Why Do Some Patients Have High Defibrillation Thresholds at Defibrillator Implantation?

Implantable cardioverter defibrillators reduce the risk of sudden cardiac death in patients with ventricular tachyarrhythmias. However, for the few patients with unacceptably high defibrillation thresholds at implantation the risk of sudden death may remain high. If a small number of defibrillation attempts are used to determine a defibrillation threshold, then a high defibrillation threshold may occur in some patients due to the probabilistic nature of defibrillation: a small percentage of shocks will fail even at optimal shock strengths. Basic investigations have suggested mechanisms for high defibrillation thresholds in other patients. The extracellular potential gradients produced by a shock correlate with ability to defibrillate and may be used to classify mechanisms for high defibrillation thresholds. Computerized mapping studies have demonstrated that extracellular potential gradient fields produced by defibrillation shocks are uneven with high gradient areas close to the electrodes and low gradient areas distant from the electrodes. A high defibrillation threshold may occur because: (1) a shock creates a subthreshold potential gradient in the low gradient areas; (2) a patient has a higher minimum potential gradient threshold than other patients; or (3) a shock leads to refibrillation in the high gradient areas. This article reviews experimental evidence to support each of these three possibilities then suggests experimental and clinical investigations that may clarify the causes of high defibrillation thresholds in patients.     

Late Manifestation of Coronary Sinus and Left Atrial Perforation of a Left Ventricular Pacemaker Lead at Extraction

A 56‐year‐old man presented for lead extraction of a left ventricular (LV) lead that had been deactivated due to hiccups and of a right ventricular (RV) lead with a high threshold. Pus was noted upon entering the pocket. The right atrial and RV leads were extracted, but traction on the LV lead caused ischemia and was not performed. An echocardiogram demonstrated the lead in the left atrium and a robotic‐assisted thoracotomy was used to remove the lead that had unroofed the coronary sinus, gone into the left atrium, and perforated through the free wall into the pericardium.     

Magnesium Adjunctive Therapy in Atrial Arrhythmias

Magnesium (Mg) is an important intracellular ion with cardiac metabolism and electrophysiologic properties. A large percentage of patients with arrhythmias have an intracellular Mg deficiency, which is out of line with serum Mg concentrations, and this may explain the rationale for Mg's benefits as an atrial antiarrhythmic agent. A current limitation of antiarrhythmic therapy is that the potential for cardiac risk offsets some of the benefits of therapy. Mg enhances the balance of benefits to harms by enhancing atrial antiarrhythmic efficacy and reducing antiarrhythmic proarrhythmia potential as well as providing direct antiarrhythmic efficacy when used as monotherapy in patients undergoing cardiothoracic surgery.     

Ventricular Arrhythmia Inducibility Predicts Subsequent ICD Activation in Nonischemic Cardiomyopathy Patients: A DEFINITE Substudy

Objectives: We evaluated whether electrophysiologic (EP) inducibility predicts the subsequent occurrence of spontaneous ventricular tachycardia (VT) or ventricular fibrillation (VF) in the Defibrillators in Nonischemic Cardiomyopathy Treatment Evaluation (DEFINITE) trial.
Background: Inducibility of ventricular arrhythmias has been widely used as a risk marker to select implantable cardioverter defibrillator (ICD) candidates, but is believed not to be predictive in nonischemic cardiomyopathy patients.
Methods: In DEFINITE, patients randomized to the ICD arm, but not the conventional arm, underwent noninvasive EP testing via the ICD shortly after ICD implantation using up to three extrastimuli at three cycle lengths plus burst pacing. Inducibility was defined as monomorphic or polymorphic VT or VF lasting 15 seconds. Patients were followed for a median of 29 ± 14 months (interquartile range = 2–41). An independent committee, blinded to inducibility status, characterized the rhythm triggering ICD shocks.
Results: Inducibility, found in 29 of 204 patients (VT in 13, VF in 16), was associated with diabetes (41.4% vs 20.6%, P = 0.014) and a slightly higher ejection fraction (23.2 ± 5.9 vs 20.5 ± 5.7, P = 0.021). In follow-up, 34.5% of the inducible group (10 of 29) experienced ICD therapy for VT or VF or arrhythmic death versus 12.0% (21 of 175) noninducible patients (hazard ratio = 2.60, P = 0.014).
Conclusions: In DEFINITE patients, inducibility of either VT or VF was associated with an increased likelihood of subsequent ICD therapy for VT or VF, and should be one factor considered in risk stratifying nonischemic cardiomyopathy patients.     

