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European Journal of Heart Failure 2007 9(6-7):644-650; doi:10.1016/j.ejheart.2007.01.009
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© 2007 European Society of Cardiology

Electromechanical effects of cardiac resynchronization therapy during rest and stress in patients with heart failure

Cinzia Valzaniaa,*, Fredrik Gadlerb, Maria J. Erikssonc, Arne Olssonc, Giuseppe Boriania and Frieder Braunschweigb

a Institute of Cardiology of the University of Bologna, Bologna, Italy
b Department of Cardiology, Karolinska University Hospital, Karolinska Institute, Stockholm, Sweden
c Department of Clinical Physiology, Karolinska University Hospital, Karolinska Institute, Stockholm, Sweden

* Corresponding author. Institute of Cardiology, University of Bologna, Via Massarenti 9, 40138 Bologna, Italy. Tel: +39 051 349858; fax: +39 051 344859. E-mail address: cinzia.valzania2{at}studio.unibo.it


   Abstract
Top
 Notes
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 
Background: Haemodynamic and functional effects of cardiac resynchronization therapy (CRT) have been studied mostly at rest. CRT effects on left ventricular (LV) dyssynchrony and function during stress have not been evaluated in detail.

Aims: We studied the electromechanical effects of CRT at rest and during Dobutamine stress echocardiography (DSE), during active and withheld CRT.

Methods: Twenty-one responders to CRT (62±12 yr) were assessed by walking test, quality of life, and BNP with active CRT ("off") and 2 weeks after pacing withdrawal ("off"). DSE (10 µ/kg/min) was performed both at "on" and "off" to evaluate dyssynchrony parameters, systolic and diastolic function.

Results: At rest, CRT withdrawal was associated with an increased interventricular mechanical delay (IVMD, from 21±18 ms to 49±24 ms, p<0.001) and impaired intraventricular synchrony. There was a significant decrease in LV systolic function and LV filling time. Dobutamine infusion had no impact on inter- and intraventricular synchrony. During stress, there was an improvement in LV performance both at "on" and "off" However, LV dp/dt, aortic VTI, cardiac output, mean systolic peak velocities and LV filling time during dobutamine stress were significantly greater with CRT "on".

Conclusion: In long-term responders, CRT improves LV performance both at rest and during dobutamine stress. This is attributable to an improvement in LV synchrony, which is maintained during stress.

Key Words: Cardiac resynchronization therapy • Heart failure • Dyssynchrony • Dobutamine stress echocardiography

Received July 18, 2006; Revised November 21, 2006; Accepted January 11, 2007


   1. Introduction
Top
 Notes
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 
Cardiac resynchronization therapy (CRT) is firmly established as a valuable additive treatment for patients with drug-refractory heart failure and electromechanical dyssynchrony [1]. In these patients, CRT has been found to improve myocardial function, heart failure symptoms, exercise capacity and survival [1-5]. These beneficial effects are attributable to an improvement in interventricular and intraventricular synchrony leading to improved left ventricular (LV) systolic function [6,7]. Moreover, the temporary cessation of long-term CRT treatment results in the progressive loss of functional and haemodynamic improvements [6,8,9], confirming that the increase in LV performance is truly pacing dependent.

The acute and chronic electromechanical effects of CRT have been described in many reports by means of invasive haemodynamics [10,11], Doppler echocardiography and Tissue Doppler Imaging (TDI) [12,13]. However, though typical clinical and pathophysiological manifestations of heart failure are particularly apparent during exercise, most of these studies were performed in the resting patient. Hence, it has not been demonstrated yet if CRT effects on the parameters of ventricular synchrony and function are sustained during stress. This would provide an important link between experimental electromechanical data and the clinical evidence of improved exercise capacity by CRT. The aim of the present study was, therefore, to compare the echocardiographic, clinical and neurohormonal response to CRT at rest and during low-dose dobutamine stress during active CRT and after 2 weeks of treatment cessation.


