Of Biventricular Pacemaker

9.1. Implantation Considerations

The key procedure for implementation of biventricular pacing is placement of the left ventricular lead. Clinical studies on multisite ventricular pacing in humans have mainly consisted of biventricular pacing modalities with transvenous left ventricular pacing via a coronary venous lead or, less commonly, a left ventricular epicardial lead through thoracotomy. It is usually necessary to target specific sites (posterolateral or lateral walls of the left ventricle) to maximize hemodynamic benefits. However, numerous limitations exist as technical challenges for current transvenous permanent pacing lead systems, such as: (1) patient safety, (2) potential of lead dislodge-ment, (3) altered pacing thresholds, (4) general lead handling, and (5) ease of use, in addition to variations of coronary vein anatomy (41).

A coronary venogram using two views is usually first obtained to provide a "road map" of the venous anatomy and to guide selection of the appropriate coronary sinus leads. Several delivery systems (including steerable catheters, venogram balloon catheters, and guiding sheaths with different curves) have been developed. An over-the-wire lead system has been described that is able to reduce the implant failure rate and avoid other more invasive means of left ventricular lead implantation (56). Many technical aspects in coronary sinus lead placement remain to be improved, although significant advances continue to be rapid in this field.

It is noteworthy that some investigators have implanted the left ventricular lead through the transseptal approach for left ventricular endocardial pacing (57). This method seems more reasonable to use in patients who require chronic anticoagulation. Another approach reported is the lead placement into the anterior paraseptum, which may be used to pace both ventricles synchronously; this approach has been associated with increased cardiac outputs (58).

9.2. Intraprocedural Testing

Figure 7 shows a device approved by the Food and Drug Administration for biventricular pacing (InSync® model 8040, Medtronic, Minneapolis, MN). This pacemaker has three lead outlets with a Y-adaptor in the header for insertion of both right and left ventricular leads. Most commercially available left ventricular leads are unipolar with a shared common ring of a bipolar sensing/pacing right ventricular lead. During lead positioning, pacing thresholds should be measured in a unipolar manner. Final thresholds should be determined using the intended pacing configuration similar to device connection.

The pacing threshold for left ventricular pacing from the coronary sinus is consistently higher compared to that during endocardial right ventricular pacing. However, it seems that the shape of the strength-duration curve during coronary vein left ventricular pacing is not significantly different from that during endocardial right ventricular pacing (Fig. 8) (unpublished data). The combined lead impedance is less than that of either lead and is typically in the range of 400 Q. The combined sensing of the two ventricles (which may be separated by 80 to 150 ms) requires additional consideration in device programming to avoid double sensing. This is particularly important during defibrillator implantation.

9.3. Risk and Complication

Of the 528 patients who underwent successful implantation in the MIRACLE study, the median duration of the procedure was 2.7 h (range 0.9-7.3 h). Implantation of the device was unsuccessful in 8% of the patients and was complicated by refractory hypotension, bradycardia, or asystole in 4 patients (2 of whom died) and by perforation of the coronary sinus requiring pericardiocentesis in 2 other patients. Dissection or perforation of the coronary sinus or cardiac vein occurred in about 6% of the patients, and other serious complications (including complete heart block, hemopericardium, and car diac arrest) presented in about 1.2% of the patients. Other complications included subsequent lead revision (6%) and explantation because of infection (1.3%).

10. PREDICTION OF EFFICACY OF BIVENTRICULAR PACING

A majority (70-80%) of patients respond well to biventricular pacing. In the MIRACLE trial, biventricular pacing was associated with more patients who elicited improvement (67 vs 39%) and fewer patients who progressively worsened (16 vs 27%) at the end of 6-month follow-up (16). Interestingly, basal demographic, clinical, and functional characteristics have failed to predict the effects of biventricular pacing. Limited data showed that there is increasing benefit for the use of biven-tricular pacing in patients with a wider QRS duration and severe left ventricular dysfunction (42,59).

However, it must be considered that a significantly enlarged heart may not respond to biventricular pacing, and some of them may not even tolerate the implantation procedure. Unfortunately, early hemodynamic testing does not necessarily predict potential chronic effects of biventricular pacing. Patients who show no improvement are more likely to have had a large myocardial infarction, low cardiac output, and no significant mitral regurgitation as those patients experiencing beneficial results (60).

Reduction in QRS duration induced by biventricular pacing is commonly seen in cardiac resynchronization therapy (39). However, QRS narrowing induced by biventricular pacing does not necessarily correlate with maximal mechanical hemody-namic benefits (37,59,61). For example, one animal study has indicated that improved mechanical synchrony and function do not require electrical synchrony (62). Further, dual-site right ventricular pacing (right ventricular apex and outflow tract) does not improve hemodynamics despite a narrowing QRS duration (63).

Yet, mechanical coordination is considered to play a dominant role in global systolic improvement in left ventricular-based pacing. In patients with complete right bundle branch block and drug-resistant heart failure, only patients with a right bundle branch block associated with a major left intra-ventricular asynchrony have been shown to respond well to biventricular pacing therapy (64). Importantly, Doppler tissue imaging can be a very useful technique for detecting regional electromechanical delays (defined as the time delay from the onset of the QRS to the onset of the systolic S wave of a given ventricular wall) to guide lead placement to optimize biven-tricular pacing (61,64,65).

At present, only echocardiographic or other image characteristics associated with left ventricular asynchrony have been reported to predict the effects of biventricular pacing. One of the major challenges in the field of biventricular pacing for congestive heart failure is the predictive clinical identification of patients who will benefit from biventricular pacing.

11. FUTURE PERSPECTIVE

There is growing acceptance for biventricular pacing as a treatment for patients with moderate-to-severe congestive heart failure in the presence of an intraventricular conduction

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment