Derivation and Verification of the Relationship between Ablation Index and Baseline Impedance

Abstract

Objective. To explore the quantitative adjustment of ablation index (AI) under different baseline impedance to achieve similar lesion dimensions. Methods. (1) Keeping the AIs relatively constant, the lesion dimensions in different baseline impedances were studied. (2) According to Joule's law, Q = I2RT, keeping the current (I) unchanged, the powers corresponding to different baseline impedances can be obtained. Under different baseline impedances and corresponding powers, the swine hearts were ablated for 30 s in simulated human circumstances. The baseline impedances, the lesion dimensions, and AIs were recorded. And the derivation of empirical formula was achieved according to the AIs and baseline impedance values in similar lesions dimension. (3) Basic AI and baseline impedance (AI0/R0) were set as 400/120 Ω in the common AI groups and 550/120 Ω in the high AI groups, AI values in different baseline impedances were calculated using the empirical formula, and the corresponding lesion dimensions were measured to verify this formula. Results. (1) Higher baseline impedances were related to smaller lesion dimensions at similar AIs. (2) The lesion dimensions were roughly the same after modulating the baseline impedance and power to keep the electric current relatively constant. The relationship between AI and R fitted with experimental data is AI = 1.9933R + 203.61 (r = 0.9649), and the formula derived is ΔAI = (AI0 − 203)/R0 × ΔR. (3) Under the guidance of the empirical formula, there was no significant difference in lesion dimensions between the standard group and the formula guiding groups when AI0 = 400, but there was a shrinking tendence when AI > 700. Conclusion. The lesion depths are negatively correlated with baseline impedance at a certain AI. The relationship between baseline impedance and AI is “ΔAI = (AI0 − 203)/R0 × ΔR”. It is verified that when the AI is not too high, the empirical formula can be used to guide the quantitative adjustment of AIs at different baseline impedance, and the lesion depths achieved are roughly the same.

Effects of different ablation settings on lesion dimensions in an ex vivo swine heart model: Baseline impedance, irrigant, and electrode configuration

“…Consistent with the results of previous studies, 6,7,10,11 using various statical approaches, our ex vivo experiments progressively demonstrated that at similar AI values, baseline impedance exhibited a strong negative linear correlation with lesion dimensions, particularly lesion depth. Namely, ablation at lower baseline impedance resulted in larger lesion dimensions created by the same level of AI, which could be explained by a simple linear formula, albeit necessitating further investigation under other conditions.…”

“…Consistent with the results of previous studies, 6,7,10,11 also reported a similar conclusion that impedance could influence ablation lesions, however, the impedance values in their experiment were distinctly lower than ours, which were more consistent with the real clinical practice. In addition, our study has revealed a threshold impedance-150 Ω, over which the predictive accuracy of AI seems to be compromised, given that the AI formula prospectively predicted lesion depth with high accuracy of ±1 mm in the context of AI-guided ablation.…”

“…Several limitations should also be acknowledged. First, the major limitation of this study was inherent to its ex vivo animal model nature; however, the obtained data were repeatable and consistent with other related studies 11,17 . Second, many other parameters were controlled to unmask the influence of baseline impedance; however it was confirmed that baseline impedance has an important influence on lesion creation in all experimental settings, albeit it is not logistically possible to lower it substantially to increase lesion volume in many clinical cases.…”

“…First, the major limitation of this study was inherent to its ex vivo animal model nature; however, the obtained data were repeatable and consistent with other related studies. 11,17 Second, many other parameters were controlled to unmask the influence of baseline impedance; however it was confirmed that baseline impedance has an important influence on lesion creation in all experimental settings, albeit it is not logistically possible to lower it substantially to increase lesion volume in many clinical cases. Third, there could inevitably be error in the assessment of lesion dimensions; nonetheless, we repeatedly assessed each ablation sites by two different researchers, minimizing this error as much as possible.…”