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J Am Coll Cardiol, 2002; 39:1163-1169
© 2002 by the American College of Cardiology Foundation
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CLINICAL STUDY: HEART FAILURE

Cardiac resynchronization therapy restores optimal atrioventricular mechanical timing in heart failure patients with ventricular conduction delay

Angelo Auricchio, MD, PhD*,*, Jiang Ding, PhD{dagger}, Julio C. Spinelli, PhD{dagger}, Andrew P. Kramer, PhD{dagger}, Rodney W. Salo, MSc{dagger}, Walter Hoersch, BE{dagger}, Bruce H. KenKnight, PhD{dagger}, Helmut U. Klein, MD* for the PATH-CHF Study Group1

* Division of Cardiology, University Hospital, Magdeburg, Germany
{dagger} Guidant Corporation, St. Paul, MinnesotaUSA

Manuscript received February 15, 2001; revised manuscript received December 21, 2001, accepted January 10, 2002.

* Reprint requests and correspondence: Dr. Angelo Auricchio, Division of Cardiology, University Hospital, Leipzigerstr. 44, 39120 Magdeburg, Germany.
angelo.auricchio{at}medizin.uni-magdeburg.de


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
OBJECTIVES: We characterized the relationship between systolic ventricular function and left ventricular (LV) end-diastolic pressure (LVEDP) in patients with heart failure (HF) and baseline asynchrony during ventricular stimulation.

BACKGROUND: The role of preload in the systolic performance improvement that can be obtained in HF patients with LV stimulation is uncertain.

METHODS: We measured the maximum rate of increase of LV pressure, LVEDP, aortic pulse pressure (PP) and the atrioventricular mechanical latency (AVL) between left atrial systole and LV pressure onset in 39 patients with HF. Two subgroups were identified: "responder" if PP improved, or "nonresponder."

RESULTS: Maximum hemodynamic improvement occurred at an atrioventricular (AV) delay that did not decrease LVEDP. Left ventricular and biventricular (BV) stimulation increased systolic hemodynamics significantly, despite no significant increase in LVEDP. All parameters decreased when the LVEDP was decreased by shorter AV delay. Left ventricular and BV stimulation provided better hemodynamics than right ventricular (RV) stimulation. For the nonresponder subgroup, systolic hemodynamics only worsened during AV delay shortening. For the responder subgroup, optimum PP was achieved when AVL was near zero.

CONCLUSIONS: Restoration of optimal left atrial-ventricular mechanical timing partly contributes to the hemodynamic improvements observed in this patient subgroup. However, preload alone cannot explain the differences seen between RV and BV stimulation and the contradictory PP decreases even at maximal preload in the nonresponder subgroup. These results may be explained by a site-dependent mechanism such as the degree of ventricular synchrony. Caution should be taken in these patients when optimizing AV delays using echocardiography techniques that focus on LV inflow.

Abbreviations and Acronyms
  AV
  atrioventricular
  AVL
  atrioventricular mechanical latency (RA-LS – RA-AP)
  BV
  biventricular
  CRT
  cardiac resynchronization therapy
  dP/dtmax
  maximum rate of increase of left ventricular pressure
  HF
  heart failure
  LV
  left ventricle/ventricular
  LVEDP
  left ventricular end-diastolic pressure
  PATH-CHF
  Pacing Therapies for Congestive Heart Failure study
  PP
  aortic pulse pressure
  RA
  right atrium
  RA-AP
  time from sensed activation in the right atrium to the peak of the atrial contraction as seen in the left ventricular pressure (AP)
  RA-LS
  time from right atrial sense to the start of left ventricular contraction (LS)
  RV
  right ventricle/ventricular

Both ventricular stimulation site (1–3) and the atrioventricular (AV) delay (3) influence improvements of systolic function during cardiac resynchronization therapy (CRT). The mechanical AV delay affects systolic performance by modulating preload (4). Furthermore, the mechanical AV delay is prolonged with respect to the electrical AV delay in patients with heart failure (HF), and the prolongation is highly variable across individuals (5). Nevertheless, little data have been published on the role that AV delay and preload play in the improvements seen with CRT. Furthermore, despite the usage of inflow-based AV delay optimization techniques (6,7), no studies have tested whether systolic function is optimized by these techniques and, in particular, whether they might negatively impact patients that do not exhibit hemodynamic increases with CRT.

