Ann Thorac Surg 1995;59:416-418
© 1995 The Society of Thoracic Surgeons
Effect on Blood Flow of Rotation and Position of the Internal Mammary Artery Pedicle
Robin P. Brown, FRCS
Alfred Hospital, Melbourne, Australia
Accepted for publication October 3, 1994.
 |
Abstract
|
|---|
The internal mammary artery is the preferred conduit for coronary artery bypass grafts. The effect on flow through the artery of both rotation and position of the artery pedicle was tested in an in-vitro setting on 20 arteries using a perfusate closely approximating the normothermic viscosity of blood. It was found that the effect of rotation on blood flow was highly significant (p < 0.0004) and that this occurs after a degree of rotation that is dependent on the pedicle length. The critical degree of rotation required to significantly affect blood flow is 30 degrees for each centimeter of pedicle length. The degree of rotation required to stop flow altogether is 45 degrees per unit length. Flow through the arteries was found to be inversely proportional to the length of the artery but was not affected by changes in the position of the artery provided there was not a rotational component. Tension of the pedicle did not influence flow in either a linear or rotated position. These results were verified in a clinical setting. This study supports that a 180-degree rotation of the internal mammary artery pedicle in any clinical setting will not alter blood flow. For rotations of 360 degrees or greater, however, the effect on flow becomes significant when this is 12 cm or less. Although flow is independent of linear pedicle deformity, as flow is inversely proportional to length, excess pedicle length is best avoided.
|
Introduction
|
|---|
Internal mammary artery (IMA) grafting is now routine for the majority of coronary artery bypass procedures performed throughout the world. The final resting position of the pedicle will depend on the target vessel grafted, the length of the pedicle, its route through or over the pericardium, and the extent of pleural opening. Before anastomosis, the pedicle may be rotated through 180 degrees for ease of anastomosis or inadvertently through 360 degrees or greater. Despite the frequency with which this operation is performed and concern among surgeons regarding the adequacy of IMA flow that is usually supplying a critical area of myocardium, little attention has been paid to factors regarding pedicle position that influence its flow. This is indeed remarkable as many techniques have been described to optimize flow through to the IMA. The aim of this study was to determine the effect of rotation and of different linear deformities of the IMA pedicle on its blood flow.
|
Material and Methods
|
|---|
A total of 20 IMA pedicles were obtained from the autopsy room. Those of appropriate length were left intact and others shortened so as to give a range of lengths from 4 cm to 20 cm (Table 1
). All arteries were stored in normal saline solution at 4°C until testing, which always was performed within 12 hours. To approximate the clinical situation, 26.75% glycerol in normal saline solution was used as an infusate as this approximates the viscosity of blood at normothermia [1]. This solution was infused through the artery from a suspended container. The pressure generated is a function of the height of the infusate above the origin of the artery and can be calculated by knowing the density of the infusate to be 1.07 g/mL [2]. A container with a large surface area was used to ensure that the pressure generated would remain approximately constant with the volume decrease that occurred during the experiment.
The flow through an artery is directly proportional to the pressure gradient over the length of the artery. This gradient in a clinical setting after bypass grafting is unknown as although the pressure of blood entering the artery will approximate the systemic pressure, the pressure at the distal end of the artery is less well known. When the isolated arteries are tested, the distal pressure is known to be atmospheric, so to approximate the clinical gradient, the height of the infusate reservoir was adjusted to produce a flow comparable with known IMA flow rates. By doing so, the gradient was indirectly approximated. This was adjusted to produce average flow rates comparable with those obtained by duplex imaging of intact IMAs [3]. A flow rate of approximately 300 mL/min through the apparatus before attaching the artery was chosen, and this corresponded to an infusate height of 28 cm.
