Mechanically-Induced Ectopy via Stretch-Activated Cation-Nonselective Channels is Caused by Local Tissue Deformation and Results in Ventricular Fibrillation if Triggered on the Repolarization Wave-Edge (Commotio cordis)

Abstract

Background

External chest impacts (Commotio cordis, CC) can cause mechanically-induced premature ventricular excitation (PVE_M) and, rarely, ventricular fibrillation (VF). Since block of stretch-sensitive ATP-inactivated potassium channels (K_ATP) curtailed VF-occurrence in a porcine model of CC, VF has been suggested to arise from abnormal repolarization caused by stretch-activation of K_ATP. Alternatively, VF could result from abnormal excitation by PVE_M, overlapping with normal repolarization-related electrical heterogeneity. Here we investigate mechanisms and determinants of PVE_M induction and its potential role in CC-induced VF.

Methods and Results

Sub-contusional mechanical stimuli were applied to isolated rabbit hearts during optical voltage mapping, combined with pharmacological block of K_ATP or stretch-activated cation-nonselective channels (SAC_NS). We demonstrate that local mechanical stimulation reliably triggers PVE_M at the contact site, with inducibility predicted by local tissue indentation. PVE_M induction is diminished by SAC_NS-block. In hearts where T-waves involve a well-defined repolarization edge traversing the epicardium, PVE_M can reliably provoke VF if, and only if, the mechanical stimulation site overlaps the repolarization wave-edge. In contrast, application of short-lived intra-ventricular pressure surges neither triggers PVE_M nor changes repolarization. K_ATP block has no effect on PVE_M inducibility per se, but shifts it to later time-points by delaying repolarization and prolonging refractoriness.

Conclusions

Local mechanical tissue deformation determines PVE_M induction via SAC_NS activation, with VF induction requiring PVE_M overlap with the trailing edge of a normal repolarization wave. This defines a narrow, subject-specific vulnerable window for CC-induced VF that exists both in time and in space.

Introduction

For over a century, it has been known that the heart is an exquisitely mechano-sensitive organ.¹ Sub-contusional mechanical stimulation of the heart (Commotio cordis, CC), whether by intra-cardiac device-tissue interactions or by chest impacts, can result in a variety of heart rhythm changes, including ectopy (e.g., mechanically-induced premature ventricular excitation, PVE_M) and sustained rhythm disturbances (e.g., ventricular fibrillation, VF).²

PVE_M is common in patients with central monitoring catheters^{3, 4} or intra-cardiac pacing wires.^5–7 It is also presumed to be common in CC-prone sports such as baseball or ice hockey, and CC-induced VF is one of the most common causes of death in youth athletes in the US.⁸ However, mechanisms of PVE_M genesis and determinants for VF induction are ill-understood.

What is known is that stretch can trigger PVE_M in myocardial cells,⁹ tissue,¹⁰ and whole heart.¹¹ This ‘mechano-electric feedback’^12–14 can be explained by ‘stretch-activated cation-nonselective channels’ (SAC_NS),¹⁵ which depolarize diastolic transmembrane potential (V_m) in cardiomyocytes¹⁶ and whole hearts.¹⁷ During the action potential (AP) plateau, stretch accelerates V_m repolarization by activating SAC_NS^{17, 18} (their reversal potential is approximately half-way between peak and resting V_m levels¹⁹), or by opening stretch-sensitive potassium-selective currents,²⁰ causing early AP shortening. In CC, the mechano-sensitive ATP-inactivated potassium (K_ATP) current²¹ has been implicated in VF induction, as administration of glibenclamide (a non-specific K_ATP-blocker) reduced mechanical VF-induction in a porcine model of CC.²²

Electrophysiological outcomes of CC are modulated by mechanical stimulus characteristics such as location, area, and duration.²³ Studies in the porcine model have characterized mechanical inducibility of VF as inversely-related to impact area and duration, rising with projectile stiffness and, most strikingly, occurring only during a narrow vulnerable period: 15-30ms prior to the ECG T-wave peak.^{8, 24, 25}