To address these issues and the relative importance of preload and resynchronization as mechanisms of action of CRT, we evaluated the relationship among aortic pulse pressure (PP), the maximum rate of left ventricular (LV) pressure increase (dP/dtmax) and LV end-diastolic pressure (LVEDP) as a function of the programmed electrical AV delay in the patients enrolled in the Pacing Therapies for Congestive Heart Failure (PATH-CHF) study (8). We tested the hypothesis that, in the subgroup of patients that respond positively to CRT, the maximum increases in PP are obtained when the AV delay maximizes LV preload, as evaluated by LVEDP.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Patient population.   Thirty-nine of 42 patients enrolled in the PATH-CHF study, who had technically valid data, represent the study population. The PATH-CHF study was a multicenter prospective randomized crossover sequential study to evaluate the effects of CRT and stimulation site on acute and chronic hemodynamic function in patients with New York Heart Association functional class III to IV HF and ventricular conduction delay. The complete PATH-CHF study inclusion and exclusion criteria as well as the study design and end points have been presented elsewhere (8,9). The ethical committees at all participating centers and the competent authorities of their respective countries approved the protocol. All patients provided written informed consent before participating in the study.

Acute data collection.   The acute test procedure and data collection have been previously described (3). Briefly, under general anesthesia, permanent pacing leads were inserted (Sweet-Tip, Guidant Corp. CRM, St. Paul, Minnesota) in the right atrial appendage and right ventricle (RV), and an epicardial screw-in lead was attached to the LV via a limited thoracotomy. Patients were instrumented for acute study by placing two 8F dual transducer Millar micromanometer catheters (Millar Instruments, Houston, Texas) for measuring aortic, RV and LV pressures. Each patient was tested in univentricular (RV and LV) and biventricular (BV) configurations, using five AV delays, that divided the intrinsic AV interval into five equal duration segments ranging from 0% to 86% of this interval. Each site/AV delay combination was repeated five times in random order. Data were digitized (16-bit resolution, 500 Hz sampling rate, TEAC, Montebello, California) for offline analysis.

Hemodynamic parameters and responses to CRT.   Aortic diastolic and systolic pressures, PP, LVEDP and dP/dtmax were extracted using custom software (Guidant Corp.). Aortic pulse pressure was defined as the difference between aortic systolic and diastolic pressure; LVEDP was measured as the LV pressure at the beginning of LV mechanical contraction. Absolute values and changes from baseline were evaluated.

Patients for whom there was an increase in PP with respect to their intrinsic baseline by more than 5% for any stimulation mode and AV delay combination were placed in a responder subgroup. The remaining patients were placed in a nonresponder subgroup.

Atrial and ventricular electrical/mechanical events.   Figure 1 illustrates the electrical and mechanical events and the timing intervals measured. Right atrial electrical activation is designated as right atrium (RA). The peak of a small change in LV pressure before ventricular systole reflects LA systole (AP) as seen in the LV (10). Thus, the RA-AP interval is a complex electromechanical delay comprising the time elapsed from RA to the mechanical activation of the left atrium, which, in turn, creates a pressure increase in the LV. The start of LV contraction is marked as LS. During intrinsic rhythm, RA-LS represents the electromechanical AV delay. The interval between left atrial systole and the beginning of the LV contraction is defined as the AV mechanical latency (AVL). During intrinsic beats, AVL reflects the lag between the end of left atrial contraction, as seen in the LV, and the beginning of LV contraction. A positive AVL (meaning left atrium precedes LV) indicates that the left atrium contributes to the LV filling process; a negative value, which can occur when the ventricle is paced with a sufficiently short AV delay, indicates ventricular contraction preceding the peak of atrial filling.



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  		Figure 1 Example of systolic left ventricular (LV) pressure during pacing (a) and intrinsic condition (b), intrinsic LV electrogram (c) and intrinsic right atrial (RA) electrogram (d) recorded from one patient. Also shown here is the presystolic peak (AP) due to atrial contraction and the start of pressure development in the LV (LS), the latter obtained as the point that first attained a slope ≥10% of maximum rate of increase of LV pressure. The interval (APLS) between AP and LS is defined as atrioventricular mechanical latency (AVL). When the ventricle is pre-excited with pacing, the LS point moves to the left, as shown here in curve a. To obtain the LS point in paced condition, the pressure curves in pacing and intrinsic condition are aligned at the right atrium electrical activation (RA). Thereafter, the difference between the two curves is obtained. The LS is the first point on the difference curve at which the slope is 10% of the maximum slope.