   2. Methods
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 5. Conclusions
 References
 
2.1. Patient population
Twenty-one patients, successfully treated with CRT for at least 6 months, participated in the study. The initial indication for CRT was drug-refractory NYHA class III or IV heart failure with wide QRS complexes (>130 ms), a LV ejection fraction (LVEF) below 35% and a LV end-diastolic diameter (LVEDD) above 55 mm. All patients had stable sinus rhythm, except 2 subjects with permanent atrial fibrillation and previous AV-nodal junctional ablation. All patients had conventional CRT systems with the left ventricular leads placed in a lateral or posterolateral position. After pacemaker implantation, basic heart rates were programmed to 60 or 70 bpm according to clinical judgment and in all patients AV delays were optimized using the Doppler mitral inflow pattern. The offset between atrial sensed and paced AV delays was set to 30 ms in all patients. Treatment response was defined as any improvement in functional class, walking distance and quality of life after 6 months of CRT. Pharmacological treatment was kept constant three months prior to inclusion and during the present study. The investigation conformed to the principles outlined in the Declaration of Helsinki. The local ethics committee approved the study protocol. All patients provided written informed consent to participation.

2.2. Study protocol
Patients were evaluated twice: 1) during active biventricular pacing ("on") and 2) after biventricular pacing had been switched off for two weeks ("off"). At "on" and "off", patients were assessed by echocardiography, physical examination, a 12 lead ECG, the six minute walk test [14] and quality of life according to the Minnesota Living with Heart Failure Questionnaire [15]. Plasma for brain natriuretic peptide (BNP) measurement was taken into EDTA containing glass tubes on iced water and centrifuged at +4 °C. Plasma was then immediately frozen and stored at –70 °C prior to analysis using a specific immunoradiometric assay (Shionogi, Osaka, Japan). During the "off" period, the pacemaker was programmed to atrial inhibited pacing (AAI) in order to maintain the same back up heart rate throughout the study. To ensure patient safety during the "off" period, the study coordinator was always available by a pager and heart failure status was assessed at the outpatient clinic after one week of inactive CRT.

2.3. Echocardiography
Standard transthoracic Doppler echocardiography and pulsed TDI were performed using a 2.5-MHz transducer (System V, Vingmed A/S, Horten, Norway) at rest and during DSE. After resting images had been acquired, dobutamine infusion was given at 5 µg/kg/min for three minutes and then increased to 10 µg/kg/min for an additional 5 min before the acquisition of stress images was started. Echocardiography sequences were always carried out in the same order and included parasternal long- and short-axis, apical 4- and 2-chamber views. The images were digitized and analyzed offline (EchoPac 6.3.6, Vingmed-General Electric) as the average of at least 3 consecutive beats. The off-line analysis was performed by one author (CV) who was blinded both to the pacemaker programming and to the condition of rest or stress. However, due to the presence or absence of ventricular pacing spikes on the ECG, the actual pacing mode could be recognized. Right arm blood pressure and heart rate were measured at rest and after 10 min of dobutamine infusion.

LV dimensions were determined from two-dimensional images of the parasternal long axis view. From the apical 4-chamber view the LV volumes were measured and LVEF was calculated according to the Simpson's equation. The degree of mitral regurgitation was expressed on a scale of 0 to 4 score with steps of 0.5, based on the percentage jet area relative to the left atrial size in the apical 4-chamber view [13]. The rate of systolic LV pressure rise (LV dp/dt) was determined from the continuous wave Doppler mitral regurgitation velocity curve [16]. Pulsed Doppler echocardiography was used to assess LV diastolic function and cardiac output. The interventricular mechanical delay (IVMD) was calculated as the difference between the aortic and pulmonary pre-ejection time [17,18] measured from QRS onset to start of Doppler flow in the aortic and pulmonary outflow tract, respectively. Pulsed TDI images were recorded from the apical 2- and 4-chamber views, with the Doppler sample volume placed at the basal level of the LV lateral, septal, anterior and inferior walls. Peak tissue velocities during systolic contraction (Sm), early diastolic (Em) and late diastolic filling during atrial contraction (Am) were measured as the average value from these four locations. For each wall, regional electromechanical delays were expressed by the time to peak Sm (Ts), measured from QRS onset to Sm peak [17,18]. Intraventricular dyssynchrony was expressed by the difference between Ts of septum and Ts of lateral wall [18], and by the maximal intra-LV electromechanical delay, defined as the time difference between the shortest and longest electromechanical delays among the four LV walls [19]. The intra-observer variability, expressed by the coefficient of variation between two series of separate measurements, was 2.1% for the LV filling time, 5.0% for the IVMD, and 6.2% for the LVEF.

