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.
Highlights
While implanted electrical pacemakers are an effective means of sustained cardiac pacing, mechanical pacing is of interest for the emergency resuscitation setting, as it represents a rapidly available, noninvasive and generally well-tolerated method for triggering cardiac excitation in the asystolic or severely bradycardic heart.[1]Localized mechanical stimulation (MS) of the human heart, whether by direct tissue contact of intra-cardiac devices[2] or by extracorporeal impact (Commotio cordis,[3] precordial thump4) can cause mechanically-induced ventricular excitation (VEM), resulting in competent ventricular contraction
Using optical mapping of direct local MS of the ventricular epicardium in rat[21] and rabbit[22] isolated hearts, we have shown that VEM originates focally from the stimulation site, spreading downstream from the point of earliest activation in a manner that is indistinguishable from electrically paced beats
Mechanical pacing occurs by activation of cation-non-selective stretch-activated channels (SACNS),[23] as it is prevented by SACNS block with Grammostola spatulata MechanoToxin-4 (GsMTx-4).[22]
Summary
While implanted electrical pacemakers are an effective means of sustained cardiac pacing, mechanical pacing is of interest for the emergency resuscitation setting, as it represents a rapidly available, noninvasive and generally well-tolerated method for triggering cardiac excitation in the asystolic or severely bradycardic heart.[1]Localized mechanical stimulation (MS) of the human heart, whether by direct tissue contact of intra-cardiac devices (catheters, pacing leads)[2] or by extracorporeal impact (Commotio cordis,[3] precordial thump4) can cause mechanically-induced ventricular excitation (VEM), resulting in competent ventricular contraction. If applied rhythmically to the precordium (‘precordial percussion’),[5] repetitive thumps have been shown to be an effective means for extracorporeal pacing of the asystolic[6,7] or bradycardic[8] heart, while benefits of MS for cardioversion of tachycardias is limited.[9] As mechanically-induced heartbeats have a greater hemodynamic effect than external chest compressions,[10] they can maintain consciousness in patients during extended periods of ventricular standstill (cases as long as 2 h 45 min have been reported).[6,11] This ‘mechano-electric feedback’ effect[12] was exploited by resuscitation pioneer Paul Zoll in the design of a mechanical pacing device for external stimulation of heart beats in emergency settings (‘cardiac thumper’).[13] In dogs with normal sinus rhythm and high-degree atrioventricular block, repetitive heartbeats were evoked using this device, with no cases of mechanically-induced sustained arrhythmia (such as tachycardia or fibrillation, which can occur following VEM14), even when stimulation occurred during the relative refractory period and at energies up to 10 times VEM threshold. The device was shown to be effective in patients with asystole after ventricular fibrillation, with atrial fibrillation, or with implanted pacemakers for atrioventricular block.[13]
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