 
Timing measurements.   The intrinsic timing of LS was determined directly from an intrinsic LV pressure curve as the first point (before the peak of systole) having a slope ≥0.1 dP/dtmax. As illustrated in Figure 1, for each paced beat, the last intrinsic pressure curve of each sequence was aligned with each of the corresponding five paced ventricular pressure curves at a fidutial point coinciding with RA. Then, the LS point for that paced beat was determined from the difference of the two pressure curves using the slope technique described for an intrinsic beat.

The LS points, their corresponding LVEDP, PP and dP/dtmax from all the paced beats with a same stimulation mode and AV delay were averaged. The timing of intrinsic LS and AP were averaged from the first set of 15 intrinsic beats. Only beats with intrinsic AV intervals within ±1 SD of the group mean were included in the analysis. The AP timing was assumed to remain unchanged during the duration of the procedure and to remain unaffected by atrial synchronous ventricular stimulation.

Statistics.   A two-tailed unpaired t test was used to evaluate the differences in baseline data between responder and nonresponder subgroups. A two-tailed paired t test was employed to test the significance of changes in PP and LVEDP created by different AV delays and modes. To account for multiple comparisons, a p value of ≤0.01 was considered significant. The correlation coefficient of a linear regression between RA-AP and RA-LS was used to evaluate the relationship between the start of LV systole as seen in the LV pressure and AP at the AV delay that provided the optimum hemodynamics. In addition, a one-sided binomial test was used to verify that the association between AVL and optimum PP did not occur by chance alone; a p value of <0.05 was considered significant. Results are expressed as the mean ± SD in the text and mean ± SE in the plots.


   Results
Top
 Abstract
 Methods
 Results
Discussion
 References
 
Table 1 contains the demographic data for the study population. Most patients had left bundle branch block. During the acute testing, all patients except one (who required DDD pacing due to bradycardia) were paced in VDD mode. Of 39 patients, 27 were in the responder subgroup and 12 in the nonresponder subgroup. Nonresponder patients had a shorter QRS duration and larger dP/dtmax than the responders (Table 1).


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  		Table 1 Patient Demographics

 
Preload dependence of the hemodynamic changes.   For the group of all patients, the maximum increases in PP and dP/dtmax occurred at 43% of the intrinsic AV interval (Fig. 2). At this AV delay, PP and dP/dtmax increases were significant for LV and BV stimulation and significantly larger than the increases seen for RV stimulation, while LVEDP did not change with LV or BV stimulation but significantly increased with RV stimulation (Table 2). Shortening the AV delay (from 43% to 0%) decreased PP, dP/dtmax and LVEDP (Table 2). The LVEDP decrease had a significantly larger impact on PP than on dP/dtmax (p ≤ 0.01), for LV (–9% vs. –6%), RV (–7% vs. –4%) and BV (–10% vs. –5%) stimulation.



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  		Figure 2 The average aortic pulse pressure (PP), maximum rate of increase of left ventricular pressure (dP/dtmax) and left ventricular (LV) end-diastolic pressure (LVEDP) obtained during LV (left) and right ventricular (RV) (right) stimulation at each tested atrioventricular (AV) delay and at baseline (i.e., 100%) for the population, the responder subgroup and the nonresponder subgroup. Actual AV delays were normalized to baseline intrinsic AV interval (AVI) to simultaneously represent both the effect of short AVIs and pre-excitation present in the individual patients.

 

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  		Table 2 Impact of AV Delay Changes on Hemodynamics for the Population, the Responder Subgroup and Nonresponders

 
Responders.   The responder subgroup (Fig. 2, Table 2), showed the same changes as the group of all patients, but, due to its definition, the increases were larger. Also in this subgroup, PP decreased about twice as much as dP/dtmax (p < 0.001), when the AV delay was decreased from 43% to 0%.