2.4. Statistical analysis
All analyses were performed using a commercially available statistical package (Statistica, StatSoft Inc., Tulsa, Oklahoma). Data are expressed as mean±standard deviation. For the comparison of different parametric variables at different time points, all measures underwent 2-sided, paired t test. For comparisons between data obtained with biventricular pacing "on" and "off", at rest and during stress, repeated ANOVA with two within factors was applied. P values≤0.05 were considered significant.


   3. Results
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 5. Conclusions
 References
 
Patient characteristics at the time of enrollment (day 0) are listed in Table 1. All patients completed the protocol. There was a slight clinical deterioration from "on" to "off", reflected by a significant decrease in walking distance, a trend towards impaired QoL and an increase in BNP after two weeks of inactive CRT (Table 2). However, the cessation of CRT was not associated with any overt cardiac decompensation, need to adjust medication or seeking medical care. DSE was well tolerated by the patients.


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  		Table 1 Demographic and clinical characteristics of the study population at baseline (n=21)

 


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  		Table 2 Variations in clinical and functional parameters associated with 2-week CRT withdrawal

 
3.1. Echocardiographic changes at rest during CRT "on" and "off"
Table 3 reports the mean values of echocardiographic parameters, blood pressure and heart rate, measured at rest and stress, both during active CRT and after withdrawal of biventricular pacing.


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  		Table 3 LV function and synchrony parameters derived during rest and stress with biventricular pacing "on" and "off"

 
Between "on" and "off" the IVMD increased from 21±18 ms to 49±24 ms (p<0.001), reflecting the transition from a well-synchronized contraction pattern to interventricular dyssynchrony. Likewise, Ts (lateral-septal) increased from 34±22 ms to 64±50 ms (p=0.013) after withdrawal of biventricular pacing, indicating a loss of intraventricular synchrony. Accordingly, the maximal intra-LV electromechanical delay increased from 60±21 ms to 92±37 ms (p=0.007).

The loss of intraventricular synchrony during "off" was associated with a reduced LV systolic performance, reflected by a decrease in LVEF and aortic VTI (Table 3). Accordingly, LV dp/dt, which could be assessed in 13 (62%) of the 21 patients with significant mitral regurgitation, showed a significant decrease (Table 3). The reduction in LV systolic function after the withdrawal of biventricular pacing was also associated with a significant increase in LV diameters, LV systolic volume and left atrium area. Mitral regurgitation increased from 1.6±1.2 at rest during "on" to 2.0±1.3 during "off", p=0.001. No significant changes in systolic tissue velocities were observed after withholding CRT.

With regards to diastolic function, between "on" and "off" LV filling time decreased from 400±110 ms to 367±92 ms (p=0.027). Accordingly, LV filling time/RR decreased from 0.42±0.14 to 0.38±0.13 (p=0.031). No changes in deceleration time and in peak velocities of early (E) and late (A) diastolic filling or the E/A ratio were observed after withdrawal of biventricular pacing.

3.2. Echocardiographic changes during stress with CRT "on"
Low-dose dobutamine infusion induced a small but significant increase in heart rate and systolic blood pressure (Table 3). In the "on" stage, IVMD, Ts (lateral-septal) and the maximal intra-LV electromechanical delay were low at rest and remained unchanged after dobutamine infusion, suggesting the persistence of inter- and intraventricular synchrony during stress. Dobutamine infusion during "on" was associated with a significant increase in LV systolic performance. LVEF increased from 33±7% to 38±7% (p<0.001), aortic VTI from 15±3 cm to 17±4 cm (p<0.001), cardiac output from 3.4±1.0 L/min to 4.5±1.1 L/min (p<0.001), LV dp/dt from 553±349 mmHg/s to 643±391 mmHg/s (p=0.002) and mean Sm from 5.0±1.1 cm/s to 6.2±1.5 cm/s (p<0.001). There was also a significant reduction in LV dimensions and LV end-systolic volume, however, LV diastolic volume remained unchanged. The mean degree of mitral regurgitation, already low at rest, did not further decrease during dobutamine infusion. Furthermore, in the "on" condition, LV filling time/RR was significantly improved during stress, whereas no change in deceleration time and peak velocities of early (E) and late (A) diastolic filling was observed.