Generally, VF vulnerability during the T-wave is related to repolarization dynamics. In the case of locally-acting mechanical stimuli, as ventricular repolarization is spatially non-uniform, timing relative to the ECG will not translate into a similarly defined timing relative to the V_m of affected cardiomyocytes.²⁶ Computational simulations have suggested that mechanically-induced VF can occur only when a supra-threshold mechanical stimulus occurs at the trailing edge of the preceding normal repolarization wave.^{27, 28} In this situation, PVE_M in excitable tissue arises directly adjacent to refractory tissue, resulting in a region of functional conduction block around which re-entry can ensue. As a wave of repolarization travels across the ventricles, the condition for such overlap will be met in different locations at different times, and for very brief time-periods only. From this ‘site-dependence’ of critical mechanical stimulation timing follows the hypothesis that the vulnerable window for CC-induced VF exists both in time and in space. Experimental assessment of this computationally-predicted mechanism is still outstanding.

An interesting additional question concerns the role of the large, but short-lived, intra-ventricular pressure surge that accompanies precordial impacts. In the porcine CC-model, VF-inducing impacts cause pressure surges whose amplitude correlates with both impact severity and probability of VF induction.²⁹ Yet similarly pronounced intra-ventricular pressure surges are seen with coughing,³⁰ where they are not normally associated with induction of sustained arrhythmias. Therefore, the question as to whether the intra-ventricular pressure surge is causal for CC-induced VF, or is a covariate of those impact properties that determine arrhythmogenicity, requires experimental verification.

Thus, the goals of our study were to assess, in the Langendorff-perfused rabbit heart (in which mechano-electric feedback responses are well-preserved³¹), mechanisms of PVE_M induction, and their potential role in triggering VF. Our specific aims were to determine whether: (i) PVE_M induction depends on the degree of local tissue deformation, deformation rate, applied force, stress, or intra-ventricular pressure surges; (ii) CC effects on cardiac electrophysiology are caused by SAC_NS or K_ATP; and (iii) spatio-temporal overlap of mechanical stimulation with the trailing edge of the repolarization wave is critical for VF induction. These aims were addressed by controlled application of sub-contusional local epicardial mechanical stimuli, or intra-ventricular volume bursts, during optical V_m mapping in the absence or presence of suitable pharmacological blockers of SAC_NS or K_ATP.

Methods

Ethical Approval

This study was carried out, under local ethical approval, in strict accordance with the UK Home Office Animals (Scientific Procedures) Act of 1986. Details of experimental protocols have been reported following the Minimum Information about a Cardiac Electrophysiology Experiment (MICEE) reporting standard,³² see repository (https://www.micee.org/?q=node/00001374). Methods are briefly described here; see Supplemental Methods for detail.

Heart Preparation

Langendorff-perfused hearts from female New Zealand White rabbits were instrumented with custom-made polyethylene balloons in the LV connected to a transducer for measurement of intra-ventricular pressure and a servomotor-controlled syringe for rapid bi-directional volume alteration. Surface ECG was recorded using two spring-loaded monopolar Ag/AgCl pellet electrodes. The experimental setup can be seen in Supplemental Fig. S1.

Voltage Optical Mapping

Hearts were loaded with a voltage-sensitive dye (di-4-ANBDQPQ) and an excitation-contraction uncoupler (blebbistatin). Optical mapping was performed with a previously described system using light emitting diodes for illumination and a 511 frames/second camera for observation.³³

Mechanical Stimulation

Local mechanical stimuli were applied to the ventricular epicardium using the Soft Tissue Impact Characterisation Kit,³⁴ with the area of indentation determined by impact probe-surface and the magnitude of indentation monitored at ~35 m resolution using an optical grating. Images and characteristics of a typical stimulation are shown in Fig. 1 and Supplemental Movie S1. For all stimuli, local excitation was assessed by optical V_m mapping. Intra-ventricular pressure surges were applied by brief bursts in intra-ventricular balloon volume (active in- and deflation). Local mechanical stimulation and pressure surge timing relative to the ECG were controlled. Tissue integrity was assessed by analysis of creatine kinase activity in coronary effluent.³⁴

Figure 1.

