Long Pulses Reduce Phrenic Nerve Stimulation in LV Pacing
Long Pulses Reduce Phrenic Nerve Stimulation in LV Pacing
Long Pulses in LV Pacing.
Introduction: Phrenic nerve stimulation is a major obstacle in cardiac resynchronization therapy (CRT). Activation characteristics of the heart and phrenic nerve are different with higher chronaxie for the heart. Therefore, longer pulse durations could be beneficial in preventing phrenic nerve stimulation during CRT due to a decreased threshold for the heart compared with the phrenic nerve.
We investigated if long pulse durations decreased left ventricular (LV) thresholds relatively to phrenic nerve thresholds in humans.
Methods and Results: Eleven patients, with indication for CRT and phrenic nerve stimulation at the intended pacing site, underwent determination of thresholds for the heart and phrenic nerve at different pulse durations (0.3–2.9 milliseconds). The resulting strength duration curves were analyzed by determining chronaxie and rheobase. Comparisons for those parameters were made between the heart and phrenic nerve, and between the models of Weiss and Lapicque as well.
In 9 of 11 cases, the thresholds decreased faster for the LV than for the phrenic nerve with increasing pulse duration. In 3 cases, the thresholds changed from unfavorable for LV stimulation to more than a factor 2 in favor of the LV. The greatest change occurred for pulse durations up to 1.5 milliseconds. The chronaxie of the heart was significantly higher than the chronaxie of the phrenic nerve (0.47 milliseconds vs. 0.22 milliseconds [P = 0.029, Lapicque] and 0.79 milliseconds vs. 0.27 milliseconds [P = 0.033, Weiss]).
Conclusion: Long pulse durations lead to a decreased threshold of the heart relatively to the phrenic nerve and may prevent stimulation of the phrenic nerve in a clinical setting.
Cardiac resynchronization therapy (CRT), possibly with a defibrillator backup (CRT-D), on top of optimal medical therapy leads to improved outcomes in patients with heart failure (HF) and New York Heart Association functional class III and IV. Other patient groups may benefit from CRT as well.
In addition to a right ventricular (RV) lead, the placement of a left ventricular (LV) pacing lead in one of the coronary sinus (CS) branches, preferably in a posterolateral position, is pivotal in CRT.
The anatomy of the CS branches is challenging, as they may be in close proximity to the left phrenic nerve. Therefore, unintended phrenic nerve stimulation is a well-known side effect of CRT. Huizar et al. demonstrated different strength–duration curves for the phrenic nerve when changing the pacing vector in LV pacing. Thus, when confronted with phrenic nerve stimulation, the operator may move the LV pacing lead to a different position or change the pacing polarity between different pacing sites, e.g., LV tip/ring versus LV tip/coil versus LV ring/coil. However, this may prove cumbersome and time consuming and may not lead to an optimal pacing site.
Apart from prohibiting the use of the optimal stimulation site, phrenic nerve stimulation may also occur postoperatively when polarity change is the only option available to avoid a new surgical procedure. Jastrzebski et al. showed that reprogramming often solved phrenic nerve stimulation, but in 3 of 201 cases reoperation was necessary. Biffi et al. also reported the necessity of reoperation in 1.6% of the cases in a cohort study, including 1,307 patients. An excellent overview of intraoperative phrenic nerve stimulation problems and phrenic nerve stimulation during follow-up is given by Biffi et al.
Another option to solve the problem of phrenic nerve stimulation may be based on the differential activation characteristics of muscle and nerve: It has been known, for over 100 years, that stimulus thresholds for different tissues change differently in response to a change of the stimulus pulse duration. The relation between pulse duration and stimulus threshold was modeled by Weiss (as a hyperbolic function in its formulation by Lapicque [1905]) and by Lapicque (1909). Both models are described by 2 parameters: the rheobase and the chronaxie. The rheobase is the stimulus intensity below which no stimulation can be achieved at any pulse duration. The chronaxie is the shortest pulse duration at which threshold for stimulation can be achieved if the stimulus intensity equals twice the rheobase, i.e., the chronaxie defines the rate at which the stimulus threshold decreases with pulse duration. As heart muscle has a higher chronaxie than nerve, it can be expected that a prolongation of the pulse duration decreases the LV threshold more than the phrenic nerve threshold. This was confirmed by a recent independent report based on animal studies, which also indicated a possible benefit of long pacing pulse durations to decrease phrenic nerve stimulation.
We hypothesized that long pacing pulses would decrease the LV threshold relative to the phrenic nerve threshold, thus decreasing the tendency for phrenic nerve stimulation in humans undergoing implantation of a CRT system.
