الفهرس 
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Abstract
Pacemakers proved to have a significant impact on the quality of life of patients suffering from conduction problems (1). However, pacing itself comes with the price of inducing alterations in myocardial functions that may have deleterious clinical consequences (2). Pacing from implantable cardiac devices differs from the natural heart conduction in having slower electrical conduction velocity as well as altering the natural sequences of myocardial activation (3).
One of the serious consequences of RV pacing is the development of pacing-induced cardiomyopathy (PICM) (4). Dys-synchrony is considered one of several mechanisms that RV pacing can induce myocardial dysfunction. Right ventricular apical (RVA) pacing can induce both inter-ventricular as well as intraventricular dys-synchrony (5). In addition, the presence of mechanical dys-synchrony after long-term RVA pacing is associated with reduced LV systolic function and adverse clinical outcome (2).
RVA pacing was shown to induce a decrease in regional strain near pacing sites (apex) compared to remote regions (middle and base) indicating that RVA pacing has an unfavorable impact on the LV performance (7). In addition, RVA pacing appeared to affect the RV performance by worsening tricuspid regurge, an effect which might be caused by elevated LV filling pressure due to LV dysfunction (9). Furthermore, RA pacing was shown to significantly increase interatrial and intra-left atrial mechanical delay and dys-synchrony which can lead to suboptimal LV performance (7) (11) (12).
Aim of the study
The purpose of this work was to study the effects of variable pacing modes of dual chamber pacemakers on myocardial strain using two-dimensional speckle tracking echocardiography (2D STE).
First of all, we planned to study the effects of 6 months of RV apical pacing on LV and RV longitudinal systolic strain (LSS) in comparison to controls. In addition, we aimed to compare the effects of changing pacing modes from AS-VP to AP-VP to VVI on global LV LSS. Finally, we investigated the effect of RAA pacing on time to peak (TTP) atrial LSS to evaluate the inter-atrial and intra-left atrial mechanical delay.
Subjects
This study was carried out on sixty subjects in the cardiology department of the faculty of medicine, Menoufia University. Subjects included thirty patients and thirty age and sex matched controls. During the period between February 2018 and February 2020, the patients enrolled were identified as individuals implanted with dual chamber pacemakers (DDD) in our department for the management of permanent third-degree atrio-ventricular (A-V) block. The patients were evaluated after 6 months of device implantation during their routine programming session. Subjects were included in this study after obtaining their written informed consent and acquiring the approval of the Ethics Committee of the Menoufia University. The subjects were segregated into the following groups;
group I (n=30): Controls with normal sinus rhythm and normal A-V conduction
group II (n=30): Patients implanted with DDD pacemakers for the treatment of permanent 3rd degree A-V block. All patients
underwent different programming modes for their devices and were thereby represented as 3 groups:
- group (II-a: AS-VP mode): in which AS: atrial sense and VP: ventricular pace. This was the baseline pacing mode for the patients and was used for echocardiographic comparison with controls.
- group (II-b: AP-VP mode): in which AP: atrial pace and VP: ventricular pace “maintaining A-V synchrony” at a rate of 100 b/min.
- group (II-c: VVI mode): in which VV: ventricular pacing and sensing and I: inhibition of discharge by sensed ventricular event i.e. asynchronous ventricular pacing mode at 100 b/min.
Methods
All subjects underwent full history taking, clinical examination and ECG. All subjects underwent conventional and two-dimensional speckle tracking echocardiography examination with vivid 9, general electric health care (GE Vingemed, Norway) machine equipped with a harmonic M5S variable frequency (1.7-4MHz) phased – array transducer. Offline analysis was done using EchoPac version 113. Conventional echocardiography included M Mode assessment of the LV dimensions and EF. 2D Simpson’s technique was also used to assess LV EF. Right ventricular basal dimension and TAPSE were also evaluated. Doppler was done for the valves (CW and PW). Measurement of aortic pre ejection delay, pulmonary pre ejection delay and interventricular mechanical delay was also done. Two-dimensional speckle tracking echocardiography was used to assess the global and segmental longitudinal systolic strain of the RV and the LV at the time of aortic valve closure. Also, time to peak longitudinal systolic strain (TTP LSS) of the left atrial lateral wall
and interatrial septum were assessed to evaluate the interatrial and intra left atrial mechanical delay.