Sub-contusional local epicardial mechanical stimulation. A, Images of a typical epicardial mechanical stimulus (from Supplemental Movie S1). B, Mechanical stimulation characteristics, showing: (i) probe velocity (blue lines indicate period of probe acceleration; green, red, and purple lines show time of initial tissue contact [0ms in A], full probe deceleration [7ms in A] followed by reversal of probe direction initially by tissue recoil, and subsequently by retractor arm activation [16ms in A, dashed outline showing resting arm position]), along with calculated pre-contact probe acceleration, velocity, and kinetic energy; peak and mean force; peak negative acceleration; mean stress; and extent and mean rate of tissue deformation (see Supplemental Methods for calculations); (ii) surface ECG showing mechanically-induced premature ventricular excitation; and (iii) intra-ventricular balloon pressure, along with pressure amplitude and rise/fall times.

Experimental Protocols

Four experimental series were performed.

Series 1 tested inducibility of PVE_M and VF by local mechanical stimulation (n=32; n-numbers = number of heart preparations). Sub-contusional mechanical stimuli of ~0.5mJ were applied to 3.1mm² contact areas across the LV-freewall, and additional locations in some of these hearts: LV-apex (n=4); LV-base (n=4); LV/RV-border (n=4); RV-freewall (with an RV intraventricular balloon, n=11). Coupling interval to the preceding sinus-beat was shortened (5ms steps) from late-diastole until either VF was induced or PVE_M ceased to be elicited (entering the electrical refractory period).

Series 2 investigated determinants of PVE_M threshold and compared properties of mechanically- and electrically-induced ventricular excitation (n=7). In all hearts, local mechanical stimuli were applied to various locations of the LV- and RV-freewall using 3.1 or 28.3mm² probes in randomized order during late diastole (at ~75% of the cycle length). Stimulation energy was reduced from ~0.5mJ until PVE_M ceased to occur (i.e., when below threshold). At each of the mechanically targeted LV locations, hearts were stimulated electrically, using a concentric bipolar stimulation electrode.

Series 3 assessed the roles of SAC_NS and K_ATP channels in mechanically-induced responses using pharmacological blockers of SAC_NS (50, 250, and 500μM streptomycin, n=6; 500nM Grammostola spatulata MechanoToxin-4 [GsMTx-4], n=5) and K_ATP channels (5 and 10μM glibenclamide, n=6).

Series 4 examined electrophysiological effects of intra-ventricular pressure surges (n=6). A rapid change in LV balloon volume (20μL), producing pressure amplitudes that mimicked those seen during ~0.5mJ epicardial mechanical stimulation, was applied by active balloon in- and deflation during diastole. This was repeated with volumes raised by 22μL increments, normally up to 130μL.

Statistics

Data analysis was performed using custom programs in Matlab. Values are presented as mean ± standard deviation. For statistical tests, p<0.05 was taken to signify statistically significant differences between means. Comparison of local mechanically- and electrically-induced excitation and the effects of glibenclamide application were assessed by Wilconox signed-rank test (as distribution normality cannot be assumed). Mechanical stimulation characteristics at PVE_M threshold were compared by two-way ANOVA with Tukey-Kramer post-hoc test. Linear correlation between the change in intra-ventricular balloon volume and pressure surge amplitude, rise-time, or fall-time was assessed by Pearson’s correlation.

Results

PVE_M and VF Induction

All local mechanical stimuli applied to the ventricular epicardium with an amplitude of ~0.5mJ and a coupling interval outside the refractory period (n=32 hearts; m=409 stimuli analyzed, all with confirmed lack of elevated creatine kinase activity), irrespective of stimulation location, resulted in PVE_M. This was associated with a change in activation pattern compared to sinus rhythm (compare Fig. 2A/B) which, in terms of epicardially-mapped V_m dynamics, generally propagated in an apico-basal direction, frequently involving multiple sites of epicardial breakthrough (Fig. 2A; Supplemental Movie S2). In 6 of these hearts, the inducibility of PVE_M was also tested prior to application of blebbistatin (i.e., while still beating), which similarly showed 100% incidence (m=60), demonstrating that PVE_M induction is not conditional upon electro-mechanical uncoupling.