3.3. Echocardiographic changes during stress, "on" vs. "off"
During "off", interventricular dyssynchrony persisted during stress, as indicated by an unchanged IVMD (49±24 ms vs. 46±36 ms, p=0.70). Likewise, parameters of intraventricular dyssynchrony, Ts (lateral-septal) and the maximal intra-LV electromechanical delay, remained prolonged during dobutamine infusion (64±50 ms vs. 56±34 ms, p=0.34, and 92±37 ms vs. 84±34 ms, p=0.30, respectively). Dobutamine infusion during "off" was associated with an increase in parameters of LV systolic function. Changes in functional parameters from rest to stress, i.e. delta values (Table 3), were of similar extent during "on" and "off" pacing. However, during stress, peak values of LV dp/dt, aortic VTI, cardiac output and mean Sm were lower during "off" as compared to "on". A trend towards lower LVEF values during stress at "off" (38±7% at "on" vs. 36±7% at "off", p=0.09) was also observed. During stress, there was a reduction in LV diameters and a trend towards a decrease in LV volumes, although they were still greater during "off". Mitral regurgitation remained unchanged during stress, and greater during "off" (2.2±1.3 cmq at "off" vs. 1.5±1.1 cmq at "on", p<0.001). LV filing time/RR improved during stress but it was lower than during active biventricular pacing. No changes in deceleration time, E and A wave were observed during dobutamine infusion.

3.4. Correlations between walking distance and functional parameters
Concerning the relationship between clinical parameters and functional echocardiographic data, the increase in walking distance between "on" and "off" correlated with a prolongation of the LV filling time/RR ratio (r=0.49, p=0.037). Furthermore, the increase in BNP from "on" to "off" was related to the decrease in LVEF (r=–0.61, p=0.004).


   4. Discussion
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
5. Conclusions
 References
 
In patients with chronic heart failure, LV dysfunction and major dyssynchrony, CRT has been widely shown to improve echocardiographic parameters of LV synchrony and function at rest [6,7,17,20]. However, the response to CRT during stress has not been studied in detail. The main finding of the present study is that the significant improvement in LV performance, accomplished by CRT at rest, is sustained during dobutamine-induced stress. The underlying mechanism seems to be that the improvement in inter- and intraventricular synchrony during rest is maintained during stress. Thus, our data provide a link between findings on improved exercise capacity by CRT from clinical trials [3,4] and the functional response to CRT by means of low-dose DSE.

The study included treatment responders to CRT, defined by functional and clinical improvement after 6 months of continuous biventricular pacing. Accordingly, and in agreement with previous studies of CRT withdrawal [6,8,9], temporary cessation of CRT induced intra- and interventricular asynchrony, which was associated with an impairment of LV systolic function and an increase in LV volumes. This was reflected by a clinical deterioration, indicated by decreased walking distance and a trend towards a worsened QoL. These functional and clinical findings were further supported by a borderline significant increase in plasma-BNP after 2 weeks with CRT "off". The latter finding is in agreement with previous observations [21].

Dobutamine infusion has a primary impact on contractility with increased oxygen demand, mimicking the effects of exercise [22]. In our study, low-dose dobutamine had only a moderate effect on heart rate and blood pressure, resulting in an increase in the rate pressure product by 34% and 31% during "on" and "off", respectively. Probably, appropriate beta-blocker treatment and the programming of basic heart rates ≥60 bpm limited the magnitude of RPP response. However, both during active CRT and after pacing withdrawal, dobutamine stress test was associated with decreased LV systolic volumes and an improvement in LV systolic function, reflected by increased LVEF and cardiac output. The direct inotropic effect of dobutamine on the failing heart was also evidenced by an increase in TDI peak systolic velocities and LV dp/dt. Even if dobutamine infusion improved LV contractility to a similar extent in both pacing conditions, LV systolic and diastolic function was still significantly greater during "on", as reflected by greater values of LV dp/dt, cardiac output, mean Sm and LV filling time/RR ratio. Of note, basic characteristics of inter- and intraventricular synchrony were not affected by low-dose dobutamine infusion regardless of whether the pacemaker was "on" or "off", suggesting that the adverse impact of electromechanical dyssynchrony as well as its improvement by CRT persist during stress. These observations also underline the fact that the benefit of CRT is independent from and additive to a purely inotropic effect.

