In this study, programmed premature stimulation S2 synchronized P wave positively swept ventricle and ECG of antegradely conducting AP was successfully simulated in the rabbit model including complete preexcitation, incomplete (typical) preexcitation, incomplete latent preexcitation and complete latent preexcitation. This confirmed that the degree of preexcitation depended on the time difference of conduction through AP and normal pathway.
Feasibility in this study
The canine model of preexcitation syndrome has been reported in clinical ECG study [8]. In our rabbit model, sensing electrode was placed to high right atrium (sense sinus P wave) and stimulating electrode was placed to atrioventricular groove of left ventricular anterior wall (simulate preexcitation through a left anterior AP). Programmed premature S2 which synchronized P wave (PS2 interval was equal to the conduction time through AP) performed stimulation and built the animal model of preexcitation syndrome in which sinus P wave can simultaneously conduct through normal pathway and left anterior AP. Various degree of preexcitation (complete preexcitation, incomplete preexcitation, incomplete latent preexcitation and complete latent preexcitation) were obtained during changing PS2 interval, confirming this study was a feasibility study. It can apply to the study of clinical ECG of preexcitation syndrome [9].
The manifestation and influencing factor of antegrade conduction of AP in preexcitation syndrome
The preexcitation syndrome with AP capable of antegrade conduction could be divided into overt preexcitation and latent preexcitation. In latent preexcitation, the preexcitation is absent on the resting 12-lead ECG, which could be induced by transesophageal atrial pacing. Though there is no delta wave, the prevalence of arrhythmia and the risk of malignancy are same as the overt preexcitation [10,11,12,13]. In this study we successfully imitated the ECG manifestation of 4 type preexcitation including complete ventricular preexcitation, incomplete ventricular preexcitation, incomplete latent preexcitation, complete latent preexciation. The finding of this study further confirms that the ECG manifestation of antegrade conduction of AP depends on the relative time that the atrial impulse is conducted down the AP into ventricle as opposed to the normal pathway. (1) When the AP conduction is faster than the AV node conduction, the overt preexcitation is observed on surface ECG. The degree of preexcitation depends on the how much faster the atrial impulse is conducted down the AP into ventricle as opposed to the normal pathway. If the relative time that the atrial impulse is conducted down the AP into ventricle as opposed to the normal pathway is longer than the time that impulse transmits from the ventricle that is directly connected to AP to the normal pathway (the time of this group is 47.00 ± 7.53 ms), the atrial impulse can’t conduct into ventricle via the normal pathway, resulting in the complete ventricular preexcitation. If the relative time that the atrial impulse is conducted down the AP into ventricle as opposed to the normal pathway is no longer than the time that impulse transmits from the ventricle that is directly connected to AP to the normal pathway, the impulse can conduct into ventricle by the normal pathway and AP forming the monophyletic ventricular fusion, resulting in incomplete ventricular preexcitation (Fig. 2). (2) When the AP conduction is slower than the AV node conduction, the delta wave is absent. If the relative time that the atrial impulse is conducted down the AP into ventricle as opposed to the normal pathway is no longer than the time that impulse transmits from the normal pathway the ventricle that is directly connected to AP (the time of this group is 13.00 ± 3.50 ms), the AP is still able to produce premature ventricular depolarization in the part of the ventricle that is directly connected to the AP forming the ventricular fusion, resulting in the change of terminal QRS vector. During this time the incomplete latent preexcitation could be seen in the ECG (Fig. 3). If the relative time that the atrial impulse is conducted down the AP into ventricle as opposed to the normal pathway is longer or equal to the time that impulse transmits from the normal pathway the ventricle that is directly connected to AP, the impulse can’t conduct into ventricle via AP (encounter the effective refractory period of the ventricle), showing complete latent preexcitation in the ECG.
The clinical significance of different degree of ventricle preexcitation
In this study the overt preexcitation was divided into complete and incomplete ventricular preexcitation based on the ECG manifestation. The clinical significance is as follows: as to the incomplete ventricular preexcitation which is the classical manifestation of preexcitation syndrome, it is easy to identify; as to the complete ventricular preexcitation, it is easy to misdiagnose resulting from lack of recognition. (1) When AP closes to the pacemaker (atrial premature beats), the intra-atrial conduction time is significantly shortened, resulting in the superimposition of P′ wave and QRS complex. During this time the PR interval is indistinguishable, and the premature atrial contraction is often misdiagnosed as premature ventricular contraction [14]. Furthermore, the polarity of initial QRS vector is identical to the delta wave, the finding of premature P′ wave is helpful for the diagnosis. (2) When the AV node conduction is obviously slower than AP conduction, such as first or third degree atrioventricular block (AVB) in normal pathway with complete ventricular preexcitation, the AVB in normal pathway is masked which is usually misdiagnosed in clinic [15,16,17]. During this time, the QRS complex is obviously widened and the PJ interval should be measured. The PJ interval prolongation provides that there is AVB in the normal pathway [15, 16, 18]. It is important for preoperative preparation and postoperative medical malpractice prevention to identify the presence of AVB according to ECG analysis before ablation.
In this study the latent preexcitation was divided into “incomplete latent preexcitation” and “complete latent preexcitation”. The clinical significance is as follows: the absence of delta wave is not means that the impulse can’t conduct down AP into ventricle. In patients with incomplete latent preexcitation syndrome, the delta wave is absent, but the impulse can conduct into ventricle via AP resulting in the change of terminal QRS vector [6]. The recognition of this new type preexcitation theoretically updates the recognition of delta wave. Meanwhile, it is helpful for the diagnosis of preexcitation syndrome mainly with a change of terminal QRS vector and the analysis of curative effect of bypass ablation [19,20,21]. However, it is not easy to observe the change of terminal QRS vector which could be found in comparison to the ECG during atrioventricular reentrant tachycardia or after ablation.
The multiple leads were not synchronously detected, and it had a certain influence on the accurate analysis of the terminal QRS vector. It will be helpful to analyze the relationship between the terminal QRS vector and the AP location if the multiple leads can be simulated. Besides, in previous study [22,23,24], researchers have observed the effects of ionic channel, gene, oxidative stress and inflammation on arrhythmias, and ventricular depolarization. Therefore, further studies are needed to assess whether these factors have influences on the electrocardiogram of antegradely conducting accessory pathway in future.