Soundness of Mind and ‘Mental Setting’

Book reviewed in this article: Mental Disabilities and Criminal Responsibility . By Herbert Fingarette and Ann Fingarette Hasse.     

Measuring capillary refill time is useless

The objectives of this study were to define the range of normal capillary refill times for an adult population and to assess the contribution of age, gender and environmental temperature to any variability shown. Data from 331 healthy adult volunteers were collected including age, gender, environmental temperature and capillary refill time by a standard method. Multiple regression analyses were used to assess the contribution of temperature, age and gender to the variation in capillary refill times. A wide range of capillary refill times were found. Taking the 95th centile as the upper limit of normal, the upper limit of normal for males aged 14–65 years was 2.2 seconds, for females in the same age group 3.0 seconds; for males over 65 years 3.6 seconds and for females over 65,3.8 seconds. Capillary refill times varied significantly with age, gender and environmental temperature (p<0.01), however, taken together, these factors accounted for only 20% of the observed variance. From this study and a review of the available literature it is concluded that measurement of capillary refill time does not perform satisfactorily as a test nor does it give useful information about the patient's circulatory status. Its use as an assessment tool in trauma should be discontinued.     

BONE MASS IN HIRSUTE WOMEN WITH ANDROGEN EXCESS

The spinal and femoral bone mass of 32 hirsute women with oligomenorrhoea and androgen excess was measured using dual photon absorptiometry and compared with the bone mass of 32 control women with regular menstrual cycles. Despite significantly lower oestradiol levels in the hirsute population there was no significant difference in the bone mass. Furthermore there was no significant difference in bone mass in five hirsute women with undetectable levels of oestradiol. It is concluded that androgen excess can maintain normal bone mass in the face of low or undetectable oestradiol levels.     

Changes in Cardiac Repolarization Following Short Periods of Ventricular Pacing

Cardiac Memory. Introduction: “Cardiac memory” (primary T wave change) is thought to occur after 15 minutes to several hours of right ventricular (RV) pacing. The two components of the temporal change in repolarization are memory and accumulation. The purpose of this study was to examine quantitatively the effect of short periods of ventricular pacing on the human cardiac action potential, using monophasic action potential (MAP) recordings. Methods and Results: Thirty-one patients (ages 43 ± 14 years) with structurally normal hearts undergoing a clinically indicated electrophysiologic procedure were enrolled. Catheters were placed in the right atrium (RA) and RV, and a MAP catheter was positioned at the RV septum. APD₉₀ was calculated from digitized MAP recordings. MAP morphology comparisons were performed using the root mean square (RMS) of the difference between complexes. All pacing was at 500-msec cycle length. There were four pacing protocols: (1) RA pacing was performed for approximately 15 minutes to evaluate temporal stability of the MAP recordings (5 pts); (2) to evaluate the memory phenomenon, four successive 1-minute episodes of RV pacing were interspersed with 2 minutes of RA pacing (5 pts); (3) the accumulation phenomenon was evaluated by assessing the effects of 1, 5, 10, and 15 minutes of RV pacing on the MAP during RA pacing (16 pts); and (4) 20 minutes of RV pacing was followed by 10 minutes of RA pacing to correlate visually apparent T wave changes with changes in MAP recordings (5 pts). In the control patients, no changes in APD₉₀ or RMS analysis were noted during 14.9 ± 1.4 minutes of RA pacing. In the second protocol, RMS of the difference between the baseline MAP complexes and the signal average of the first 50 beats following each of four 1-minute RV pacing trains demonstrated progressively greater differences in morphology after successive episodes of RV pacing. In protocol 3, RMS analysis identified a progressively greater difference between the baseline MAP recording and the average of the first 50 beats after 1,5, 10, and 15 minutes of RV pacing. In protocol 4, visually apparent changes in T waves occurred in parallel with the RMS of the difference between the baseline MAP recordings and the average of the first 50 beats after 20 minutes of RV pacing. Similar changes also were demonstrated by APD₉₀ analysis. Conclusion: This study is the first to demonstrate that episodes of abnormal ventricular activation as short as 1 minute in duration may exert lingering effects on the repolarization process once normal ventricular activation resumes.     

Incessant Tachycardia Using a Concealed Atrionodal Bypass Tract

Atrionodal Bypass Tract. A case of incessant supraventricular tachycardia continuing despite AV block is reported. Atrial tachycardia and AV nodal reentrant tachycardia were excluded, as was orthodromic tachycardia using a concealed accessory AV pathway. The earliest retrograde atrial activation was at the posterolateral tricuspid annulus, and the tachycardia was eliminated by ablation at this site. The findings in this case are explained only by a concealed atrionodal pathway.