In patients with AVNRT undergoing slow pathway ablation, junctional ectopy during the application of RF energy is a sensitive but nonspecific marker of successful ablation. The bursts of junctional ectopy are significantly longer at effective target sites than at ineffective sites. VA conduction should be expected during the junctional ectopy that accompanies slow pathway ablation, even when there is poor VA conduction during baseline ventricular pacing. VA block during junctional ectopy is a harbinger of AV block in patients undergoing RF ablation of the slow pathway. If energy applications are discontinued as soon as VA block occurs, the risk of AV block may be markedly reduced.

The effects of coronary capacitance on instantaneous pressure-flow (P/F) relationships were analyzed using a theoretical model of coronary flow during diastole that included capacitance. The magnitude of the discrepancy between actual intramural and instantaneously derived P/F relationships was predicted to be dependent on the ratio of two natural decay constants (central aortic decay constant/intrinsic coronary decay constant). The effects of coronary capacitance are eliminated using constant pressure conditions. The instantaneous (dynamic) and constant pressure (static) P/F relationships were compared experimentally using a reservoir to provide constant pressure perfusion during prolonged diastoles in heart blocked dogs. In the presence of coronary tone, zero flow pressure intercepts (Pzf) of 27.1 +/- 6.6 and 11.0 +/- 3.0 mm Hg were obtained under dynamic and constant pressure conditions respectively, P less than 0.001. After maximal vasodilation, Pzf of 14.2 +/- 4.5 mmHg and 10.7 +/- 2.4 mmHg were obtained under dynamic and constant pressure conditions, respectively, P = NS. Pzf derived under constant pressure conditions were independent of the state of coronary vasomotor tone with a value about 11 mmHg. The slopes of the dynamic P/F relationships tended to be greater than those derived from constant pressure conditions. This may suggest an additional component of increasing coronary resistance during diastole that could not be readily assessed under dynamic conditions. We conclude that coronary capacitive effects and resistance changes during diastole severely limit the interpretation of instantaneous dynamic P/F relationships. Diastolic coronary perfusion ceases at about 11 mm Hg and is independent of coronary tone when capacitive effects are eliminated.

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Defibrillation thresholds with monophasic shocks are approximately 30% lower with the distal electrode as the anode. The use of anodal shocks may obviate the need for a subcutaneous patch and allow more frequent implantation of a transvenous lead system.

Until recently, coronary pressure-flow relationships have been considered only in terms of resistance characteristics. However, evidence suggests that the zero flow pressure intercept (Pzf) is of significant magnitude and must also be considered. BeUamy (1) analyzed the instantaneous diastolic coronary pressure-flow relationships in chronically instrumented dogs. Pressure-flow relationships were found to be linear with P~ as high as 50 mmHg in the presence of coronary tone. During reactive hyperemia or adenosine induced vasodilation, the instantaneous pressure-flow relationships revealed a greater slope (interpreted as coronary conductance) and a decrease in the Pz~ to about 20 mmHg. These results have been confirmed by other groups (2) in open-chest anesthetized preparations.The question remains as to the correct interpretation and the physiologic significance of these high diastolic pressure intercepts. If coronary inflow as measured by an epicardial electromagnetic flowmeter correctly represents the flow in the microcirculation at every instant of time, then these high zero flow pressure intercepts could represent the downstream back pressure of a vascular waterfall (3). Critical closing pressures of this magnitude during diastole would certainly play a major role in the regulation and distribution of coronary flow. However, the assumed instantaneous equivalence of coronary inflow and intramural flow cannot be made in the presence of reactive components of the coronary system (capacitance and inertia). In particular, capacitive properties of the coronary system could provide continued intramural flow despite zero coronary inflow as measured by a flow meter. This underestimation of intramural flow can result in overestimation of the true zero flow pressure intercept. Figure 1 illustrates the consequences of coronary capacitance on the instantaneous pressure-flow relationship. In the electrical analog representation of the coronary system during diastole PA denotes the aortic driving pressure and P0 denotes the back pressure which can represent either coronary sinus pressure or the critical closing pressure whichever is greater. Coronary capacitance, C, is in parallel to the intramural resistance, Supported in part by NIH grant #HL 23171 991