Clinical Study

We prospectively recruited 23 children (9 males), who underwent an electrophysiology (EP) study with radiofrequency ablation at the Stollery Children’s Hospital for supraventricular arrhythmias. Patients with preoperative echocardiographic findings of abnormal heart structure, a greater than mild myocardial dysfunction, or ventricular dysfunction due to significant dyssynchrony and those not in sinus rhythm or with a pacemaker were not recruited for the study.

All EP studies were performed under general anesthesia and mechanical ventilation. Vascular sheaths were introduced in femoral veins and were then used to position EP catheters in a standard manner for diagnostic and interventional purposes with the use of a right atrial lead for pacing. Following the successful clinical procedure (ablation), the patients were submitted to a standardized provocative testing period that included atrial pacing and inotropic stimulation with isoprenaline infusion. Study echocardiograms were performed using a Vivid 7 ultrasound machine (GE Healthcare, Milwaukee, WI) with a 5- or 7-MHz probe optimized for temporal resolution for STE (frame rate, mean 106 ± 16 Hz). Echocardiograms were performed at baseline HR and repeated following provocative testing, which consisted of (1) atrial pacing to 130% of baseline HR and, separately, (2) isoprenaline infusion titrated to achieve 130% of baseline HR. Echocardiographic measurements were performed offline using EchoPAC BT12 software (GE Healthcare) following standardized guidelines. The variables collected included HR, noninvasive blood pressure, LV dimensions, FS, color TDI–derived IVA, and STE-derived GLS and SR.

Two-dimensional (2D) parasternal long-axis and color TDI data sets in the apical four-chamber view were recorded. The isovolumic contraction period was determined by using M-mode from a parasternal long-axis view to identify mitral valve closure and aortic valve opening. In the apical four-chamber color TDI data set, a sample volume was placed at the septal mitral annulus and separately at the lateral mitral annulus. The software derived a velocity-time curve, and the isovolumic period was zoomed and checked to ensure that at least four frames were contained within the upstroke of the isovolumic velocity curve ( Figure 1 ). Septal and lateral IVA was measured by tracing the slope from zero velocity to the peak of velocity of the isovolumic spike for three consecutive beats by a single observer (S.A.; Figure 1 ). The averaged values at baseline HR, atrial pacing, and isoprenaline infusion were reported. While color TDI was used to measure IVA, ventricular strain and SR measures were not made using color Doppler methods.

Mitral annulus color TDI–derived velocity curves. The sample volume is placed at the mitral annulus (septal in this case), and the software derives the velocity ( y axis) curves over time ( x axis). Three consecutive IVAs were averaged. The insert is a zoom to better show the IVA measurement. The IVA ( white arrow ) was automatically calculated by the tracing of the slope ( blue line ) during isovolumic contraction time; the observers used at least four frames ( small yellow squares ) for an accurate reading.

Optimized 2D four-chamber views were acquired for measurement of GLS ( Figure 2 ) and SR ( Figure 3 ) using STE at baseline HR, atrial pacing, and isoprenaline infusion. The endocardium was traced, and the onset of the Q wave was set as the beginning of the cardiac cycle. GLS was calculated automatically by the software, and SR was derived and recorded as the peak deformation rate during systole by a single observer (M.A.).

LV strain tracing derived from STE. The LV endocardium was traced, and the region of interest was optimized to ensure adequate tracking of the myocardium. The onset of the Q-wave was defined as the beginning of the cardiac cycle. The aortic valve closure was defined previously on the M-mode of parasternal long axis. Global and segmental strains were automatically calculated by the software (EchoPAC). The white-dotted line represents the GLS, and the colored lines represent the longitudinal strain for each of the LV segments. Peak GLS was defined as the most negative value in systole.

LV SR curve was automatically generated by the EchoPAC speckle-tracking algorithm. The white-dotted curve is the GLS rate over time, and the colored lines are the SR for the longitudinal LV segments. Global peak SR was defined as the most negative value in systole.

Interobserver reproducibility of IVA, GLS, and SR was evaluated. Efforts were made to blind observers to the conditions, however, this was limited by differences in HR during atrial pacing and inotropic stimulation.

Reproducibility assessment for GLS and SR were performed in 10 patients at baseline, five patients during pacing and five patients during isoprenaline infusion, randomly selected by two observers (M.A. and S.A.). Each observer proceeded to independently select the best cardiac cycle, trace the endocardium, and adjust the region of interest, before the software algorithm for speckle-tracking was applied.

Reproducibility for IVA was performed in 10 randomly selected patients at baseline by two observers (S.A. and K.M.). In an apical four-chamber color-TDI data set, the observers independently placed sample volume at the septal and lateral mitral annulus and independently chose three consecutive beats for analysis.

Neonatal-Infant Piglets

Landrace cross piglets were studied under general anesthesia and mechanical ventilation. Vascular sheaths (5 to 6.5 Fr) were introduced in the common carotids and external jugular veins and were then used for catheter insertion and manipulation. Fluid-filled catheters (3.5 Fr) were positioned in the superior vena cava and the common carotid artery for arterial pressure monitoring. A pacemaker lead (4 Fr) was introduced into the right atrial appendage and connected to an external pacemaker (Medtronic, Minneapolis, MN). A high-fidelity catheter (3.5 or 5 Fr Millar Instruments, Houston, TX) was positioned in the left ventricle (LV) midcavity. Catheter placement was performed under both fluoroscopic and echocardiographic guidance. Blood gas analysis was performed before and immediately after the pacing protocol (iStat system, Abbott Point of Care, Princeton, NJ). Rectal temperature and pulse oximetry were continuously monitored during the experiments.

