Background: Changes in resting membrane potential due to extracellular potassium ([K+]
e) accumulation are thought to be responsible for TQ segment depression in ischemia. However, the nature of the [K+]
e-TQ relationship remains to be fully elucidated. Methods: We created a carotid-coronary shunt in 21 pigs, and recorded [K+]
e and TQ-segment potentials simultaneously during graded left anterior descending artery (LAD) flow reduction via 4-6 K
+-sensitive electrodes placed in the LAD. Only data from K
+ electrodes with calibration slopes of 55-65 mV/decade change in K
+ were used. Results: While the correlation between the changes in potassium equilibrium potential (E
K) and the TQ shift was linear, the regression slopes initially increased and then decreased during graded flow reduction (S = -0.338, Q = 30 mL/minute, S = -3.253, Q = 10 mL/minute, S = -0.312, Q = 0 mL/minute), i.e., TQ depression at all E
K values became larger, then smaller as the flow was decreased in a stepwise manner. The inhomogeneity of changes in [K+]
e and TQ potential changes and their relationship also decreased initially then increased during graded flow reduction (R = -0.237, LAD flow = 30 mL/minute; R = -0.819, LAD flow = 15 mL/minute; R = -0.115, LAD flow = 0 mL/minute). Conclusions: Although [K+]
e and the TQ shift are related linearly, there is large variability in their relationship in the setting of graded coronary flow reduction. Therefore, local TQ-segment potentials cannot be used as indices of the severity of ischemic changes.
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