Perspectives and future trends in cellular electrophysiology: Implications for the clinician

As a result of single fiber electrophysiologic studies, the clinical approach to the electrical behavior of the heart has improved. Three areas are examined: 1) the electrocardiographic waveform, 2) normal and abnormal cardiac rhythms, and 3) the mechanism of action of antiarrhythmic drugs. In each area, the results of single fiber studies have provided a conceptual framework for diagnostic and therapeutic decisions. These studies have also enabled investigators to test hypotheses formulated from clinical observations. It may be only a slight exaggeration to attribute many of our recent advances in each of the three areas to the development and use of the microelectrode

Lack of specificity of new negative U waves for anterior myocardial ischemia as evidenced by intracoronary electrogram during balloon angioplasty

Negative U waves on the surface electrocardiogram are reported to be a specific marker of myocardial disease. In the setting of ischemia, they correlate with stenosis of the left main and left anterior descending coronary arteries. To determine whether U wave changes are unique for anterior ischemia, the development of new U waves on the intracoronary electrogram was correlated with the location and magnitude of ischemia during coronary balloon angioplasty. Recordings were obtained during dilation of 43 vessels in 37 patients. New negative U waves developed during dilation of 12 vessels (7 of the left anterior descending, 4 of the left circumflex and 1 of the right coronary artery). New positive U waves developed during dilation of 18 vessels (12 of the left anterior descending, 3 of the left circumflex and 3 of the right coronary artery). The magnitude of ST segment change was 10.9 +/- 6.7 mm in the presence of a new U wave but only 3.4 +/- 2.8 mm in the absence of a new U wave (p less than 0.001). It is concluded that 1) negative U waves on the intracoronary electrogram are not specific for anterior ischemia; 2) new positive U waves on the intracoronary electrogram are as sensitive as new negative U waves for acute ischemia; 3) the development of a new positive or negative U wave is associated with the magnitude of myocardial ischemia; and 4) the recording of U waves may be related to the proximity of the recording leads to the location of ischemia

Exercise-induced distal atrioventricular block

Three patients with 1:1 atrioventricular (AV) conduction at rest developed fixed 2:1 or 3:1 AV block during treadmill exercise testing. Electrophysiologic study documented block distal to the AV node in all three patients, and suggested that the exercise-induced block occurred because of increased atrial rate and abnormal refractoriness of the His-Purkinje conduction system. The findings in these three patients suggest that high grade AV block appearing during exercise reflects conduction disease of the His-Purkinje system rather than of the AV node, even in the absence of bundle branch block. Patients with this diagnosis should be considered for permanent cardiac pacing

Repolarization heterogeneity and rate dependency in a canine rapid pacing model of heart failure

Repolarization heterogeneity and rate dependency have long been established as factors contributing to arrhythmogenic risk. However there are conflicting observations regarding the nature and extent of ventricular repolarization heterogeneity that complicate understanding of arrhythmogenic mechanisms. In order to explore these disparate findings, we studied ventricular repolarization heterogeneity and rate dependency in a canine, rapid pacing model of heart failure

Identification of false positive exercise tests with use of electrocardiographic criteria: A possible role for atrial repolarization waves

Atrial repolarization waves are opposite in direction to P waves, may have a magnitude of 100 to 200 mu V and may extend into the ST segment and T wave. It was postulated that exaggerated atrial repolarization waves during exercise could produce ST segment depression mimicking myocardial ischemia. The P waves, PR segments and ST segments were studied in leads II, III, aVF and V4 to V6 in 69 patients whose exercise electrocardiogram (ECG) suggested ischemia (100 mu V horizontal or 150 mu V upsloping ST depression 80 ms after the J point). All had a normal ECG at rest. The exercise test in 25 patients (52% male, mean age 53 years) was deemed false positive because of normal coronary arteriograms and left ventricular function (5 patients) or normal stress single photon emission computed tomographic thallium or gated blood pool scans (16 patients), or both (4 patients). Forty-four patients with a similar age and gender distribution, anginal chest pain and at least one coronary stenosis greater than or equal to 80% served as a true positive control group. The false positive group was characterized by (1) markedly downsloping PR segments at peak exercise, (2) longer exercise time and more rapidmore » peak exercise heart rate than those of the true positive group, and (3) absence of exercise-induced chest pain. The false positive group also displayed significantly greater absolute P wave amplitudes at peak exercise and greater augmentation of P wave amplitude by exercise in all six ECG leads than were observed in the true positive group.« les

ACC/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities: A report of the American College of Cardiology/American Heart Association Task Force on practice guidelines (Writing Committee to revise the ACC/AHA/NASPE 2002 guideline update for implantation of cardiac pacemakers and antiarrhythmia devices)

This revision of the “ACC/AHA/NASPE Guidelines for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices” updates the previous versions published in 1984, 1991, 1998, and 2002. Revision of the statement was deemed necessary for multiple reasons: 1) Major studies have been reported that have advanced our knowledge of the natural history of bradyarrhythmias and tachyarrhythmias, which may be treated optimally with device therapy; 2) there have been tremendous changes in the management of heart failure that involve both drug and device therapy; and 3) major advances in the technology of devices to treat, delay, and even prevent morbidity and mortality from bradyarrhythmias, tachyarrhythmias, and heart failure have occurred

Initial and Secondary ST-T Alternans During Acute Myocardial Ischemia in the In-Situ Pig Heart

The factors responsible for the ST-T wave alternans (STTA) and associated arrhythmias during acute ischemia have not been clarified.In acutely ischemic porcine myocardium, we recorded transmural unipolar and bipolar electrocardiograms and mid-myocardial extracellular K(+) ([K(+)]e) from the center of the ischemic zone during 8-minute episodes of ischemia. Two different STTAs occurred. The initial STTA, which occurred at 4 minutes 15 seconds ± 12 seconds of ischemia during sinus rhythm, was most prominent in the subendocardium, independent of [K(+)]e and activation block, and heart rate dependent. It occurred in 13/19 (68%) occlusions at heart rates ≤ 100 bpm and in 22/23 (96%) at > 100 bpm. The second STTA was more obvious and greatest in the subepicardium. It began in the later phase of ischemia and was also heart rate dependent (5/19 [26%] occlusions at heart rates ≤ 100 bpm and 10/23 [44%] at > 100 bpm). This STTA was consistently associated with 2:1 change in the bipolar electrogram morphology, possibly due to 2:1 conduction block. Ventricular fibrillation (VF) occurred only at > 100 bpm.The initial STTA may be independent of conduction abnormalities and represent primary repolarization alternans. The second STTA may be secondary to and indicative of 2:1 activation block or marked alternans of the action potential amplitude/duration. The associated VF most likely reflects the underlying conduction abnormality