High stored iron and the risk of ischemic heart disease

Relationship between the upper limit of vulnerability determined in normal sinus rhythm and the defibrillation threshold in patients with implantable cardioverter defibrillators

The upper limit of vulnerability is the strength above which ventricular fibrillation is no longer inducible with a shock delivered during the vulnerable phase of the cardiac cycle. It has been demonstrated that the upper limit of vulnerability correlates with the defibrillation threshold in a paced rhythm. The purpose of this study is to evaluate the correlation of the upper limit of vulnerability determined in normal sinus rhythm with the defibrillation threshold using a simplified protocol in patients undergoing placement of an ICD. We studied 28 patients who underwent ICD implantation. CPI generators and Endotak leads were used in all patients. Device-based testing was used to determined the defibrillation threshold and the upper limit of vulnerability. The upper limit of vulnerability was tested with three shocks delivered at 0, 20, and 40 ms before the peak of the T wave during normal sinus rhythm. The defibrillation threshold was determined by a simple step up-down protocol. The upper limit of vulnerability (9.0 +/- 4.5 J) did not significantly differ from the defibrillation threshold (9.9 +/- 4.0 J), P = NS. A close correlation was present, correlation coefficient = 0.75, P < 0.0001. The upper limit of vulnerability was within 5 J of the defibrillation threshold in 27 (96%) of the 28 patients. The upper limit of vulnerability underestimated the defibrillation threshold by 10 J in one patient who had a defibrillation threshold of 15 J. The upper limit of vulnerability determined in normal sinus rhythm correlates significantly with the defibrillation threshold in patients undergoing ICD implantation. The protocol is simple and easily implemented clinically.

Myocardial vulnerability to T wave shocks: relation to shock strength, shock coupling interval, and dispersion of ventricular repolarization

INTRODUCTION

Induction of ventricular fibrillation (VF) by T wave shocks is of clinical interest due to the correlation between the upper limit of vulnerability (ULV) and the defibrillation threshold (DFT). However, the ULV has not yet been defined precisely in reference to the entire "area of vulnerability" (AOV), which is defined bifunctionally by both shock strengths and shock coupling intervals, nor has it been related to the dispersion of ventricular repolarization, considered to be an important determinant of vulnerability.

METHODS AND RESULTS

In 11 isolated perfused rabbit hearts immersed in a tissue bath containing a 3-lead ECG recording system and two opposite plate electrodes for field shock administration, 7 monophasic action potentials (MAPs) were recorded simultaneously from different epicardial and endocardial regions of the right and left ventricles. An average of 90 +/- 25 monophasic waveform shocks of varying shock strengths and coupling intervals were delivered to each heart to determine the horizontal and vertical boundaries of the AOV. The AOV approximated a rhomboid with homogenous VF inducibility. The ULV and lower limit of vulnerability (LLV) represented discrete corners of the AOV with significant changes in VF inducibility if either shock coupling intervals or shock strength were changed by only 10 msec or 10 V, respectively (P < 0.001). The ULV occurred at 7 +/- 10 msec shorter coupling intervals than the LLV (P < 0.05), and VF-inducing shock strengths at the left corner of the AOV were 50 +/- 67 V higher as compared to the right corner (P < 0.01). The maximal range of VF-inducing coupling intervals coincided (within < 2 msec) with the dispersion of MAPs at 70% repolarization, and the ULV coupling interval coincided (within < 4 msec) with the longest repolarization at 50%.

CONCLUSIONS

(1) VF vulnerability to monophasic T wave shocks is defined by an AOV that has the shape of a leftward tilted rhomboid. (2) Both the ULV and LLV are sharply defined upper and lower corners of the AOV rhomboid. (3) The width of the AOV corresponds to the dispersion of ventricular repolarization at the 70% level. (4) Considering the dispersion of ventricular repolarization may yield more precise ULV determinations and a better understanding of the correlation between the ULV and DFT.

Clinical aspects of preconditioning and implications for the cardiac surgeon

Ischemic preconditioning is one of the most powerful means to reduce myocardial ischemic cell death in the experimental laboratory. Data are now emerging suggesting that ischemic preconditioning also can occur in the human heart. Studies performed on human myocardial biopsies, angioplasty studies, clinical studies assessing acute tolerance to angina, and some studies evaluating the effect of angina prior to myocardial infarction, lend support to the concept that the human heart can be preconditioned. The ultimate objective is to develop preconditioning-mimetic agents that can be administered prophylactically prior to the time of cardiopulmonary bypass surgery or administered to hearts that have been harvested for transplant in order to better preserve the ischemically jeopardized myocyte.