Inappropriate Implantable Cardioverter-Defibrillator Therapy Due to the Detection of Premature Ventricular Complexes

Inappropriate shocks delivered by implantable cardioverter-defibrillators may occur in 15%-40% of patients treated with these devices. This article describes a rare cause for inappropriate shocks. Two patients received inappropriate shocks due to the presence of premature ventricular complexes during attempted reconfirmation of ventricular fibrillation. Knowledge of device algorithms for detection and reconfirmation of ventricular fibrillation, designed to have a high degree of sensitivity and therefore lower specificity, allows for reprogramming to avoid further inappropriate shocks.     

Electrophysiological Studies on Cardiac Catheter Ablation

Clinical and animal investigations have pointed out that high energy electrical shocks are associated with the development of cardiac arrhythmias and with variable success in permanent ablation. The effects of electrode configuration and location on the size of the recorded electrogram was investigated to help explain variable catheter ablation results. We analyzed the cellular effects of catheter ablation shocks and found depression of resting potential, action potential amplitude, dV/dt and action potential duration. The most severe effects were noted with high current densities in tissues located between the cathode and anode. Damage was worse nearest the cathode. Similar cellular studies were completed using argon laser photoablation. Again, there was a decrease in resting potential, action potential amplitude and dV/dt. Laser energy led to a more focal region of myocardium void of action potentials and the border zone of injury was smaller. We also investigated the effects of lower energy shocks)1 to 10 joule) on cardiac tissues. Using microelectrodes, we observed that the membrane potential can "hang up" at the depolarized levels for varying periods of time and that conduction is altered during this membrane "hang-up" period. The duration and membrane hang-up level correlated with shock intensity and shock duration. Sequential shocks resulted in additive membrane "hang-up". We believe that membrane hang-up may be associated with brief arrhythmias observed following catheter ablation since conduction, refractoriness and excitability are all altered.     

Inappropriate Discharges by the Implantable Cardioverter Defibrillator During Postoperative Testing: Implications for Intraoperative Assessment

Inappropriate shocks were delivered to a patient while in sinus rhythm by an implantable cardioverter defibrillator (ICD) during routine prehospital discharge testing. This was induced by the standard programmer when the "read" telemetry sequence was initiated. The ICD was removed and found to suffer from electrical artifact that was sensed as ventricular tachycardia during telemetry. To avoid inadvertent telemetry-induced shocks during routine testing, all ICDs should be interrogated, using a standard programmer, intraoperatively, with the unit in "defibrillation on" mode.     

Observations From Intraatrial Recordings on the Termination of Electrically Induced Atrial Fibrillation in Humans

Background: The circulating wavelet hypothesis suggests that atrial fibrillation could terminate by either progressive fusion or simultaneous block of all wavelets. Methods: Intraatrial recordings from the right atrial free wall were made during procainamide induced (n = 8) or spontaneous (n = 7) termination of electrically induced atrial fibrillation in 14 patients. Atrial rate, mean magnitude squared coherence, and direction of activation during sequential electrograms were measured. Rate and coherence were calculated from the earliest point within 5 minutes prior to termination as well as from the 4-second interval just prior to termination. Results: Termination was directly to sinus rhythm (13 episodes) or to atrial flutter (2 episodes). For the eight procainamide induced terminations, rate decreased between the first measurement and the measurement just prior to termination, from 443 ±127 beats/ min to 322 ± 119 beats/min. For the seven spontaneous terminations, rate also decreased from 373 ± 119 beats/min to 323 ± 88 beats/min; however, a slight increase in atrial rate prior to termination was observed in three episodes. No specific patterns of atrial cycle lengths were seen during the final few seconds of fibrillation. No increase in coherence was observed. In seven episodes, recordings were made using orthogonal bipoles in the x, y, and z directions, allowing direction of activation of wavefronts to be measured. Three episodes showed multiple instances where direction of activation remained similar over several electrograms as we have previously reported for chronic fibrillation. However, no such instances precipitated termination in any of the seven episodes. Conclusions: Atrial fibrillation usually terminates directly to sinus rhythm and does so abruptly and without forewarning. While we and others have previously reported that the rate of atrial fibrillation decreases with procainamide infusion, a decrease in the rate of atrial fibrillation is not required for the rhythm to terminate and consequently may not be a part of the termination process at all. Coherence does not demonstrate a progressive increase in the organization of atrial fibrillation prior to termination. Lack of stabilization in the direction of activation of wavefronts in the final few seconds also fails to support fusion of wavefronts as the mechanism of termination of atrial fibrillation. Simultaneous block of all wavelets is consistent with, but not proven by our observations.     

