Comparing maximum rate and sustainability of pacing by mechanical vs. electrical stimulation in the Langendorff-perfused rabbit heart

Abstract

AimsMechanical stimulation (MS) represents a readily available, non-invasive means of pacing the asystolic or bradycardic heart in patients, but benefits of MS at higher heart rates are unclear. Our aim was to assess the maximum rate and sustainability of excitation by MS vs. electrical stimulation (ES) in the isolated heart under normal physiological conditions.Methods and resultsTrains of local MS or ES at rates exceeding intrinsic sinus rhythm (overdrive pacing; lowest pacing rates 2.5±0.5 Hz) were applied to the same mid-left ventricular free-wall site on the epicardium of Langendorff-perfused rabbit hearts. Stimulation rates were progressively increased, with a recovery period of normal sinus rhythm between each stimulation period. Trains of MS caused repeated focal ventricular excitation from the site of stimulation. The maximum rate at which MS achieved 1:1 capture was lower than during ES (4.2±0.2 vs. 5.9±0.2 Hz, respectively). At all overdrive pacing rates for which repetitive MS was possible, 1:1 capture was reversibly lost after a finite number of cycles, even though same-site capture by ES remained possible. The number of MS cycles until loss of capture decreased with rising stimulation rate. If interspersed with ES, the number of MS to failure of capture was lower than for MS only.ConclusionIn this study, we demonstrate that the maximum pacing rate at which MS can be sustained is lower than that for same-site ES in isolated heart, and that, in contrast to ES, the sustainability of successful 1:1 capture by MS is limited. The mechanism(s) of differences in MS vs. ES pacing ability, potentially important for emergency heart rhythm management, are currently unknown, thus warranting further investigation.

Effects of gadolinium on cardiac mechanosensitivity in whole isolated swine hearts

“…In our experiments, we observed mechanical rundown at the beginning of each stimulation protocol (increased D for the second stimulus compared to the first), but the phenomenon was not observed as mechanical stimulation approached failure (no difference between D of the two stimuli preceding a failed mechanical stimulus). This is consistent with Quinn and Kohl’s observation that rundown was pronounced after the first mechanical stimulation but less prominent thereafter 32 . It is therefore unlikely that mechanical rundown played an important role in our results.…”

“…Use-dependent decrease of mechanical stimulation efficacy (also called “mechanical rundown”) has been reported 32 , 33 and is a potential cause for failed mechanical stimulation in repetitive protocols such as ours. The sustainability of successful mechanical stimulation depends on the presence of background electrical pacing and decreases as pacing rate increases 32 . In our study, mechanical stimuli were interspersed with electrical stimuli at a ratio ≥3:1 (electrical:mechanical) and the hearts were paced at a rate lower than normal pig sinus rhythm 38 .…”

“…To investigate possible loss of mechanical stimulation efficacy with repeated delivery (“mechanical rundown”) 32 , 33 , we compared the stimulation-activation delay D between the first and second mechanical stimuli in each stimulation protocol and also between the two stimuli immediately preceding each failed mechanical stimulus. D was significantly shorter for the first stimulus compared to the second (154 ± 43 vs. 202 ± 48 ms, mean ± stdev, p < 0.001, paired t -test, n = 46 pairs), but there was not a significant difference between D of the two stimuli preceding failure (209 ± 54 vs. 211 ± 56 ms, mean ± stdev, p = 0.62, paired t -test, n = 214 pairs).…”

“…In our study, mechanical stimuli were interspersed with electrical stimuli at a ratio ≥3:1 (electrical:mechanical) and the hearts were paced at a rate lower than normal pig sinus rhythm 38 . This pacing condition facilitates the sustainability of successful mechanical stimulation 32 . In our experiments, we observed mechanical rundown at the beginning of each stimulation protocol (increased D for the second stimulus compared to the first), but the phenomenon was not observed as mechanical stimulation approached failure (no difference between D of the two stimuli preceding a failed mechanical stimulus).…”

“…reported that creatine kinase elevation, an indicator for muscle tissue damage, was observed when focal mechanical impact exceeded a certain energy level 41 . However, the energy level required for triggering electrical activation generally does not cause such tissue damage 20 , 32 . In the present study, the mechanical stimulation strength was maintained at a level just sufficient to elicit electrical activation.…”