The contribution of atrial contraction to cardiac output (CO) has been the subject of extensive research but has yet to be quantified adequately in children and adolescents. Patients with third-degree atrioventricular (AV) block treated with pacemakers (PMs) are ideal candidates to assess the atrial contribution to CO by repeated measurements in single-chamber pacing mode (VVIR) and dual-chamber pacing mode (DDD/VDD). Hemodynamic measurements in children are often complicated by technical restrictions, but more recently a noninvasive method involving inert gas rebreathing has become available, which is an excellent tool for this age group. We examined 10 patients (6 female patients, mean age 14.5 ± 2.5 years, range 11 to 18) with congenital complete AV block treated with dual-chamber PM. Using an inert gas rebreathing device (Innocor) we measured CO in DDD/VDD with optimized AV delays. Devices were subsequently set to VVIR with matched heart rates and after 20 minutes the CO measurement was repeated. Mean CO of 6.4 ± 1.8 L/min was significantly higher in DDD/VDD than in VVIR, where it averaged 5.2 ± 1.4 L/min (p <0.001). Fractional increase of CO gained through sequential ventricular contraction was 18% (p <0.001). In VVIR, 8 patients reported PM-related symptoms. In conclusion, our data strongly suggest that pediatric patients with congenital complete AV block may benefit from AV synchrony with respect to hemodynamics and tolerability. Therefore, preferred use of DDD/VDD with optimized AV conduction delays should be considered.
In patients with third-degree atrioventricular (AV) block the active atrial component of ventricular filling is absent due to a disrupted AV conduction. That can be re-established and treated by dual-chamber cardiac pacing mode (DDD/VDD). This makes this patient group an excellent collective for measuring the atrial contribution to cardiac output (CO) because switching dual-chamber pacemakers (PMs) to a rate-responsive single-chamber mode (VVIR) results in a hemodynamic situation that omits ventricular filling through co-ordinated atrial systole. For routine CO assessment, methods such as thermodilution and quantitative magnetic resonance imaging have limited utility in pediatric patients due to technical complexity, invasiveness, or radiation. As an alternative to the frequently used echocardiographic method, the noninvasive concept of inert gas rebreathing has more recently become available, providing reliable and accurate results. Because it can be performed repeatedly in children from school age on, taking only 3 to 5 minutes to complete, it was the technique of choice. The aim of this study was to measure the effects of co-ordinated atrial emptying on CO with an inert gas rebreathing method in generally healthy children and adolescents with dual-chamber PM therapy.
Methods
This prospective observational study was conducted in a tertiary children’s and adolescents’ pacemaker outpatient clinic at the University Hospital of Vienna (Vienna, Austria) from July 2009 to March 2010. Patients 6 to 20 years of age and treated with a dual-chamber PM due to congenital complete AV block were considered eligible. All patients had a full echocardiographic examination before baseline testing of CO, including measurement of left ventricular ejection fraction using the modified Simpson rule. Exclusion criteria were (1) hemodynamically significant cardiac defects and (2) concomitant sick sinus syndrome due to a concern of an inadequately low heart rate. The study was approved by the medical ethics committee of the hospital and written informed consent was obtained from each patient and a relative.
Hemodynamic parameters were assessed noninvasively using an Innocor INN00500 device (Innovision A/S, Odense, Denmark), which is based on an oxygen-enriched, physiologically inert composition of soluble (0.5% nitrous oxide) and insoluble (0.1% sulfur hexafluoride) gases. This mixture is respired within a closed system and quantified continuously by photoacoustic analyzers over a period of 4 to 5 breaths. The concentration of soluble gas decreases proportionally with pulmonary blood flow, which is assumed to be equal to CO in the absence of intrapulmonary shunts. Stroke volume was calculated by dividing CO by heart rate, and the cardiac index and stroke index were obtained by dividing CO and stroke volume, respectively, by a patient’s body surface area.
In the present patient cohort with congenital complete AV block, we previously optimized AV delays using the electrocardiography-based method described by Koglek et al, in which the interval between the end of the P wave and the amplitude of the S wave should be 100 ms. This was achieved by shortening AV delays under electrocardiographic monitoring. Results of optimization were verified echocardiographically.
After an initial CO measurement in DDD/VDD, PMs were switched to VVIR and after 20 minutes, which was allowed for patient adaptation to the new setting, the test was repeated. During the waiting period patients were asked to document any symptoms on a specially designed questionnaire. For VVIR, heart rates were matched to those recorded in DDD/VDD to rule out CO changes through heart rate variation. After completion of all CO measurements, PMs were reset to their individually optimized DDD/VDDs.
All statistical operations were performed using Predictive Analytics 17.0 for the Macintosh (SPSS, Inc., Chicago, Illinois). Hemodynamic data were compared between pacing modes using Student’s t test for paired samples and expressed as mean ± SD. Statistical significance was assumed for p values <0.05.
Results
Of 24 children and adolescents with dual-chamber PMs due to congenital and postoperative AV block attending our outpatient clinic during the inclusion period, 18 were examined. Only data from a subgroup of 10 patients with congenital complete AV block were analyzed for this study (6 female patients, mean age 14.5 ± 2.5 years, range 11 to 18). Mean duration of PM therapy was 6.4 ± 4.1 years ( Table 1 ). Cardiac function was generally excellent with a mean ejection fraction of 68 ± 5%. Mean CO of 6.4 ± 1.8 L/min was significantly higher in synchronous AV pacing than in VVIR, where it averaged 5.2 ± 1.4 L/min (p <0.001; Figure 1 ). This computed to a mean fractional increase of 18%, with a 30% maximum, due to co-ordinated atrial contraction. Consistent with this finding, mean cardiac index was also higher in DDD/VDD, measuring 4.2 ± 1.0 versus 3.4 ± 0.6 L/min/m 2 (p = 0.001). Mean heart rates did not differ between DDD/VDD and VVIR, averaging 76 ± 14 and 72 ± 6 beats/min, respectively (p >0.05). In contrast, mean stroke volume and stroke index were significantly higher in DDD/VDD, measuring 88 ± 33 ml and 57 ± 13 ml/m 2 versus 73 ± 24 ml and 47 ± 9 ml/m 2 , respectively (p ≤0.005 for the 2 comparisons).
| Patient No. | Age (years)/Sex | Pacing (years) | Paced AV Delay (ms) ⁎ | AP (%) | VP (%) | EF (%) |
|---|---|---|---|---|---|---|
| 1 | 10/F | 4.1 | 120 | 1 | 100 | 67 |
| 2 | 11/F | 4.8 | 100 | 0 | 100 | 66 |
| 3 | 11/F | 6.1 | 100 | 1 | 100 | 80 |
| 4 | 12/F | 2.2 | 120 | 0 | 100 | 71 |
| 5 | 15/M | 2.9 | 90 | 1 | 100 | 62 |
| 6 | 15/M | 10.9 | 110 | 2 | 100 | 65 |
| 7 | 15/M | 10.5 | 100 | 8 | 100 | 70 |
| 8 | 17/F | 11.5 | 100 | 1 | 100 | 65 |
| 9 | 17/M | 0.6 | 90 | 1 | 100 | 69 |
| 10 | 18/F | 10.4 | 100 | 6 | 100 | 65 |
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