The most important achievement of our research was the proof that the presence of IABs impair the synchrony of LA contraction, which we proved after analyzing LA regional strain. This method is complementary to precise ECG assessment. Moreover, we defined the factors which directly influence PALS and PACS.
Pathophysiology of IABs
When it comes to pathophysiology of IABs, we should first discuss different conduction pathologies described by Bayes de Luna (REF 6,8,9).
1. The full conduction block (A-IAB) happens, when the roof of LA, or the junction between LA and RA are blocked. The exact spot blockade is crucial, because it determines if the impulse has already entered LA before the blockade, or if it “has to” chose alternative pathways to enter LA. The site of the complete block is however rarely located in the junction, but rather in the roof of the LA (REF 13,14). What happens to the signal after it has entered LA, but then encountered the blockade in the roof of LA? This case was graphically presented in Figure 5 p.3. The impulses travel anteriorly, and inferiorly, surrounding the blocked region, which results in a caudocranial activation in the area of PV. So far, this phenomenon has been confirmed by Ramdat, who focused on the electrocardiographic mapping of the complete transversal conduction block of Bachmann bundle (A-IAB) (REF 15). The authors presented the pattern of conduction in a multicolor panel. (Supp. Fig. 1,2). What’s interesting, we acquired the similar patterns of contraction, basing only on the regional LA strain (Fig. 5 p.3). It means that the LA regions contract in the order which is consistent with the spreading wavefront in the PV area. This fact is important because echocardiography is incomparably less invasive than the epicardial mapping, and yet the results seem to be complementary.
When it comes to the block in the LA/RA junction, we should discuss the alternative ways of conduction, which – in our opinion - are more characteristic for the P-IAB, or to be more precise – remain in the grey zone between P-IAB and A-IAB. In that case the activation happens through the coronary sinus, or through the region of foremen ovale (REF 16). It is possible, because the Bachmann bundle is anatomically connected at the septal raphe with the fibers running from the rim of oval fossa (REF 17). The existence of the alternative ways of conduction were proven in the research by Jurkko (REF 18). The authors used the technology of intra-cardiac electroanatomic mapping. The electrical breakthrough to LA occurred through Bachmann’s bundle, margin of fossa ovalis (FO), coronary sinus ostial region (CS), or their combinations by invasive reference.
2. The other kind of IAB is a partial block of a Bachmann bundle (P-IAB). In this type, the activation slows down in the roof of LA, due to the damaged conduction fibers, which is registered in ECG in as a prolonged, double peaked P-wave (REF 19). The first peak reflects the activation of RA, and the other one – LA. However sometimes there happens an additional, parallel activation through foremen oval or the fibers of coronary sinus. This profile of activation results in a long, flat, and irregular P wave, due to multiple electric currents which activate LA from above and down (REF 20). Basing on our observations, such “long and flat” morphology in P-IAB happens more frequently than the typical double notched P wave. So far, the problem of numerous different P wave morphologies has been observed by Platonov (REF 16), but the questions about the hemodynamic followings and the prognostic values still remain unanswered.
It’s interesting that the same alternative ways of conduction potentially exist in both A-IAB and P-IAB, as we described in paragraph 1. We also used the expression of “the grey zone” between IABs, and judging from our results, it’s not far from reality. The statistic dependency was mainly significant between the normal condition, and any kind of IAB. At the same time, there was hardly any dependency between P-IAB and A-IAB.
Strain technology and measurements
The examination of strains is a method based on speckle tracking technology which registers the change in length of separate regions in time. (REF 21). The technology has been developed since 1991, and has grown more and more advanced (REF 22,23,24). In 2021 Kupczyńska published a review on the current knowledge on LA strain, basing on the overview of the previous works, which was helpful in determining the reference values and measurements (REF 25). The research which were included in the overview proved i.a. the correlation between LA strain measurements and the diastolic dysfunction of LV which can be helpful in diagnosing HFpEF on early stages. (REF 26,27). This correlation revealed the clear dependency between the condition of LV and LA strain values, which prompted us to include LV EF in our study. So far, most of the researches on LA strain measurements have been focusing on the raw percentage values and their correlations with the other parameters (REF 28). Interestingly, Watanabe presented the opposite approach and discussed the LA mechanic dispersion in their work published in 2015. (REF 29) This phenomenon was defined as the standard deviation of the time to peak positive strain corrected by the R-R interval. According to the authors, this parameter could be used to evaluate disturbances in the timing of LA contraction, which reflects the atrial fibrosis and electrophysiological disorders. It was one of the inspirations for our research, however we focused on the order of LA regional strain curves in order to confirm the profile of LA contraction in IABs.