Verapamil or Adenosine for the Immediate Treatment of Supraventricular Tachycardia

A comparison of verapamil with adenosine for the immediate treatmentof supraventricular tachycardia was made from a retrospectivereview of 164 spontaneous episodes of paroxysmal tachycardiain 43 patients. Verapamil administered to 33 patients restoredsinus rhythm in 91 of 112 episodes (81 per cent). Hypotensionoccurred in 9 per cent of episodes. Adenosine terminated 94per cent of episodes of supraventricular tachycardia in 25 patients.The arrhythmia recurred shortly after adenosine restored sinusrhythm in 20 episodes. Transient side effects were common. Fifteenpatients were treated with both agents. Adenosine was successfulin all, but verapamil failed to restore sinus rhythm at leastonce in seven of the 15 patients. Early recurrence of tachycardiaoccurred in five of these after adenosine, but in only one afterverapamil. Verapamil and adenosine are both effective in the treatmentof supraventricular tachycardia; adenosine has the higher successrate and is safer, but transient symptoms are common and arrhythmiasmay recur.     

An Automatic Microcomputer System for Analysis of Monophasic Action Potentials

A computer system for rapid measurement and analysis of monophasic action potentials (MAPs) recorded in vivo was developed. MAPs recorded from the epicardium of mongrel dogs using a contact electrode were digitized by analog-to-digital conversion at a sampling rate of 1 kHz per channel for computer data acquisition. Activation time was detected using a sliding 10-point window at the location where the average positive dV/dT exceeds an adjustable threshold value in order to eliminate spurious detection due to baseline variability or motion artifact. Action potential duration (APD) was determined at 50% and 90% (APD50, APD90) repolarization levels at the first sample point below these detection levels. In addition, a tangent algorithm (APDtan) that detects peak negative dV/dT during repolarization was developed. APDtan was determined from the location of onset of activation to the intersection of tangent and baseline. APDtan allowed estimation of APD in the presence of subsequent premature beats when APD90 was not measurable. To validate activation time measurements, 4,600 action potentials were analyzed during fixed rate pacing. Over a range of paced coupling intervals from 200 to 1000 msec, an R2 value of 0.99968 and a slope of 0.9959 were obtained by linear regression between paced and calculated intervals. To validate APD measurements, 5035 action potentials were analyzed in five animals during fixed rate pacing (longer than 3 minutes) when action potential duration should be constant. Average coefficient of variation of 1.25%, 1.65%, and 1.14% were obtained for APD50, APD90, and APDtan, respectively. This algorithm provides a rapid and accurate method to analyze MAP activation and duration for basic physiological studies such as the determination of initiation of arrhythmias.     

Hydroxyurea therapy in β-thalassaemia intermedia: improvement in haematological parameters due to enhanced β-globin synthesis

Summary. The β-thalassaemias represent a heterogenous group of diseases resulting from decreased erythroid β-globin mRNA expression and imbalanced a/β-globin chain synthesis which are manifest clinically by ineffective erythropoiesis and excessive haemolysis. Increasing levels of haemoglobin F (HbF) by pharmacological agents has been proposed to ameliorate the severity of the disease by improving the balance in globin chain synthesis. Hydroxyurea (HU), as an effective agent with low toxicity for activating 7-globin gene, has been shown to enhance HbF synthesis in experimental animals and in patients with sickle cell anaemia. However, previous trials of HU in β-thalassaemia patients are ambiguous, with a small number having increased HbF synthesis. In a recent study of HU effects in Chinese j3 -thalassaemia patients we unexpectedly found that two unrelated patients with β-thalassaemia intermedia demonstrated an improvement in the effectiveness of erythropoiesis reflected by an increase in haemoglobin concentration (from 4-1 to 6-3 g/dl, patient 1; from 6-5 to 97 g/dl, patient 2) and in red cell volume (from 68 to 104 fl, patient 1; from 68 to 85 fl, patient 2) after a period of excess of 300 d of low-dosage HU treatment. These effects, however, appear to be due to increased,3-globin biosynthesis, because the percentage of HbF decreased in each patient as total Hb increased. This was reflected by changes in the β/a ratio (from 0'301 to 0-581, patient 1; from 0'348 to 0-487, patient 2) with minimal changes in 7-globin biosynthesis. We conclude that in addition to its known effects in stimulating 7-globin production, hydroxyurea may have a more general role in augmenting globin synthesis, including β-globin in some thalassaemia intermedia patients who maintain the capacity to express normal β-globin chains.