Results
There was no statistical difference between the controls (group I) and the patients (group II) regarding age, gender and risk factors as smoking, diabetes mellitus or hypertension (P value > 0.05). The QRS duration in patients during AS-VP mode (group II-a) was longer than the controls (group I) (P value < 0.001). Regarding LV dimensions and EF by M Mode, there was no statistical difference between the two groups (P value > 0.05). On the other hand, EF by 2D biplane Simpson’s technique showed that the patients during AS-VP mode (group II-a) had a lower value than the controls (group I) (P value < 0.05). The RV basal dimensions did not show a difference between the patients (group II-a: AS-VP) and controls (group I) (P value > 0.05), however the TAPSE was lower in the patients during AS-VP mode (group II-a) (P value < 0.001). The measurement of aortic pre-ejection time (APET) revealed that the patients during AS-VP mode (group II-a) had a longer duration than controls (group I) (P value < 0.001). This difference is most probably due to prolonged conduction time in the left ventricular myocardium as a result of RVA pacing of the patients during AS-VP mode (group II-a) in comparison to fast conduction through the natural His-Purkinje system in controls (group I). This slow conduction further delays the ventricular activation and contraction which in addition to the dys-synchronous septal activation increases the aortic pre-ejection time (APET). The pulmonary pre-ejection time (APET) of the patients during AS-VP mode (group II-a) also had a longer duration than the controls (group I) (P value < 0.001). The measurement of the inter-ventricular mechanical
delay (IVMD) was done by subtracting the PPET from the APET. Results showed that patients during AS-VP mode (group II-a) had a longer duration than controls (group I) indicating interventricular dys-synchrony (P value <0.001).
Two-dimensional speckle tracking echocardiography demonstrated that in comparison to the controls (group I), RVA pacing in the patients during AS-VP mode (group II-a) resulted in the reduction of the global LV LSS (P value < 0.001). The global RV LSS was also reduced (P value < 0.001). Segmental analysis showed that the apical segments were the most significantly reduced (P value < 0.001).
Changing the pacing mode from AS-VP mode (group II-a) to AP-VP mode (group II-b) resulted in the reduction of the global LV LSS (P value < 0.05). Reprogramming of the device to VVI mode (group II-c) also resulted in a more significant reduction of the LSS when compared to AS-VP mode (P value <0.001). Two-dimensional atrial strain demonstrated that the time to peak longitudinal systolic strain (TTP LSS) of the left atrial lateral wall had a longer duration in patients during AP-VP mode (group II-b) than during AS-VP mode (group II-a) indicating the presence of interatrial mechanical delay (P value < 0.001). Inter left atrial mechanical delay calculated from the difference between TPP LSS of the left atrial lateral wall and the interatrial septum was found to have a longer duration in the patients during AP-VP mode (group II-b) than during AS-VP mode (group II-a) (P value < 0.001). This indicated that atrial pacing led to conduction delay in the atrial tissue that resulted in dys-synchronous mechanical contraction.
Conclusion
In conclusion, RVA pacing induced interventricular and intraventricular mechanical delay. In addition, the right and left ventricles showed reduction in the global LSS when compared to controls. The apical segments showed the greatest reduction. Changing the pacing mode of the patients from AS-VP to AP-VP resulted in the reduction of the global LV LSS. Reprogramming of the device to VVI mode also resulted in a more significant reduction of the LSS. By the use of TTP LSS, RA pacing in AP-VP mode resulted in inter-atrial and intra-left atrial mechanical delay.