Nonresponders.   The nonresponder subgroup showed a monotonic decrease in PP and dP/dtmax when the AV delay was shortened. This decrease occurred with no significant decrease in LVEDP until AV delays were shorter than 43% of the intrinsic AV interval (Table 2). At middle AV delays, PP and dP/dtmax were significantly worse for RV stimulation compared with LV and BV stimulation. The percentage decreases in PP and dP/dtmax were similar for short AV delays.

AV time lag measurements.   A measurable atrial peak (Fig. 1) was present in the LV pressure in 29/39 patients (19 responders and 10 nonresponders). The size of the atrial peak ranged between 0.5 mm Hg and 6.0 mm Hg. The intrinsic AVL varied from 33 ms to 140 ms, lasting on the average 65 ± 24 ms. The LV stimulation that resulted in the largest increase in PP in the responder subgroup occurred when AVL was in the range of ±25 ms (p < 0.0001 vs. chance alone) (Fig. 3). For values of AVL outside of this range, the PP changed rapidly, declining symmetrically to near or below baseline (Fig. 3). For the nonresponder subgroup, however, the largest PP consistently occurred at the longest AV delay irrespective of the value of AVL (Fig. 3). A similar relationship between AVL and PP was observed for BV and RV stimulation modes.



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  		Figure 3 (A) Relationship between aortic pulse pressure (PP) and atrioventricular mechanical latency (AVL) during left ventricle (LV) stimulation in the responder subgroup. Measurements are shown for all 19 patients with measurable and stable presystolic peaks. Pulse pressure changes are normalized to the maximum PP increases in each patient. Note that the AVL was calculated not only at five individual paced atrioventricular (AV) delays but also at the interpolated optimal AV delay for PP. The interpolation was based on a fourth order polynomial fit to the response curve. (B) Distribution of AVL values that corresponded to maximum increase in PP during LV stimulation in A. Each bin represents a 10 ms range of AVL values, and the number under each bin is the center value of that bin. (C) Same as A for 10 nonresponder patients. (D) Same as B for the nonresponder subgroup.

 
The correspondence for all stimulation modes of the maximum PP increase in the responder subgroup with a nearly zero AVL is quantified in Figure 4A. The RA-LS was linearly correlated with the RA-AP (r2 = 0.7, slope = 1.1, intercept = –28 ms), indicating that at optimum AV delay AVL is small. The average AVL for maximum PP increase was 3 ± 17 ms for LV stimulation, –3 ± 15 ms for BV stimulation and –3 ± 22 ms for RV stimulation (p = NS). The relationship between RA-AP and RA-LS was weaker (r2 = 0.2) when dP/dtmax was maximized (Fig. 4B).



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  		Figure 4 Correlation of time from sensed activation in the right atrium (RA) to the peak of the atrial contraction as seen in the left ventricular (LV) pressure (RA-AP) and time from RA sense to the start of LV contraction (RA-LS) intervals that were calculated at the maximum aortic pulse pressure (PP) (A) and maximum rate of increase of LV pressure (dP/dtmax) (B). The difference between the RA-LS and RA-AP intervals is the atrioventricular mechanical latency (AVL). The identity line (dashed line) indicates an AVL of 0 ms. Points below the identity line occur when the onset of LV systolic pressure precedes the atrial systolic peak. Measurements are combined from patients with a measurable and stable presystolic peak and with maximum stimulation-induced PP and dP/dtmax increments of at least 5%. Stimulation chamber is identified in the legend. The linear regression was applied to all data points in each plot, and the regression equation and the correlation coefficient were displayed accordingly. BV = biventricular; RV = right ventricle/ventricular.

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
This study suggests that there are at least two electromechanical determinants of the PP and dP/dtmax changes: 1) the active contribution of the left atrial systole to LV filling and 2) the improvement of contractile function due to resynchronization of inter/intraventricular contractions by LV or BV pre-excitation (11). The fact that for all stimulation modes the maximum dP/dtmax and PP were obtained at an AV delay that preserved the baseline values of LVEDP suggests that preload plays a significant role in determining the point at which the optimum acute hemodynamic impact of CRT is achieved. This role is further supported by the behavior observed in the responder subgroup, where PP is consistently the optimum when AVL is near zero independently of the pacing site or site combination. It is likely that improved cooperative ventricular contraction amplifies the PP and dP/dtmax increases in responder patients, since both the optimum PP and dP/dtmax are significantly larger than intrinsic for both LV and BV stimulation, which, in turn, are larger than RV stimulation, even though all three types of stimulation equivalently advance the start of LV contraction and achieve a near-zero AVL. The observation that the AV delay that optimizes dP/dtmax is associated with a poor correlation between R-LS and R-AP(Fig. 4B) and that the sensitivity of dP/dtmax to decreases in LVEDP is less than for PP suggests that dP/dtmax may be more associated with a contractile function mechanism, such as the degree of intraventricular synchronization, which is less preload-dependent.