Abstract and Introduction
Abstract
Long Pulses in LV Pacing.
Introduction: Phrenic nerve stimulation is a major obstacle in cardiac resynchronization therapy (CRT). Activation characteristics of the heart and phrenic nerve are different with higher chronaxie for the heart. Therefore, longer pulse durations could be beneficial in preventing phrenic nerve stimulation during CRT due to a decreased threshold for the heart compared with the phrenic nerve.
We investigated if long pulse durations decreased left ventricular (LV) thresholds relatively to phrenic nerve thresholds in humans.
Methods and Results: Eleven patients, with indication for CRT and phrenic nerve stimulation at the intended pacing site, underwent determination of thresholds for the heart and phrenic nerve at different pulse durations (0.3–2.9 milliseconds). The resulting strength duration curves were analyzed by determining chronaxie and rheobase. Comparisons for those parameters were made between the heart and phrenic nerve, and between the models of Weiss and Lapicque as well.
In 9 of 11 cases, the thresholds decreased faster for the LV than for the phrenic nerve with increasing pulse duration. In 3 cases, the thresholds changed from unfavorable for LV stimulation to more than a factor 2 in favor of the LV. The greatest change occurred for pulse durations up to 1.5 milliseconds. The chronaxie of the heart was significantly higher than the chronaxie of the phrenic nerve (0.47 milliseconds vs. 0.22 milliseconds [P = 0.029, Lapicque] and 0.79 milliseconds vs. 0.27 milliseconds [P = 0.033, Weiss]).
Conclusion: Long pulse durations lead to a decreased threshold of the heart relatively to the phrenic nerve and may prevent stimulation of the phrenic nerve in a clinical setting.
Introduction
Cardiac resynchronization therapy (CRT), possibly with a defibrillator backup (CRT-D), on top of optimal medical therapy leads to improved outcomes in patients with heart failure (HF) and New York Heart Association functional class III and IV. Other patient groups may benefit from CRT as well.
In addition to a right ventricular (RV) lead, the placement of a left ventricular (LV) pacing lead in one of the coronary sinus (CS) branches, preferably in a posterolateral position, is pivotal in CRT.
The anatomy of the CS branches is challenging, as they may be in close proximity to the left phrenic nerve. Therefore, unintended phrenic nerve stimulation is a well-known side effect of CRT. Huizar et al. demonstrated different strength–duration curves for the phrenic nerve when changing the pacing vector in LV pacing. Thus, when confronted with phrenic nerve stimulation, the operator may move the LV pacing lead to a different position or change the pacing polarity between different pacing sites, e.g., LV tip/ring versus LV tip/coil versus LV ring/coil. However, this may prove cumbersome and time consuming and may not lead to an optimal pacing site.
Apart from prohibiting the use of the optimal stimulation site, phrenic nerve stimulation may also occur postoperatively when polarity change is the only option available to avoid a new surgical procedure. Jastrzebski et al. showed that reprogramming often solved phrenic nerve stimulation, but in 3 of 201 cases reoperation was necessary. Biffi et al. also reported the necessity of reoperation in 1.6% of the cases in a cohort study, including 1,307 patients. An excellent overview of intraoperative phrenic nerve stimulation problems and phrenic nerve stimulation during follow-up is given by Biffi et al.
Another option to solve the problem of phrenic nerve stimulation may be based on the differential activation characteristics of muscle and nerve: It has been known, for over 100 years, that stimulus thresholds for different tissues change differently in response to a change of the stimulus pulse duration. The relation between pulse duration and stimulus threshold was modeled by Weiss (as a hyperbolic function in its formulation by Lapicque [1905]) and by Lapicque (1909). Both models are described by 2 parameters: the rheobase and the chronaxie. The rheobase is the stimulus intensity below which no stimulation can be achieved at any pulse duration. The chronaxie is the shortest pulse duration at which threshold for stimulation can be achieved if the stimulus intensity equals twice the rheobase, i.e., the chronaxie defines the rate at which the stimulus threshold decreases with pulse duration. As heart muscle has a higher chronaxie than nerve, it can be expected that a prolongation of the pulse duration decreases the LV threshold more than the phrenic nerve threshold. This was confirmed by a recent independent report based on animal studies, which also indicated a possible benefit of long pacing pulse durations to decrease phrenic nerve stimulation.
We hypothesized that long pacing pulses would decrease the LV threshold relative to the phrenic nerve threshold, thus decreasing the tendency for phrenic nerve stimulation in humans undergoing implantation of a CRT system.
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