Figure 2.

Left ventricular excitation visualized by epicardial optical mapping. Representative recordings during sinus rhythm (A) and mechanically- (B) or electrically-induced (C) excitation (from Supplemental Movies S2-S4) and maps of activation time (0ms represents earliest epicardial activation in the associated map). Mechanical and electrical stimulation were triggered at the same mid-left ventricular (LV) freewall location (green dots). Isochrones represent 2ms steps. Note that the dark region at the mechanical stimulation location is an artefact caused by the probe entering the field of view, and the notch at the LV apex is due to the surface ECG electrode.

All PVE_M, including those in the absence of an electro-mechanical uncoupler, originated focally from the stimulation site, confirmed by optical mapping (Fig. 2B; Supplemental Movie S3). Mechanically- and electrically-induced ventricular excitation, triggered at the same location, resulted in down-stream patterns of electrical activation (compare Fig. 2B/C, Supplemental Movies S3/S4) more similar to one-another than to sinus activation (see Table 1 for dV_m/dt_max, AP duration, and conduction velocity; n=6, m=18).

Table 1.

Comparison of excitation induced mechanically or electrically at the same site (mid-level LV-freewall).

Type of Stimulation	dVm/dtmax (Fn/ms)	APD50 (ms)	APD90 (ms)	CVmax (cm/s)
Mechanical	75±11	136±12	163±8	98±21
Electrical	71±8	134±11	164±10	92±27
p-Value	0.219	0.313	0.875	0.438

APD_X indicates action potential duration at X%-repolarization; CV_max, conduction velocity in ‘fiber’ direction; dV_m/dt_max, maximum rate of transmembrane potential change (in nominal units of normalized fluorescence, F_n, per ms). Measurements are averaged from 6 hearts, with 3 stimuli of each type at 3 different sites per heart. p-values are for comparisons between mechanical and electrical stimulation by Wilconox signed-rank test.

Most PVE_M-inducing stimuli resulted in a single ventricular activation. In 3 of 32 hearts, local mechanical stimulation during the T-wave gave rise to instantaneous induction of VF (Fig. 3A; Supplemental Movie S5). VF episodes lasted 30-60s before spontaneously converting to sinus rhythm. Where observed, mechanically-induced VF was repeatable (m=13), when stimulation site and coupling interval were kept constant (including attempts with reduced mechanical stimulation energy, as long as supra-threshold for PVE_M, m=5).

Figure 3.

Mechanically-induced ventricular fibrillation (VF). A, Surface ECG showing mechanical stimulation in early T-wave, resulting in instantaneous VF (see Supplemental Movie S5). B, Spatial interrelation of stimulation site and 50%-repolarization isochrone (green). C, Comparison with previously published computational two-dimensional modeling predictions,²⁷ showing mechanically-induced premature ventricular excitation (PVE_M; red) arising directly adjacent to inexcitable tissue (yellow), forming a region of functional block (black rectangle) around which sustained re-entry occurs (adapted from²⁷, with permission).

In all cases of VF induction, optical mapping showed a well-defined trailing edge of the preceding depolarization wave, traversing the epicardial surface at the time of mechanical stimulation (Supplemental Movie S5). VF-inducing PVE_M induction sites corresponded to the 50%-repolarization isochrone (Fig. 3B), supporting prior two- (Fig. 3C)²⁷ and three-dimensional²⁸ computational model predictions. A different repolarization pattern, near-uniform repolarization of the epicardial surface (Supplemental Movie S2), was seen in ~90% of hearts. In these preparations, PVE_M did not cause VF at any coupling interval or impact location.

Determinants of PVE_M Threshold

For mechanical stimuli, applied during late diastole to either the LV- or RV-freewall using different probe contact areas, PVE_M threshold corresponded to significantly different values of mean rate of local tissue deformation, peak force, mean stress, and intra-ventricular pressure surge amplitude (n=7, m=405), suggesting that none of these parameters scale well with arrhythmogenicity. However, local tissue indentation, required for PVE_M induction, was similar across all groups (Fig. 4).

Figure 4.