The roles of preload and ventricular synchrony in CRT.   Stimulating the delayed LV alone or biventricularly appears necessary to maximally resynchronize ventricular contractions, increasing cooperative contractile function and output (3). Right ventricular stimulation is less effective at pre-exciting the delayed LV, minimally improving contractile function. The small PP increases during RV stimulation despite a larger LVEDP are consistent with this hypothesis. Thus, most of the PP increases with RV stimulation might be explained by the LVEDP increases, whereas the incremental increases in PP with LV and BV stimulation are likely due to better ventricular synchronization.

The alteration of cooperative contractile function also explains the PP and dP/dtmax decreases despite maintained preload in nonresponder patients. We hypothesize that this subgroup has relatively little ventricular asynchrony so that any pre-excitation tends to desynchronize the ventricles. In fact, the nonresponder patients had higher baseline hemodynamic function than responder patients, most notably a significantly higher dP/dtmax associated with a shorter QRS duration. It is well known that RV stimulation worsens LV systolic function compared with normal activation (12), with the results being less detrimental for LV or BV (13). Alternatively, nonresponders may have baseline asynchrony, but the stimulation electrode placements are ineffective for restoring ventricular synchrony (14). By either explanation, CRT stimulation at long AV delays would be expected to have the least impact and may even be beneficial, while stimulation at short AV delays may desynchronize the ventricles worsening contractile function and decreasing PP. This hypothesis would explain why the maximum PP and dP/dtmax are observed at the longest AV delays in nonresponder patients, despite suboptimal AVL values.

AV mechanical lag and end-diastolic pressure as preload determinants.   In a normal heart, atrial and ventricular systole are temporally coordinated to optimize blood transfer (10). For HF patients with prolonged AV conduction and diastolic mitral regurgitation, filling may be compromised (15). Although LVEDP is a good surrogate for preload, it does not take into account the dynamics of AV coordination, which is an important determinant of the efficiency of the LV contraction. The AVL individually measures AV coordination and takes into account any LV pressure decreases between the peak of LA systole and the start of LV contraction. Such pressure drops may be created by diastolic mitral regurgitation (15,16).

Clinical implications.   Pulse pressure and dP/dtmax are indexes used to assess the LV systolic function because of their correlation with stroke volume and global contractile function, respectively, under the experimental conditions used in our study (2,4). Nevertheless, care should be taken when using these relationships because of their dependence on preload and arterial impedance. Inflow-based Doppler echocardiography techniques are gaining popularity as a method for optimizing the AV delay for CRT (6,7). Basically, the method uses Doppler mitral flow velocity recordings to optimize the left AV mechanical timing such that filling time is maximized by starting the LV contraction at the end of the A-wave. However, an AVL of zero would correspond to the time of peak velocity in the A-wave and not to the end of the A-wave, suggesting that the optimum AV delay for responder patients may be shorter than currently believed. In contrast, for nonresponder patients, for whom a zero AVL is associated with PP and dP/dtmax decreases, the optimum AV delay is longer than the AV delay obtained using the inflow Doppler technique. Because this and other preload optimization methods will not account for the apparent nonpreload determinants of PP, they should be used with discretion. If a patient can be identified as responder type, optimizing by preload methods would seem safe. However, nonresponder patients should probably not be optimized by preload alone. For these patients, it may be better to optimize ventricular resynchronization rather than preload. The degree of resynchronization may be assessed by dP/dtmax (17).

Study limitations.   Because only the peak of atrial contraction could be estimated from the presystolic component, the ventricular contraction may alter the shape or size of the presystolic component, making the determination of the atrial event less accurate. All these measurements were performed with the patients anesthetized, supine and at rest. Varying degrees of mitral regurgitation could have affected the results.