To date, only a few studies have evaluated the effects of CRT in the context of exercise. Using symptom-limited maximal bicycle exercise, Ennezat et al. [23] demonstrated that CRT induced a significant increase in LV contractility, associated with an attenuation of functional mitral regurgitation. Boriani et al. [24] found an increase in LVEF, but not in right ventricular ejection fraction, in CRT patients undergoing radionuclide angiography with exercise. Riedlbauchova et al. [25] analyzed the performance of left ventricular versus biventricular pacing during stress echocardiography by means of bicycle exercise test. For both pacing modes cardiac output was found to increase during exercise and was significantly greater during left ventricular pacing. Recently, both LV and biventricular pacing were shown to improve mitral regurgitation and LV dyssynchrony during bicycle exercise, but CRT-induced haemodynamic improvement during exercise was lower during LV pacing [26].

Our study provides further insight into the electromechanical effects of CRT during pharmacological stress and the pathophysiological changes occurring after withdrawal of pacing. In particular, the specific contribution of our study was not to evaluate the electromechanical effects by CRT in an acute comparative "on-off" setting, but at the end of a 2 week "off" period, thereby putting the echocardiographic assessment into a clinical context. The coherence of persistent LV synchrony and improved LV function during stress conditions may provide an important mechanistic background to the clinical CRT response in terms of exercise capacity. Besides enhanced systolic function, the LV filling time/RR ratio was significantly improved by CRT during stress and its improvement between "on" and "off" correlated with the increase in walking distance, underscoring the beneficial impact of CRT on diastolic filling properties. Importantly, previous studies using invasive haemodynamic methods or positron emission tomography indicated that the additional functional benefits by CRT during stress are achieved at favourable energy costs [8,27,28]. It is therefore conceivable that the observed improvement of myocardial performance during stress in our study was achieved without adverse excess oxygen consumption.

Stress echocardiography has been shown to provide important prognostic information in heart failure and ventricular function at peak exercise has been indicated as a powerful independent index for outcome [22,29]. Recently, Da Costa et al. found that a LVEF improvement of ≥25% during DSE was an independent outcome predictor in patients implanted with CRT [30]. Therefore, it may be speculated that the better LV performance during stress at "on" rather than at "off" pacing is related to a favourable prognosis in patients constantly maintained at "on" pacing.

4.1. Limitations
Since we aimed to describe the possible effects of biventricular pacing, we intentionally selected long-term responders to this treatment. In consequence, the results are not representative for a population of patients before and after implantation of a CRT device, where 30-40% are still expected not to respond to treatment. Moreover, the magnitude of changes between CRT "on" and "off" may have been mitigated by reverse remodelling processes after more than 6 months of treatment. We used pharmacological stress with low-dose dobutamine infusion rather than physiological bicycle exercise to allow for comprehensive image recording during the stress condition.


   5. Conclusions
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusions
References
 
Temporary cessation of CRT is associated with the deterioration in LV performance and a slight clinical impairment. The significant beneficial effects of CRT on parameters of synchrony, systolic function and diastolic filling time, observed at rest, were sustained during low-dose dobutamine stress. These findings provide an important link between the improved electromechanical LV function during stress echocardiography and the clinical evidence of improved exercise capacity in CRT patients.


   Acknowledgements
 
The authors thank Viveka Rendelius and Helena Karlsson for their expert technical assistance and patient management. Helene von Bibra is gratefully acknowledged for the suggestions to the study protocol and Odd Bech-Hansen for valuable criticism of the manuscript.


   Notes
Top
 Notes
Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 
{star} The study was supported by the Swedish Heart and Lung Foundation, Stockholm and by the Institute of Cardiology of the University of Bologna.


   References
Top
 Notes
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 

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Eur J Heart Fail, April 1, 2008; 10(4): 412 - 420.
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