PVE_M thresholds. Box-and-whisker plots of mechanical stimulation characteristics (pre-contact kinetic energy, mean deformation rate, peak force, mean stress, pressure surge amplitude, local tissue deformation) at PVE_M threshold with varying combinations of freewall contact location (mid-LV [blues boxes] and mid-right ventricular [RV; red boxes]) and area (3.1mm² [lighter boxes] and 28.3mm² [darker boxes]). Only local tissue indentation (far-right) forms a uniform predictor of PVE_M threshold across the various stimulation protocols. Measurements from 7 hearts. p-values are for comparison of contact location and area by two-way ANOVA with Tukey-Kramer post-hoc tests.

Role of SAC_NS and K_ATP Channels

Application of GsMTx-4 (500nM) reduced PVE_M-inducibility in all preparations, on average by 62% (n=5, m=48/77 PVE_M inductions prevented). In contrast, streptomycin (up to 500μM) had no effect on PVE_M-inducibility (n=6, m=0/270 PVE_M prevented).

The K_ATP channel blocker glibenclamide (up to 10μM) also had no effect on PVE_M-inducibility (n=6, m=0/78 PVE_M prevented), but resulted in a ~10% decrease in heart rate (from 138±10 to 125±13 beats/min; p=0.031). This was associated with slowed repolarization (Supplemental Fig. S2) and a 12% increase in refractory period for PVE_M (from 218±32 to 244±41ms; p=0.031).

Electrophysiological Effects of Intra-Ventricular Volume Pulses

Pressure surges, observed during epicardial mechanical stimulation (Fig. 1B), were mimicked by application of intra-ventricular volume bursts. Pressure surge amplitude (r=0.904, p=1.07×10^-7; Fig. 5A) and pressure rise-time (r=0.723, p=0.008) increased linearly with the size of rapid intra-ventricular balloon volume change, while pressure rise-time was not correlated with the change in balloon volume (r=0.428, p=0.166).

Figure 5.

Intra-ventricular volume pulses. A, Relationship between intra-ventricular balloon volume pulse and peak-pressure, averaged from 6 hearts. Red points indicate volumes assessed in the standard experiments (which were not associated with elevated creatine kinase release; Supplemental Fig. S3), while a PVE_M-inducing excessive volume pulse (200μL) is shown in green. The gray horizontal band indicates the range of pressure amplitudes measured during PVE_M-inducing epicardial mechanical stimulation. B, Representative pressure surges measured during epicardial mechanical stimulation (black) and with active injection/retraction of 20μL (left) or 130μL (right) intra-ventricular volume pulses (red). C, Representative optical mapping-derived data for 90%-repolarization time (0ms represents earliest repolarization in the associated map) in sinus rhythm during (i) control and application of (ii) 15mmHg or (iii) 124mmHg pressure surges (using stimuli shown in B) applied at the peak of the T-wave. Isochrones represent 5ms steps. Note that the notch at the LV apex is due to the surface ECG electrode in the field of view.

Diastolic pressure surges (all with confirmed lack of elevated creatine kinase activity), even if about an order of magnitude larger than those measured during PVE_M-inducing epicardial stimuli (101±27 vs. 15±3mmHg, induced by volume bursts of 130 vs. 20µL, respectively), did not trigger a single PVE_M (Fig. 5B; n=6, m=108). If applied during the ECG T-wave, no change in repolarization pattern was observed (Fig. 5C; n=6, m=108). In contrast, pressure surges of 178±21mmHg (induced by volume bursts of 200µL) were needed to trigger excitation (Fig. 5A).