Conclusions.   For HF patients with conduction defects, CRT with an appropriate AV delay can increase PP and dP/dtmax by restoring optimal AV mechanical timing and inter/intraventricular mechanical synchrony. The largest PP and dP/dtmax occurs at an AV delay that does not decrease LVEDP. The optimal PP for the responder subgroup patients occurs when the peak of left atrial systole coincides with the start of LV contraction. Changes in preload alone cannot explain the changes in PP observed with CRT, strongly supporting the presence of a second mechanism, probably related to the ability of LV and BV stimulation to improve the synchrony of the ventricular contraction (11,17).

The nonpreload mechanism appears to be the dominant PP and dP/dtmax determinant in the nonresponder subgroup. Thus, caution should be taken when optimizing AV delays using a method that only maximizes preload.


   Acknowledgments
 
The authors are deeply indebted to the patients, nurses and colleagues at each institution without whose help and logistic support this study would not have been possible. Dr. Auricchio wishes to thank the Heart Failure Research Group of Guidant Corp. for their outstanding technical support.


   Footnotes
 
Supported by a grant from Guidant Corporation, St. Paul, Minnesota.

1 The investigators and participating centers of the Pacing Therapies for Congestive Heart Failure (PATH-CHF) Study Group along with collaborators in the Guidant CHF research group have been listed in the Appendix of J Am Coll Cardiol 2001;38:1957–65. Back