After instrumentation, a recovery period of 30 minutes was allowed for the piglets to stabilize, defined by less than 10% variation in hemodynamic parameters and normal parameters in an arterial blood gas analysis. The baseline invasive parameters were then recorded with an arterial blood gas, and echocardiography was performed. The measurement dP/dt was generated from the LV pressure curve (derivation of the pressure curve, peak value) automatically by Ponemah Software (Data Sciences International, St. Paul, MN). Max dP/dt is the peak rate of change in pressure in the LV during systole. It was obtained by inserting a high-fidelity Millar pressure catheter through the carotid into the LV. The pressure curves were recorded continuously. The software derived the dP/dt curve, allowing for subsequent offline analysis in order to determine an average peak value over many (20-25) cycles.

Atrial pacing was subsequently started at 200 bpm and progressively increased up to 260 bpm in increments of 10 bpm. The animals were allowed to stabilize for 30 seconds before invasive recording and echocardiograms that were performed at baseline, 200, 230, and 260 bpm. Following completion of the experiments, piglets were euthanized with an overdose of pentobarbital (100 mg/kg intravenously). Data from invasive monitoring were analyzed using Ponemah software. Each data point for aortic blood pressure (systolic, diastolic, and mean) and dP/dt were averaged from 20-25 heart cycles.

Echocardiographic images were acquired in all piglets using a Vivid 7 ultrasound machine (GE Healthcare) with a 10- to 7-MHz probe optimized for temporal resolution (frame rate, mean of 247 ± 7 Hz). Ductus arteriosus closure was confirmed before the beginning of the protocol in the animal subjects. Measurements were done offline using EchoPAC BT12 software (GE Healthcare) following standardized guidelines. The variables collected included mean blood pressure (invasive), Doppler cardiac output (calculated by multiplying the stroke volume [velocity-time integral × LV outflow tract area] by the HR), LV dimensions and calculated FS and LVEF, and STE-derived basal circumferential strain and SR.

Optimized 2D parasternal short-axis views of the atrioventricular valves were acquired for measurement of global circumferential strain (GLS) and SR using STE at baseline HR and during the different atrial paced HR. The endocardium was traced, and the onset of the Q-wave was set as the beginning of the cardiac cycle. GLS was calculated automatically by the software, and SR was derived and recorded as the peak deformation rate during systole.

Observers (E.F.P. and L.M.) performed interobserver reproducibility for circumferential GLS and SR. Twelve cardiac cine loops were randomly selected from four piglets. This consisted of four loops during baseline, three loops during pacing at 200 bpm, three loops during pacing at 230 bpm, and two loops during pacing at 260 bpm. The observers, using the same cardiac loops, then independently traced the endocardium and adjusted the region of interest to optimize tracking. Strain and SR curves were automatically generated by the EchoPAC algorithm, and peak strain and SR were manually measured.

Statistical Analysis

Normality of distribution was tested by D’Agostino and Pearson omnibus normality test. Non-Gaussian distributed data were presented as median [interquartile range]. Paired Wilcoxon signed rank test was used to assess whether the variables differed in children with different conditions. Interobserver variability for SR and IVA was assessed with the intraclass correlation coefficient (ICC). Piglet data were presented as mean ± SD unless specified otherwise. One-way analysis of variance (ANOVA) with repeated measures was used to compare the evolution of parameters through the pacing protocol and applying multiple comparisons with repeated measures tests. Greenhouse-Geisser and variance-homogeneity tests were used for testing ANOVA assumptions. Statistical significance was defined as P < .05. Statistical analysis was performed using SPSS version 19.0 for windows software (SPSS Inc. Chicago, IL).

Clinical Study

Twenty-three children (9 males) participated in the clinical arm of our study (median age, 13 years; range, 7.8-17.5 years). Twelve children had atrioventricular node reentry tachycardia, seven had Wolff-Parkinson-White syndrome, three had atrioventricular reentry tachycardia, and one had permanent junctional reciprocating tachycardia. Twenty-one patients had successful radiofrequency ablation, and two patients with Wolff-Parkinson-White syndrome were deemed not successful. Clinical and echocardiographic findings and response to atrial pacing and inotropic stimulation are summarized in Table 1 . The differences in SR and septal and lateral IVA from baseline to pacing and from baseline to isoprenaline infusion in children are plotted on the graphs in Figures 4 and 5 and demonstrate the relationship to HR of both SR and IVA (septal and lateral) through a plotted graph.

Differences in SR and lateral and septal IVA between baseline versus pacing and baseline versus isoprenaline infusion in children. (A) Lateral IVA changes from baseline to pacing. (B) Septal IVA changes from baseline to pacing. (C) Global SR changes from baseline to pacing. (D) Lateral IVA changes from baseline to isoprenaline. (E) Septal IVA changes from baseline to isoprenaline. (F) Global SR changes from baseline to isoprenaline.

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