Discussion

Summary of Principal Findings

PVE_M is induced reliably in diastole, using sub-contusional local stimuli (~0.5mJ in isolated rabbit heart). While accompanied by intra-ventricular pressure surges (~15mmHg), the origin of electrical activation always coincides with the mechanical contact site (similar to repetitive local mechanical stimulation³⁵). PVE_M induction is correlated with the degree of local tissue indentation, but not with indentation rate, force, stress, or intra-ventricular pressure surge amplitude, in keeping with earlier suggestions that mechanical deformation at the contact site is a key determinant for mechano-electric signal-transduction.³⁴ Underlying mechanisms involve SAC_NS, as PVE_M induction is reliably attenuated by the specific³⁶ SAC_NS-blocker GsMTx-4. The lack of streptomycin effects, which is an efficient SAC_NS-blocker in vitro,³⁷ confirms results from a porcine model of CC³⁸ and is in keeping with earlier reports on the limited efficacy of streptomycin for acute SAC_NS-block in native myocardium.³⁹ The K_ATP-blocker glibenclamide has no effect on PVE_M-inducibility per se, but shifts PVE_M to later time-points as a result of reduced sinus rate, delayed repolarization, and prolonged electrical refractoriness. As a result, impacts applied early after the original refractory period, identified prior to drug application, can become ineffective.

With reduction of the coupling interval between preceding sinus activation and mechanical stimulation, PVE_M continues until stimulation timing enters the refractory period. If, and only if, repolarization is associated with a well-defined wave-edge, PVE_M reliably and repeatably triggers VF. This occurs only if stimulation site and timing are such that PVE_M induction overlaps with the 50%-repolarization isochrone of the preceding sinus beat.

In contrast, intra-ventricular pressure surges mimicking those seen during PVE_M-inducing epicardial stimuli do not result in a single occurrence of PVE_M, nor in noticeable changes in repolarization, even if increased in amplitude by an order of magnitude.

Taken together, these results suggest that ventricular excitation upon local mechanical stimulation is a result of SAC_NS-activation, due to local tissue deformation. The presence of a well-delineated trailing repolarization wave, and spatio-temporal co-localization of mechanically-affected tissue with the trailing edge of the preceding wave, are preconditions for VF induction. The vulnerable window for CC-induced VF is thus defined in both time and space, explaining the short period during which mechanical stimuli can induce re-entry for any given contact location, and why mechanically-induced VF is so rare in real life. Our findings may have future implications for predictive assessment of individual athletes’ CC-risk (a target which until now has been lacking⁴⁰), through measurement of ventricular repolarization patterns using non-invasive imaging techniques such as electrocardiographic imaging⁴¹ or overall dispersion of repolarization by the TpTe interval⁴² (whose prolongation has been shown to be an independent risk-marker for sudden cardiac death⁴³).

Mechanisms of PVE_M Induction

Previously reported PVE_M induction by transiently increased intra-ventricular volume in isolated hearts^{11, 44–46} occurred in different studies at remarkably similar pulse volumes (which define tissue stretch; e.g., 750μL pulses resulted in near-100% PVE_M occurrence in hearts isolated from ~2kg rabbits¹¹). However, associated changes in intra-ventricular pressure (a function of the interaction between volume pulse dynamics and tissue visco-elastic properties⁴⁷) show high variability.⁴⁵ In addition, where seen, ventricular excitation upon volume loading is focal, often originating in the posterolateral LV,¹¹ the most compliant region where tissue distension is likely to be largest.

This suggests that stretch of (part of the) myocardium is a primary determinant of electrophysiological responses to mechanical stimulation. Our results support this view, as the threshold for PVE_M, regardless of contact location (LV, RV) and area (3.1mm², 28.3mm²), correlates with local tissue deformation only (indentation depth). In our hands, independently applied intra-ventricular pressure surges up to an order-of-magnitude larger than those seen with epicardial stimulation do not result in PVE_M. Note that pressure surges in our study were achieved by rapid application of comparatively small volume pulses (20-130μL), using active balloon in- and deflation to mimic more closely the CC setting, where pressure surges: (i) are exceedingly short-lived and (ii) arise without intra-ventricular volume increase.

Our results also identify SAC_NS as a key contributor to PVE_M, as application of GsMTx-4 reduces its inducibility. This role of SAC_NS in depolarizing cells to threshold during mechanical stimulation is further reflected by the lack of an additional delay between mechanical stimulation and the onset of electrical excitation when the coupling interval is reduced (as SAC_NS are not inactivated by normal AP dynamics).

