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Journeys Inside the Heart: Fantastic Voyages, but What Will Their Impact Be?

      Origin of Cardioscopy

      In a report in 1922, Allen and Graham
      • Allen DS
      • Graham EA
      Intracardiac surgery—a new method: preliminary report.
      described the experimental use of a cardioscope to guide the surgical relief of mitral stenosis. Their cardioscope was a metal tube with a convex lens at one end and a small electric lightbulb placed against the lens to provide illumination. A knife to incise heart valves was attached to the side of the cardioscope. The cardioscope was inserted into the heart through the left atrial appendage, and when the lens was in contact with the mitral valve leaflets, excellent visualization was afforded. As those researchers wrote, the cardioscope provided a means to “go into the cavities of the heart without haste, to examine its chambers carefully, and to operate with deliberation on the interior of the heart under guidance of the eye.” In retrospect, this experiment was the beginning of cardioscopy.

      Current Experiments

      In this issue of the Mayo Clinic Proceedings (pages 629 to 635), Seward and associates describe a prototype phased-array ultrasound cardioscope that allows both ultrasonographic imaging and delivery of a catheter. These investigators use the term “cardioscope” analo gously to a laparoscope, to emphasize the point that the device facilitates imaging as well as performance of complex diagnostic or therapeutic interventions.
      The prototype cardioscope in this study is a rigid stainless steel probe that is 34 cm long and 8 mm (24 F) in diameter. The two functional components are a 7-MHz side-viewing 128-element ultrasound transducer and an 8-F diameter port that traverses the length of the device and exits near the distal end, just proximal to the ultrasound transducer.
      In this series of experiments, the cardioscope was introduced into the right heart chambers through an exposed external jugular vein in 20 dogs. With the cardioscope in the right ventricle, radio-frequency energy was applied within the left side of the heart through separate catheters. Cardioscopic monitoring of right ventricular and right atrial ablations was also completed. The authors report excellent ultrasound localization of the ablation catheter tips as well as documentation of changes in tissue density during and after ablation. These changes included development of small thrombi in ablation craters and intramyocardial hematomas. Results of cardioscopic assessment of the gross and microscopic characteristics and the distribution of the ablation injury correlated with findings on postmortem examination.

      Echocardiography Versus Fluoroscopy

      Intracardiac echocardiography has the potential to overcome several of the limitations of fluoroscopy and perhaps facilitate electrophysiologic procedures. Kalman and colleagues
      • Kaiman JM
      • Lee RJ
      • Fisher WG
      • Chin MC
      • Ursell P
      • Stillson CA
      • et al.
      Radiofrequency catheter modifications of sinus pacemaker function guided by intracardiac echocardiography.
      used intracardiac echocardiography with separate ultrasound imaging catheters in dogs for imaging the crista terminalis to supplement endocardial activation mapping to determine the sites of earliest atrial activations. The prominent ridge of the crista terminalis was imaged clearly in all animals, and direct imaging of the interface between the ablation catheter tip and the tissue was successful in 93% of energy applications. Chu and coworkers
      • Chu E
      • Fitzpatrick AP
      • Chin MC
      • Sudhir K
      • Yock PG
      • Lesh MD
      Radiofrequency catheter ablation guided by intracardiac echocardiography.
      studied the utility of intracardiac echocardiography in localizing radio-frequency ablations in various right atrial sites in dogs. They were able to apply radiofrequency energy accurately by using echocardiographic guidance to the junction of the inferior vena cava and right atrium, the junction of the superior vena cava and right atrium, the fossa ovalis, the coronary sinus ostium, and the orifice of the right atrial appendage.
      Successful radio-frequency ablation necessitates precise catheter positioning, which is currently based on intracardiac electrograms and fluoroscopic imaging. Fluoroscopy, however, does have some disadvantages and limitations. First, identification of detailed anatomic landmarks such as the crista terminalis or precise sites of electrode-endocardial tissue contact is not possible by fluoroscopy. Moreover, fluoroscopy has an inherent risk of radiation exposure, both to the patient and to the operator. A further concern is that ablation lesion formation is not amenable to assessment by fluoroscopy.
      Intracardiac echocardiography may be of greatest value in the assessment of radio-frequency ablation lesion size. Kalman and associates,
      • Kaiman JM
      • Jue J
      • Sudhir K
      • Fitzgerald P
      • Yock P
      • Lesh MD
      In vitro quantification of radiofrequency ablation lesion size using intracardiac echocardiography in dogs.
      who measured the depth and the width of radio-frequency ablations in dog hearts in vitro, found good correlation between ultrasonic and pathologic measurements of lesion depth and ablation of ventricular tachycardia, in which situation large lesions may be necessary. This modality may also be beneficial in guiding the long linear lesions necessary to ablate atrial flutter and reentrant atrial tachycardia.

