Ann Thorac Surg 1996;62:1819
© 1996 The Society of Thoracic Surgeons
Invited Commentary
Invited Commentary
Graham J. Cooper, MD, FRCS(Ed)
Department of Cardiothoracic Surgery, Northern General Hospital, Sheffield S5 7AU, England
See also page 1816.
How much should we worry about free flow through the IMA? Specifically, should we worry enough to consider a statistically significant increase in free flow from 94 to 107 mL/min to be clinically significant?
Free flow through the IMA is determined by the balance between systemic arterial pressure and the resistance to flow imposed by the artery, which in turn depends on blood viscosity and the length and cross-sectional area of the artery. Once the diameter is less than 2 mm, resistance to flow increases exponentially. The diameter of the distal, muscular portion of the IMA is within this critical range. Thus dilating this section of the IMA can be expected to substantially increase its flow. Indeed, this has been shown by several investigators using either topical or systemic vasodilators alone or in combination with mechanical methods. Not only does this dilatation and increase in free flow reassure the surgeon and facilitate the IMA-to-coronary artery anastomosis, but also there is some evidence that the incidence of hypoperfusion is less with a well-dilated artery. After anastomosis, coronary vascular resistance also affects flow through the IMA graft. There is no direct evidence that IMA free flow correlates with postanastamotic flow. What evidence there is suggests that a free flow of 100 mL/min provides a postanastomotic flow equivalent to that through a vein graft.
Arnaudov and colleagues have shown that if you resect the distal 3 cm of the IMA, systemic vasodilators have relatively little effect on free flow. As they made no measurement before trimming, they have not strictly proved their assertion that trimming the distal 3 cm improves flow. However, the control flows approach 100 mL/min, which on available evidence should be adequate and is certainly higher than the level from which others have achieved substantial increase with vasodilators. One attraction of using systemic vasodilators is the possible benefit of maintaining vasodilation in the postoperative period. However, my, admittedly empirical, view is that in the postoperative period maintenance of systemic arterial pressure is of more importance in maintaining graft flow. Arnaudov and colleagues' study lends some weight to this belief; the decrease in free flow with sodium nitroprusside reflects the associated decrease in mean arterial blood pressure.
Thus, in clinical practice, it is sensible to trim the IMA as short as is compatible with a tension-free anastomosis; if it is possible to resect 3 cm of the IMA, vasodilators provide little clinically relevant improvement in flow. Often however, it is not possible to resect this much of the IMA, and it would seem prudent to use some technique to maximize free flow provided this does not damage the artery. The optimum method of maximizing flow through IMA grafts postoperatively remains to be clarified.
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Effect of Systemic Vasodilators on Internal Mammary Flow During Coronary Bypass Grafting
- Dimitri Arnaudov, Amram J. Cohen, Deeb Zabeeda, Eli Hauptman, Lior Sasson, Arie Schachner, and Shaul Ezra
Ann. Thorac. Surg. 1996 62: 1816-1819.
[Abstract]
[Full Text]