The solution was infused through the arteries at this pressure and was collected in a measuring container over a timed interval, and the flow was calculated. The length of all arteries was measured before testing, and the flow through the artery first was measured in its linear, nonrotated state. The pedicle then was rotated through 180-degree increments and the process repeated. This was continued until there was no further flow through the artery. Five measurements were obtained and averaged for each increment of rotation, and each measurement was made both with and without tension in the pedicle for comparison. To apply tension to the pedicle, a 100-g weight was suspended from it, which corresponded to a constant force of 0.98 newtons. This amount of tension was chosen as it was the maximal force tolerated by most pedicles without sustaining pedicle damage. Each pedicle then was deformed to simulate different courses of the artery in a clinical setting from its subclavian origin to its ultimate coronary destination. This included a tortuous course through the left pleural space, a course over an intact but unclosed pericardium, and a direct line through a pericardial defect. Angulations in its course varied between 0 and 145 degrees. Five measurements again were obtained for each linear deformity. The determinants of flow were analyzed for each increment of rotation and linear deformity with nonparametric regression analysis.
|
Results
|
|---|
The flow rates through the different length arteries in their nonrotated state and with increments of rotation are shown in Table 1
. The values given are to one standard deviation from the mean of the five measurements recorded.
The flow first was plotted as a function of the length of the artery. There was a variation in flows for different length arteries, but the trend was that flow was inversely proportional to the length of the artery as described by Poiseuille's law [2]. The variations can be accounted for by differences in artery diameter, which was not measured. The relationship was more constant when different lengths of the same artery were measured.
It was in the analysis of the effect on flow of rotation of the pedicles that the results were most interesting. This was analyzed by nonparametric two-way analysis of variants and found to be highly significant (p < 0.0004). Upon rotation of the IMA pedicle, the flow through the artery remained constant until a point was reached where the flow became significantly reduced. With continued rotation of the pedicle, flow continued to diminish until a further point was reached where flow ceased altogether. This was usually one additional increment of rotation. The degree of rotation (in increments of 180 degrees) required to reach each of these points was plotted against the length of the pedicle. It was found that the degree of rotation required to affect flow through the artery is directly proportional to the length of the artery. From this, critical lengths were determined at which flow will become significantly reduced for a given degree of rotation. These lengths were 6 cm, 12 cm, and 18 cm for 180, 360, and 540 degrees of rotation, respectively. The critical degree of rotation to affect flow can be expressed as the angle per unit length and was found to be constant at 30 degrees rotation for each centimeter of artery length. The angle of rotation required to significantly reduce flow therefore can be determined by multiplying its length by 30. The critical lengths for total cessation of flow were 7 cm, 12 cm, and 16 cm for rotations of 360, 540, and 720 degrees, respectively. Cessation of flow did not occur with 180-degree rotation in the range of arteries tested. The angle per unit length for total cessation was constant at approximately 45 degrees. The flow through the arteries was found to be independent of the different linear deformities of the arteries. This was the case for all lengths of artery with their corresponding differences in flow rates. It was demonstrated that although the flow rates through the different arteries was highly variable, the flow rate through any given artery did not change significantly with different linear deformities.
To establish whether these results could be extrapolated to a clinical setting, measurements were taken in the operating room from IMA pedicles before anastomosis in 5 patients (Table 2). These verified the obtained results.
|
Comment
|
|---|
Both the degree of rotation and any linear deformity required to significantly reduce blood flow through an IMA supplying an area of myocardium are of great clinical importance. This study has demonstrated that the course taken by the artery pedicle to its coronary destination does not affect flow provided there is no rotation of the pedicle. As flow is directly proportional to the length of the artery, however, a more direct course taken by the IMA will result in a greater blood flow. A proximal anastomosis also will be beneficial to flow.
Rotation of the pedicle easily can pass unnoticed before anastomosis. This study has demonstrated that this is a highly significant factor regarding blood flow, and for a pedicle length of 12 cm or less, a 360-degree rotation will affect flow. As the average length of a left IMA pedicle to left anterior descending artery anastomosis is approximately 12 cm, a complete rotation of the pedicle before anastomosis should be avoided. A rotation of 180 degrees will be tolerated with no effect on blood flow in any clinical setting as all pedicle lengths will be greater than the critical length of 6 cm.
|
Footnotes
|
|---|
Address reprint requests to Dr Brown, 4 Fraser St, Middle Park, Victoria 3206, Australia.
|
References
|
|---|
- Gourlay T, Gibbons M, Taylor KM. Pulsatile flow compatability of a group of membrane oxygenators. Perfusion 1987;2:115–26.
- Giancoli D. Physics for scientists and engineers with modern physics. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1988:290–307, 316–8.
- Canver C, Ricotta J, Bhayana J, et al. Use of duplex imaging to assess suitability of the internal mammary artery for coronary artery surgery. J Vasc Surg 1991;13:294–301.[Medline]
Top
Abstract
Introduction
