Under Blood Visualization: An Ongoing Fulfillment of the Initial Promise

DOI: 10.19102/icrm.2011.021003

Samuel J. Asirvatham, MD, FHRS, FACC

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At times, the highest impact innovation for a particular field or procedure comes from an allied field that serves an adjunctive and enabling role. Safe and effective performance of invasive electrophysiology (EP) procedures requires a detailed knowledge of complex physiological principles as well as understanding the associated three-dimensional anatomy relevant for arrhythmia. In the pioneering days of EP, the major advantage that allowed our field to blossom did indeed detail such electrophysiological facts including the accessory pathway potential, entrainment mapping, etc.^1–3

When first developed, under blood visualization with intracardiac ultrasound had the potential of being a breakthrough innovation that impacted almost every aspect of interventional EP.^4,5 In this issue of Innovations in Cardiac Rhythm Management, Brenyo et al. provide an excellent overview of the available technology for intracardiac echocardiography (ICE) and provide us a view of what has been accomplished and what, today, is of commonplace practical value during ablation procedures.⁶

Promises Fulfilled

Either phased-array or rotational ICE imaging is commonly used today during EP procedures, especially those that require complex catheter navigation and extensive ablation.

Ease of Use

Although much of the information we get today with ICE can be obtained with transesophageal or careful transthoracic imaging, the fact that the electrophysiologists themselves perform the imaging and focus the imaging based on information needed for that particular step in the ablation process being performed has contributed to the widespread use of this technology.

Established Value

For the trainee, the value and safety added with ICE when performing transseptal puncture is obvious and is often the reason why ICE is used during the procedure by having the probe in place whenever hypotension is noted. When there is an expected change in the clinical course for the patient, as detailed by Brenyo et al., ICE imaging readily identifies significant pericardial effusion, valvular damage, and possible ischemia. Our anticoagulation management in the periablation period has been significantly impacted by the information provided by intracardiac ultrasound.^7–9 Finding significant thrombi in the left atrium minutes after transseptal and the decreased fear of performing transseptal puncture¹⁰ has combined to allow early heparinization prior to transseptal puncture or continued warfarin anticoagulation during ablation, something that would’ve been unthinkable in the pre-ICE era.

Ongoing Innovation – The Future

The key feature of the truly successful innovations in science is that the breakthrough serves as a platform technology for continued secondary innovation to truly fulfill the original potential of the invention.

Activation Mapping

So called four-dimensional echocardiography was an early promise of ICE imaging. Here, in addition to three-dimensional reconstruction in real time of the cardiac structures, activation points using the mechanical wavefront of contraction as a surrogate for electrical activation were promised. To date, however, this has not materialized as a commonplace procedure.^11–14 The reasons why this application has been relatively slow are not readily apparent. Algorithms for quick, real time, three-dimensional reconstruction have been developed, and the incredible frame rates aborted by “close” visualization with ICE should make this possible. When this application materializes in a practical way, the needed innovation likely requires even better temporal resolution and intuitive three-dimensional imaging that reflect the fluoroscopic views that ablationists have become comfortable with. Another reason for this yet unfulfilled potential may be the rise of so-called three-dimensional mapping systems¹⁵ that allow us to visualize propagation sequences. However, the limitations with such systems both in terms of defining anatomy and the difficulty associated with taking the activation points accurately need to be appreciated.^16,17

Lesion Formation

A remaining limitation of present day EP is our inability to know whether we are creating lesions when delivering energy. With three-dimensional mapping, for example, colored dots are taken to reflect lesions created in the three-dimensionally rendered anatomy. However, the lack of knowledge of catheter contact, transmurality of the lesion created, and, in fact, whether any energy has been delivered to the targeted substrate make these “dots” an attractive video game at best. If, in fact, we knew when transmural lesions are created, the ease with which gaps could be defined and incomplete linear lesions avoided would be considerable.

ICE imaging had early promise for each of these components as in contact, energy delivery at the tissue interface, and transmurality of lesion formation.^18,19 Why hasn’t ICE fulfilled this important need that in many ways the technology is best fit to do? Spatial resolution, particularly at significant distances from the imaging transducer, and the necessity for multiple imaging planes to define contact and transmurality remain obstacles that require ongoing innovation.

Understanding whether lesions are occurring is not only important in the heart tissue but for collateral damage as well. Brenyo et al. describe the potential value in avoiding atrial esophageal fistula formation.²⁰ However, while ICE can tell us how far the anterior wall of the esophagus is to our ablation catheter, since we don’t yet exactly know whether an ablation lesion has caused a full thickness atrial burn, it becomes difficult to know whether the anterior wall of the esophagus or the associated vascular are being damaged.

Cardiac Resynchronization

For the last decade, multiple potentially valuable techniques to identify patients who benefit from cardiac resynchronization therapy (CRT) as well as optimizing devices when implanted have been suggested and have appeared theoretically sound and, in fact, found useful in small studies and anecdotal reports. However, larger, randomized trials have shown no benefit from any single or conceivable combination of echocardiographic variables.^21,22 A potential primary reason for this failure of another innovative use of echocardiography to materialize may have to do with temporal resolution and frame rate. Intracardiac echo simply by virtue of the proximity to the ventricular myocardial tissue could allow this better temporal resolution, and, when combined with novel echocardiographic imaging techniques, may be an important adjunct to CRT as it has become to interventional EP. While this has been investigated in smaller studies without clear demonstration of benefit, further innovation allowing quick three-dimensional reconstruction and modeling to use the derived velocity and strain information to predict overall benefit from a particular site or type of stimulation could become an exciting method to decrease the troublesomely high non-responder rates with CRT.

Summary

In this issue of Innovations in Cardiac Rhythm Management, Brenyo et al. review some major, high impact innovations over the last 15 years in interventional EP. Their review sets the stage for ongoing and future innovation such that the initial, even greater promised device may be fulfilled.

Samuel J. Asirvatham, MD, FHRS, FACC
asirvatham.samuel@mayo.edu
Consultant, Division of Cardiovascular Diseases
and Internal Medicine, Division of Pediatric Cardiology
Professor of Medicine and Pediatrics
Mayo Clinic College of Medicine
Rochester, MN

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