   References
Top
 Abstract
 Methods
 Results
 Discussion
 References
 

  1. Blanc JJ, Etienne Y, Gilard M, et al. Evaluation of different ventricular pacing sites in patients with severe heart failure: results of an acute hemodynamic study. Circulation. 1997;96:3273–3277[Abstract/Free Full Text]
  2. Kass DA, Chen CHCurry C, et al. Improved left ventricular mechanics from acute VDD pacing in patients with dilated cardiomyopathy and ventricular conduction delay. Circulation. 1999;99:1567–1573[Abstract/Free Full Text]
  3. Auricchio A, Stellbrink C, Block M, et al. Effect of pacing chamber and atrioventricular delay on acute systolic function of paced patients with congestive heart failure. Circulation. 1999;99:2993–3001[Abstract/Free Full Text]
  4. Yu Y, Ding J, Liu L, et al. Experimental validation of pulse contour method for estimating stroke volume at pacing onset. Proc 20th Ann Int Conf IEEE/EMBS. 1998;20:401–404
  5. Auricchio A, Salo R. Acute hemodynamic improvement by pacing in patients with severe congestive heart failure. Pacing Clin Electrophysiol. 1997;20:313–323[CrossRef][Medline]
  6. Cazeau S, Leclercq C, Lavergne T, et al. Effects of multisite biventricular pacing in patients with heart failure and intraventricular conduction delay. N Engl J Med. 2001;344:873–880[Abstract/Free Full Text]
  7. Kindermann M, Frohlig G, Doerr T, Schieffer H. Optimizing the AV delay in DDD pacemaker patients with high degree AV block: mitral valve Doppler versus impedance cardiography. Pacing Clin Electrophysiol. 1997;20:2453–2462[CrossRef][Medline]
  8. Auricchio A, Stellbrink C, Sack S, et al. The Pacing Therapies for Congestive Heart Failure (PATH-CHF) study: rationale, design, and endpoints of a prospective randomized multicenter study. Am J Cardiol. 1999;83:130D–135D[CrossRef][Medline]
  9. Auricchio A, Stellbrink C, Sack S, et al. Chronic effect of hemodynamically optimized cardiac resynchronization therapy on patients with heart failure and ventricular conduction delay. J Am Coll Cardiol. 2001;38:1957–1965[Abstract/Free Full Text]
  10. Appleton C, Basnight M, Gonzalez M, Carucci M, Henry C, Olajos M. Diastolic mitral regurgitation with atrioventricular conduction abnormalities: relation of mitral flow velocity to transmitral pressure gradients in conscious dogs. J Am Coll Cardiol. 1991;18:843–849[Abstract]
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  16. Brecker S, Xiao H, Sparrow J, Gibson D. Effects of dual-chamber pacing with short atrioventricular delay in dilated cardiomyopathy. [published erratum appears in Lancet 340;1992:1482]Lancet. 1992;340:1308–1312[CrossRef][Medline]
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G. B. Bleeker, T. A.M. Kaandorp, H. J. Lamb, E. Boersma, P. Steendijk, A. de Roos, E. E. van der Wall, M. J. Schalij, and J. J. Bax
Effect of Posterolateral Scar Tissue on Clinical and Echocardiographic Improvement After Cardiac Resynchronization Therapy
Circulation, February 21, 2006; 113(7): 969 - 976.
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 Eur. J. Cardiothorac. Surg.Home page
M. J. Flynn, J. M. McComb, and J. H. Dark
Temporary left ventricular pacing improves haemodynamic performance in patients requiring epicardial pacing post cardiac surgery
Eur. J. Cardiothorac. Surg., August 1, 2005; 28(2): 250 - 253.
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M U Braun, T Rauwolf, T Zerm, M Schulze, A Schnabel, and R H Strasser
Long term biventricular resynchronisation therapy in advanced heart failure: effect on neurohormones
Heart, May 1, 2005; 91(5): 601 - 605.
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O. S. Satish, K.-H. Yeh, M.-S. Wen, and C.-C. Wang
Cardiac resynchronisation therapy versus dual site right ventricular pacing in a patient with permanent pacemaker and congestive heart failure
Europace, January 1, 2005; 7(4): 380 - 384.
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M. Heinke, R. Surber, H. Kühnert, G. Dannberg, G. Schwarz, and H. R. Figulla
Transoesophageal left ventricular pacing in heart failure patients with permanent right ventricular pacing
Europace, January 1, 2005; 7(6): 617 - 620.
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B. Lamp, J. Vogt, H. Schmidt, and D. Horstkotte
Impact of cardiopulmonary exercise testing on patient selection for cardiac resynchronisation therapy
Eur. Heart J. Suppl., August 1, 2004; 6(suppl_D): D5 - D9.
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O. A. Breithardt, L. Herbots, J. D'Hooge, P. Claus, B. Bijnens, C. Stellbrink, A. Franke, and G. R. Sutherland
Strain rate imaging in CRT candidates
Eur. Heart J. Suppl., August 1, 2004; 6(suppl_D): D16 - D24.
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A Auricchio and C M Yu
Beyond the measurement of QRS complex toward mechanical dyssynchrony: cardiac resynchronisation therapy in heart failure patients with a normal QRS duration
Heart, May 1, 2004; 90(5): 479 - 481.
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A. Auricchio and W. T. Abraham
Cardiac Resynchronization Therapy: Current State of the Art: Cost Versus Benefit
Circulation, January 27, 2004; 109(3): 300 - 307.
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A. Auricchio
Pacing the left ventricle: does underlying rhythm matter?
J. Am. Coll. Cardiol., January 21, 2004; 43(2): 239 - 240.
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Y. Yu, A. Kramer, J. Spinelli, J. Ding, W. Hoersch, and A. Auricchio
Biventricular mechanical asynchrony predicts hemodynamic effect of uni- and biventricular pacing
Am J Physiol Heart Circ Physiol, December 1, 2003; 285(6): H2788 - H2796.
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X. A. A. M. Verbeek, K. Vernooy, M. Peschar, R. N. M. Cornelussen, and F. W. Prinzen
Intra-ventricular resynchronization for optimal left ventricular function during pacing in experimental left bundle branch block
J. Am. Coll. Cardiol., August 6, 2003; 42(3): 558 - 567.
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D. J. Bradley, E. A. Bradley, K. L. Baughman, R. D. Berger, H. Calkins, S. N. Goodman, D. A. Kass, and N. R. Powe
Cardiac Resynchronization and Death From Progressive Heart Failure: A Meta-analysis of Randomized Controlled Trials
JAMA, February 12, 2003; 289(6): 730 - 740.
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P. Sogaard, H. Egeblad, A. K. Pedersen, W. Y. Kim, B. O. Kristensen, P. S. Hansen, and P. T. Mortensen
Sequential Versus Simultaneous Biventricular Resynchronization for Severe Heart Failure: Evaluation by Tissue Doppler Imaging
Circulation, October 15, 2002; 106(16): 2078 - 2084.
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