      Deficiencies of Ultrasound Cardioscopy

      Although this merger of ultrasonic and cardioscopic technologies offers the potential for new insights and localizations during ablation procedures, several practical and technical limitations require consideration.
      One problem that may be encountered with any tomographic imaging system is sampling error-that is, the higher the resolution of the system and the thinner the imaging slice, the more difficult catheter tip localization may be. Although the ultrasound cardioscope is designed so the catheter exits the tip aligned with the imaging plane, the catheter could bend within the cavity or against the wall. If the tip passes out of the imaging plane, the apparent end of the catheter on the image would not actually be the tip. This problem has been encountered not only with intracardiac imaging catheters but also with the catheter procedures done with use of transesophageal echocardiography as a guiding method. In addition, because metallic electrode tips can distort ultrasound, an apparent imaging of the catheter tip does not mean that the tip is truly within the imaging plane. Inasmuch as metal is such a strong reflector of ultrasound, reverberant or distorted echoes may be received from electrode tips adjacent to, but not within, the imaging plane. Some interesting approaches to this problem of catheter tip localization have been undertaken by Frazin and colleagues.
      • Frazin LJ
      • Vonesh MJ
      • Yaacoub AS
      • Kane BJ
      • Greene R
      • Kemper WS
      • et al.
      Doppler catheter tip localization using color enhancement.
      The method involves use of a crystal on the catheter tip emitting at a slightly different frequency, which facilitates imaging and localization of the actual catheter tip by Doppler shift.
      Another consideration is that these are likely to be expensive devices that could have limited reusability. The issue of intracardiac imaging was a major topic at the 1996 North American Society of Pacing and Electrophysiology annual meeting. The consensus was that they were still unproven devices. Improved anatomic localization is needed for a small percentage of ablations (such as atrial tachycardia or flutter) or for instances in which the anatomy is grossly altered (Ebstein's malformation or the complex congenital anomalies). In most cases, analysis of intracardiac electrophysiologic waveforms with use of multipolar catheters yields a 95% success rate.
      Surgeons, however, did not initially believe that they needed an imaging method such as transesophageal echocardiography to help them plan or evaluate open-heart repairs for valvular or congenital heart disease because they directly visualized the hearts adequately during bypass. Perhaps skepticism about anatomic localization could resolve if ultrasound guidance reduced procedure time or fluoroscopic time (or both) and therefore decreased the radiation dose.

      Applications and Challenges

      Of fundamental interest would be the demonstration by the current investigative group and others that during and immediately after the application of radio-frequency energy, the tissue effect and injury produced could be monitored and the lesion position, size, and depth assessed in a reproducible fashion. Such observations might not only improve the efficiency and results of ablation procedures but also provide basic new knowledge in human patients of the types and extent of injury necessary for success of radio-freauencv ablation.
      With reduction in size of these devices (which will probably also entail an increase in frequency), a reasonable expectation is that intracardiac echocardiography could become important in electrophysiologic procedures. The use of intracardiac echocardiography to improve the accuracy of complex ablative procedures and the task of miniaturization of these probes present extremely challenging engineering problems.

      References

        • Allen DS
        • Graham EA
        Intracardiac surgery—a new method: preliminary report.
        JAMA. 1922; 79: 1028-1030
        • Kaiman JM
        • Lee RJ
        • Fisher WG
        • Chin MC
        • Ursell P
        • Stillson CA
        • et al.
        Radiofrequency catheter modifications of sinus pacemaker function guided by intracardiac echocardiography.
        Circulation. 1995; 92: 3070-3081
        • Chu E
        • Fitzpatrick AP
        • Chin MC
        • Sudhir K
        • Yock PG
        • Lesh MD
        Radiofrequency catheter ablation guided by intracardiac echocardiography.
        Circulation. 1994; 89: 1301-1305
        • Kaiman JM
        • Jue J
        • Sudhir K
        • Fitzgerald P
        • Yock P
        • Lesh MD
        In vitro quantification of radiofrequency ablation lesion size using intracardiac echocardiography in dogs.
        Am J Cardiol. 1996; 77: 217-219
        • Frazin LJ
        • Vonesh MJ
        • Yaacoub AS
        • Kane BJ
        • Greene R
        • Kemper WS
        • et al.
        Doppler catheter tip localization using color enhancement.
        Cathet Cardiovasc Diagn. 1994; 32: 62-69