While mechanisms by which external energy delivery give rise to ectopy are clearly different for mechanical and electrical stimulation, our results confirm that, once excitation and ventricular activation occur, their downstream characteristics are similar, as previously shown in open-chest, anesthetized dogs⁴⁸ and rat isolated hearts (in which similar downstream calcium dynamics were also seen).³³ Electrical stimulation initially excites a narrower tissue region than mechanical stimulation (elongated in the locally-prevailing cell-direction; compare early activation isochrones in Fig. 2B/C and Supplemental Movies S3/S4), as local electrical stimulation affects a smaller tissue region (contact area of the stimulation electrode was one-hundredth that of the mechanical probe, 0.031mm² vs. 3.1mm²).

Mechanisms of VF Induction

In the porcine model of CC, a critical determinant of electrophysiological outcome is precordial impact timing relative to the ECG.^{8, 24, 25} Impacts just prior to the T-wave peak can induce VF, while impacts at other stages of the cardiac cycle result in different transient rhythm disturbances.⁴⁹ The vulnerable time-window for precordial impact-induced VF in pig is short: ~15ms,⁴⁹ compared to ≥100ms for extracorporeal electrical stimulation in other large animals (dogs).⁵⁰ In smaller hearts, vulnerable windows are narrower: we could not trigger VF if impact timing varied by ±5ms, while the electrical vulnerable window in rabbit is ~20-25ms.⁵¹

A pig model of CC also highlighted a correlation of VF-inducibility with the amplitude of impact-induced intra-ventricular pressure surges, suggesting that the rapid pressure increase may be causal for electrical effects.²⁹ Furthermore, block of K_ATP channels by glibenclamide reduced VF-occurrence.²² This observation motivated the hypothesis that CC-induced VF represents an acquired form of abnormal repolarization.^{8, 24}

The role of intra-ventricular pressure surges is supported by experiments with acute LV balloon inflation in rabbit isolated hearts.⁴⁴ Pressure surges of >200mmHg, caused by volume injections of 800-1600μL (a ~80-160% increase in intra-ventricular volume; no assessment of tissue integrity reported) result in increased dispersion of repolarization and, occasionally, VF (11% of volume pulses), in an amplitude/timing dependent manner. In our hands, using hearts of similar size, intra-ventricular volume increases greater than 500μL cause structural damage (assessed by creatine kinase release; Supplemental Fig. S3). In contrast, the exceedingly short-lived pressure surges during CC occur in the absence of a rise in intra-ventricular volume. Given that tissue visco-elasticity dampens the translation of mechanical stress into stretch, it is likely that CC-induced pressure surges cause little, if any, tissue distension beyond what is induced locally by precordial impact. In keeping with this suggestion, excitation in the in situ pig heart during CC-inducing impacts is focal, and originates from tissue immediately underneath the extracorporeal impact site.⁵²

In the present study, rapid infusion and active retraction of 20μL into the LV-balloon results in pressure surges similar in amplitude to those seen during PVE_M, albeit with slightly slower dynamics (as expected from an intervention that is not isovolumic per se). Increasing volume pulses up to 130μL (~10-15% of LV volume) gives rise to pressure surges up to an order-of-magnitude larger than those during PVE_M-inducing stimulation, but still does not result in excitation or marked changes in repolarization (volumes exceeding ~200μL are needed to trigger PVE_M in diastole; Fig. 5A). This suggests that precordial impact-induced pressure surges are not the main driver of mechanical VF induction, but a covariate of impact severity.

Of course, it is possible that pressure surges have effects on repolarization in whole animals that are not preserved in isolated hearts, in particular if they involve nervous system responses. Given that CC-induced VF is near-instantaneous and that mechanically-induced changes in cardiac electrophysiology persist in situ after surgical and/or pharmacological denervation,⁵³ it remains unlikely that these would be major determinants of electrophysiological outcomes.

