracic and transesophageal echocardiographic imaging showed a 5 times 5 cm oval-shaped, spherical tumor in the atrioventricular corner. The tumor was partially filled with homogenous echostructures. Coronary angiography showed aneurysmatic configuration of the right coronary artery, but no region demonstrating a mass of similar size and shape as seen by echocardiography. The finding of a partially thrombosed coronary artery aneurysm was subsequently confirmed by surgery.     

Verification of Linear Lesions Using a Noncontact Multielectrode Array Catheter versus Conventional Contact Mapping Techniques

INTRODUCTION: Creation of linear lesions is an established ablation goal. Verification of complete conduction block at the ablation line is required to determine ablation success. Conventional mapping techniques are sequential endocardial activation mapping and documentation of double potentials. Recently, a noncontact multielectrode array catheter was developed that allows instantaneous three-dimensional mapping by simultaneous reconstruction of > 3,000 electrograms. In this study, we prospectively compared the accuracy of noncontact mapping to identify discontinuities in linear lesions and to verify a conduction block with that of conventional mapping techniques. METHODS AND RESULTS: In 12 patients with atrial flutter, radiofrequency pulses were applied between the tricuspid annulus and either the inferior vena cava or the eustachian ridge. Following each application, pulse propagation at the ablation line was determined during pacing by conventional mapping techniques. The findings were compared to high-density isopotential mapping using the noncontact multielectrode array catheter. It was found that noncontact mapping reliably distinguished conduction delays from a conduction block as defined by contact mapping. In addition, noncontact mapping instantaneously identified the area where a discontinuity in the line of block was present. In these patients, complete conduction block was achieved by radiofrequency pulses guided by the noncontact mapping system. CONCLUSION: Noncontact mapping is highly accurate in distinguishing conduction delays from a complete conduction block. By providing an instantaneous high-density propagation vector at all sites along the ablation line, three-dimensional isopotential mapping is helpful in localizing discontinuities of linear lesions and, thus, may facilitate the creation of a complete conduction block.