Major correlations in our study
One of our most important achievements was the definition of the factors which directly influence the global LA strain. We found that PALS is independently correlated with the P wave duration and HF. PALS corresponds to LA early diastole, which means the longer the P wave duration is, the more impaired the conduction, as well as the ability to diastole must be. This means that PALS may potentially be considered as one of the variables for HF diagnosis. Putting in another words - the parameters of IABs, P wave measurements, LA strain, and HF correlate with each other which may be the ground for the further research of HFpEF pathomechanism. Discussing the presence of HF in co-existence of impaired atrial conduction we must mention the definition of “Bayes' Syndrome” (REF 30,31). In his study, Bayes de Luna drew the conclusion, that in patients with HF, the A-IAB predicts new-onset AF and ischemic stroke. To make the whole picture more complete, in our study there was a strong correlation between:
- The presence of IABs and the episodes of AFP (p>0.001, Table 3).
- The presence of A-IAB and the decreased PALS, PACS (Table 2, p >0.001)
- The decreased PALS, PACS and the presence of HF (reduced LV EF).
Interestingly, we found no clear correlation between the reverse flow in PV and the type of IAB. There have been no significant publications on this topic so far, nevertheless the topic is still open in our opinion. When it comes to the phenomenon of reversed flow in PV, it has been mainly raised in regard to mitral regurgitation (MR). J B Mark stated that MR jet direction isn’t likely to cause predictable alteration in PV flow patterns. (REF 32). Similarly, M.Enriques-Sarano indicated that systolic PV reversal flow is a useful sign of severe MR but of relatively low sensitivity, so it’s no surprise that the MR didn’t present any correlation in our research either (REF 33). We believe however, that the phenomenon of IABs is more complex and there might exist more subtypes of IABs which may result in some significant changes. The topic requires further research among more numerous study groups.
Correlations with comorbidities
One of our major findings was the correlation between arterial hypertension and overweight. Our study group was limited by its number, that is why it was reasonable to compare our findings with the great cross-sectional studies. For example Guozhe Sun et al. in his study on 11 956 patients found that the prevalence of IAB was higher in hypertensive individuals which is definitely consistent with our statistics (REF 34). Similarly, the presence of IAB was also higher with increasing body mass index (BMI). Considering the comorbidities, which didn’t present any relevant correlations with LA strain in our research, we can enumerate: DM2, chronic kidney disease, ischemic heart disease, asthma and chronic obstructive pulmonary disease (COPD). Interestingly, the epidemiological analysis by Vyas V. (REF 35) has shown the connection in many studies between obesity, DM 2 and the risk for AF. It was suspected the obesity remains the major factor in this case – as it was proven in a large cohort study of 67,238 patients (REF 36). Considering DM2 however, the long term hyperglycemia results myocardium fibrosis, making it more susceptible for AF onsets. These circumstances could make DM2 an independent factor for the changes in atrial strain, however there exist common fibrotic mediators (TGF-beta family) in pathomechanism of both obesity and DM2 (REF 37). The exact mechanism, however is still unclear, and there aren’t too many studies on atrial strain analysis regarding this connection, so the problem remains under research. The presence of COPD and stable coronary disease (CAD) and their negative influence on LA strain have been discussed in the study of Pavasini. (REF 38). LA strain was significantly lower in patients with COPD and stable CAD, contrasted to the control group. However, it wasn’t clearly stated how the presence of CAD influenced the independency of COPD as a factor. When it comes to our study group, we should have in mind, that our patients suffered from acute COVID 19, which means that everybody could have potential respiratory problems, despite the fact not everybody required mechanic ventilation. Such conditions question the objectivity of the influence of COPD or asthma as the independent factors in our research.