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J Thorac Cardiovasc Surg 1997;113:726-727
© 1997 Mosby, Inc.

SURGERY FOR CONGENITAL HEART DISEASE

Commentary

Although the authors discuss some of the potential limitations of the method used in their study, several aspects of it deserve further consideration. By sampling flow velocity in the native aortic root and calculating flow rate, the authors suggest that most of that flow enters the coronary circulation and is reflective of coronary blood flow. It should be realized, however, that this assumption has not been validated and the proportion of native aortic root flow entering the coronary arteries is unknown. As noted in the article, some of the blood that enters the native ascending aorta is reflected and does not enter the coronary circulation. Inasmuch as spectral Doppler echocardiography displays the summed average flow velocity and direction along the beam path, it cannot accurately resolve instantaneous flow in opposite directions. By using Doppler flowmetry in the native ascending aorta, one is currently unable to resolve the relative proportions of circular flow and coronary flow. Other inherent limitations of Doppler echocardiography might further complicate its use in the infant with aortic atresia who has undergone palliation. These technical limitations, including dependency on angle of interrogation, an assumed circular shape of the native ascending aorta, respiratory and cardiac motions that shift the region of interrogation, and other limitations, further illustrate the need for studies to determine the validity and accuracy of Doppler echocardiography in this particular anatomic-physiologic setup.

Future research on coronary blood flow dynamics and its relation to surgical treatment in patients with hypoplastic left heart syndrome may correlate parameters that are obtainable in the cardiac catheterization laboratory or during the operation with noninvasive techniques. For example, coronary blood flow can be estimated by means of transvascular intracoronary imaging and Doppler ultrasonography. This technique has been successfully used in the pediatric age group, as demonstrated by Hamaoka and Onouchi. ¹ Another approach is to measure coronary sinus blood flow using either catheter-tipped flowmetry or intravascular Doppler ultrasonography. The metabolic state of the myocardium, which reflects coronary supply/demand ratio, can also be assessed by measuring lactate, pH, and other metabolic parameters in coronary sinus blood samples. ² An attractive noninvasive alternative to Doppler echocardiography is magnetic resonance imaging (MRI). Recent technologic advances in flow imaging and quantification by MRI make this modality a promising research tool in the study of coronary flow dynamics. ³ In particular, multidimensional phase velocity mapping MRI is capable of resolving flow velocity throughout the cardiac cycle in three spatial dimensions. This technique has shown a promising prospect when applied to coronary arteries in adults. ⁴ In addition, MRI is also capable of evaluating myocardial perfusion and ventricular function. ⁵

Dr. Fogel and his colleagues took the first step in a long journey toward better understanding of a complex interplay between coronary flow dynamics and pathophysiology of the palliated cardiovascular system in patients with various forms of hypoplastic left heart syndrome. Such improved understanding is essential for future improvements in surgical management of these patients.

Tal Geva, MD

The Children's Hospital

300 Longwood Ave.

Boston, MA 02115

References

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2. Paridon SM, Fisher DJ. Regulation of myocardial blood flow and oxygen consumption. In: Carson A, Bricker JT, McNamara DG, editors. The science and practice of pediatric cardiology. Philadelphia: Lea & Febiger, 1990;250-65.
   
3. Poncelet BP, Weisskoff RM, Wedeen VJ, Brady TJ, Kantor HW. Time of flight quantification of coronary flow with echo-planar MRI. Magn Reson Med 1993;30:447-57.[Medline]
   
4. Keegan J, Firmin D, Gatehouse P, Longmore D. The application of breath hold phase velocity mapping techniques to the measurement of coronary artery blood flow velocity: phantom data and initial in vivo results. Magn Reson Med 1994;31:526-36.[Medline]
   
5. Wendland MF, Saeed M, Masui T, Demgin N, Moseley ME, Higgins CB. Echoplanar MR imaging of normal and ischemic myocardium with gadoliamide injection. Radiology 1993;186:535-42.[Abstract/Free Full Text]
   

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