In terms of possible roles of K_ATP channels in CC-induced VF, it is noteworthy that under conditions of normal oxygen supply, K_ATP channels are inactivated⁵⁴ and not responsive to mechanical stimulation.^{19, 21, 55} In our experiments, application of the K_ATP channel blocker glibenclamide has no effect on PVE_M-inducibility, which is in agreement with previous reports from anesthetized pigs.²² We observe a glibenclamide-induced slowing of sinus rate, delayed repolarization, and an increase (by ~25ms) in refractory period for PVE_M. This is in keeping with non-specific effects reported for glibenclamide.^{56, 57} The observed changes in repolarization timing and refractoriness exceed the narrow vulnerable time-window for VF-inducibility. Therefore, glibenclamide application may shift vulnerability for mechanical VF induction past critical timings, identified in control conditions. This could explain the previously reported reduction in VF-occurrence during precordial impact with glibenclamide in the pig, where impact timings that previously induced VF failed after drug application.²²

In contrast to the view that mechanically-induced VF arises primarily out of repolarization abnormalities, two-²⁷ and three-dimensional²⁸ computational modelling, including representations of SAC_NS, suggests that CC-induced re-entry is a consequence primarily of abnormal excitation. In particular, if a mechanical stimulus overlaps the trailing edge of the normal repolarization wave, it can induce VF by causing PVE_M in tissue that has regained excitability (i.e., where V_m levels are below SAC_NS reversal potential), while at the same time not only increasing electrophysiological heterogeneity by regional AP-shortening in tissue that is still excited (i.e., where V_m levels are above SAC_NS reversal potential) but, crucially, by forming a region of functional conduction-block at the intersection of the normal repolarization wave-edge and the PVE_M-induced excitation. Our results support this prediction. In all cases where epicardial stimulation resulted in VF, the mechanically-affected tissue overlapped with the 50%-repolarization isochrone, traversing the epicardial surface. The vulnerable time-window for CC-induced VF is therefore location-specific, existing both in time and in space.

Limitations

The key limitation is the low incidence of local mechanical stimulation-induced VF (n=3/32 animals, 9%), which prevented a more systematic assessment of the vulnerable window. This incidence is in keeping, however, with the exquisite dependence of VF induction on repolarization dynamics, stimulation site, and stimulation timing, whose conditions will be met only rarely, in hearts that display a specifically well-delineated repolarization wave. The necessary conditions occurred in a small sub-set of hearts only, due to inter-subject variability, but in hearts that fulfilled the preconditions, VF was reliably induced. The low incidence of VF in our study is in line with the rarity of CC-induced VF in humans⁸ and the individual susceptibility in the present gold standard in vivo model of CC.⁵⁸ In addition, in physiological conditions Langendorff-perfused rabbit hearts show an exceedingly low probability of VF induction also using electrical stimulation.⁵⁹ This could be further compounded by a reduction in arrhythmia susceptibility by blebbistatin,⁶⁰ suggesting that our observations form a conservative estimate.

Conclusion

Our findings demonstrate that local sub-contusional mechanical stimuli can reliably trigger PVE_M, originating at the probe-tissue contact site, and require SAC_NS, whose activation scales with the degree of local tissue deformation. PVE_M cause VF if, and only if, there is overlap of mechanical stimulation with the trailing edge of the preceding repolarization wave. As a result, there is a subject-specific set of vulnerable windows for CC-induced VF both in time and in space.

What is Known.

• Mechanical impact-induced changes in cardiac electrophysiology (Commotio cordis) can result in ventricular fibrillation, with a generally low incidence attributed to a short vulnerable period during the ECG T-wave.

• Possible mechanisms include abnormal repolarization caused by intraventricular pressure surge-mediated opening of stretch-activated potassium channels (SAC_K), or premature excitation caused by local tissue deformation-mediated opening of stretch-activated cation non-selective channels (SAC_NS).

What the Study Adds.

• T-wave impact-associated pressure surges have no electrophysiological effect, local deformation induces ectopic excitation that can be prevented by pharmacological block of SAC_NS, and block of SAC_K plays no relevant role except shifting the vulnerable period.

• Ventricular fibrillation occurs if, and only if, mechanically induced premature excitation occurs right on the trailing edge of the preceding normal repolarization wave as it traverses the ventricles.

• This means that the vulnerable window for mechanical induction of ventricular fibrillation is short (in time) and narrow (in space), explaining why mechanically-induced ventricular fibrillation is rare.