Using the 12-Lead ECG to Localize the Origin of Atrial and Ventricular Tachycardias: Part 2—Ventricular Tachycardia

Monomorphic ventricular tachycardia (VT) can arise from multiple different ventricular locations in the context of several different underlying myocardial substrates. Despite this variability, the surface 12-lead electrocardiograph (ECG) has proven to be a robust and reproducible initial mapping tool that can provide useful information in localizing the origin of both focal and reentrant forms of VT. The second part of this review series will look at the use of the ECG in mapping the various forms of VT encountered in clinical practice.     

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.     

Cerebellar Stimulation for Spastic Cerebral Palsy; Preliminary Report; On-going Double Blind Study

To date, June 1, 1986, 33 spastic cerebral palsy (CP) patients have taken part in a double blind study testing the safety and efficacy of chronic cerebellar stimulation (CCS) for reduction of spasticity and improvement in function. Seven U.S. surgical centers involving ten neurosurgeons have implanted the Neurolith 601 cerebellar stimulator supplied by Pacesetter Systems Inc. (Sylmar, CA). A pilot study was run with three patients at Stanford University (Stanford, CA) using taped-on real (strong) and dummy (weak) magnets to control the ON-OFF status. Following the pilot study, a magnetically controllable switch was placed in line between the Neurolith stimulator and the cerebellar lead to allow more reliable switching sequences for the study. The test battery included joint angle measurements (passive and active), motor performance testing, reaction time, hand dynamometry, grooved peg board placement, hand/foot tapping, and rotary pursuit testing. Testing only was done at presurgery. Testing and ON-OFF switching was performed following recovery from surgery and at one, two, and four months. After four months, the switch was left turned ON. Of the 30 patients using the implanted switch, 11 were dropped from the study and seven are still in progress. Of the 11 dropped from the study, four were due to switch problems and three were due to double blind protocol violations, i.e., the participants discovered the stimulus status. The remaining four were removed because of a broken lead, infection, or unrelated medical problems, or refusal to participate after implant. A preliminary analysis indicated that three-quarters of the patients have a demonstrable quantitative improvement during the time the stimulation was "ON." Three patients showed no significant change.     

Baroreflex Sensitivity as a Cardiac and Arrhythmia Mortality Risk Stratifier

The last two decades have provided clear evidence for the tight and casual relation existing between arrhythmic mortality and the autonomic nervous system, particularly with imbalances characterized by decreases in vagal and/or increases in sympathetic activity. A series of compelling experimental results has represented the driving force for the clinical evaluation of the potential prognostic value of baroreflex sensitivity (BBS), a measure that can provide information on the capability to augment vagal activity. This article reviews the methodology more commonly used to quantify the clinical evaluation of this parameter, and then focuses on the key clinical studies highlighting those performed in postmyocardial infarction patients. Among them the most informative is ATRAMI, a multicenter prospective study involving almost 1300 patients. The main conclusion is that both heart rate variability and BRS are strong and independent risk factors for post-infarction mortality, thus demonstrating the clinical usefulness of autonomic markers.     

Intravascular Lead Extraction Using Locking Stylets and Sheaths

BYRD, C.L., ET AL.: Intravascular Lead Extraction Using Locking Stylets and Sheaths. Chronic lead extraction using intravascular countertraction techniques was studied in patients with over 65 different lead models including passive and active fixation devices. Indications for removal of 115 leads implanted 5 days to 264 months (mean 58 months) in 62 patients (mean 65 years) included septicemia, subcutaneous tissue infection, preerosion, free-floating lead, lead trapped in valve, too many leads, pain, and vein thrombosis. The superior vena cava (SVC) approach was attempted in 101 leads and was successful in 82 attempts (71% of total leads). The inferior vena cava (IVC) approach via the femoral vein was required to extract 14 (12%) leads inaccessible to the SVC approach and the 19 leads that failed the SVC approach (29% of total leads). The SVC procedure includes a sized stylet locked at the tip and telescoping sheaths advanced over the lead to the heart. An IVC procedure includes placement of a 16 F sheath workstation via a femoral vein into the right atrium. A deflection catheter and Dotter snare in an 11 F sheath were advanced through the workstation into the right atrium. The lead was maneuvered into position, snared, and pulled into the workstation. For both the SVC and IVC approaches, the leads were removed by applying traction on the lead and countertraction with the sheaths. In experienced hands, these techniques have proven safe and effective for removing chronic transvenous leads.     

Cardiac Pain, Sympathetic Afferents, and Life-Threatening Arrhythmias

Cardiac Pain, Sympathetic Afferents, and Life-Threatening Arrhythmias. Myocardial ischemia activates receptors of sympathetic and vagal afferent fibers. Although both afferent pathways are excited, this article primarily focuses on the sympathetic pathways. Increased sympathetic afferent activity to cells of the spinal cord can lead to cardiac pain and reflex sympathetic hyperactivity that can be quite arrhythmogenic. Complicating factors that affect the relationship arises from activation of vagal afferent fibers. Sympathetic afferent fibers in the ventricle appear to be composed of mechanosensitive and chemosensitive receptors. Sympathetic afferent fibers excite cells of origin of the spinothalamic tract, the classical pain pathway that transmits information about noxious somatic and visceral episodes to areas of the brain involved with pain perception. Activation of selected brainstem nuclei, the vagus, and the dorsal columns can suppress afferent information from the heart that excites spinothalamic tract cells. Excitation of sympathetic afferent fibers initiates a series of cardiovascular reflexes, some of which may affect cardiac electrical stability significantly. Increased arrhythmogenesis can be reduced when C₈-T₅ dorsal roots are transected. Stimulation of cardiac nerves that involve the left stellate ganglion can produce life-threatening arrhythmias, particularly if combined with myocardial ischemia or infarction. Through reflex mechanisms, activation of sympathetic afferent fibers can suppress activity of vagal efferent fibers. If prevention of cardiac pain through reduced firing or inhibition of cells in the spinothalamic tract reduces afferent sympathetic activity that occurs during acute myocardial ischemia, it would be logical to surmise that another significant benefit would follow, namely reduced risk for malignant arrhythmias.