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Hemodynamics of venous repair in the canine hind limb
Hemodynamics of venous repair in the canine hind limb Creighton B. Wright, Major, MC, USA, and Kenneth G. Swan, Lieutenant Colonel, MC, USA, Washington, D. C.
Ligation has been the treatment of choice for wounds of major veins during wartime. Because of anatomic relationships, venous injuries are usually accompanied by arterial injuries." 8, 10 The surgeon usually directs his greatest concern, in terms of therapy, to reconstruction of the injured artery rather than the associated vein.v 3, 7, 10, 12 In a recent report of vascular wounds in Vietnam, 33 per cent of the injured veins were repaired, and the remainder were ligated." Reports which predate this analysis indicate that very few venous repairs were undertaken." 3, 7, 8, 12 It has been assumed that, because of extensive collateral circulation, ligation of major veins would be tolerated by the leg. In general, this is true; however, it has been demonstrated recently that ligation of the femoral vein causes a marked reduction in femoral arterial blood flow. This reduction in arterial blood flow would presumably jeopardize the patency rate of arterial reconstructive procedures.': 2, 4, 5 The present study was designed to assess the effects of autogenous vein grafts placed in the femoral venous system, as well as the effects of primary venous anastomoses, upon arterial blood flow. Two questions are to be
Adult mongrel dogs, with a mean weight of 22 kilograms, were anesthetized with pentobarbital (30 mg. per kilogram given intravenously). The femoral artery and vein were exposed in each inguinal region. After a segment of an external jugular vein was excised and flushed with heparinized saline, its branches were ligated. Femoral arterial blood flow was measured with an electromagnetic blood flowmeter (Biotronex Laboratory, Inc., Silver Spring, Md.), connected to a flowmeter transducer (In Vivo Metric Systems, Los Angeles, Calif.) , and calibrated in vitro and in vivo with whole blood. Zero blood flow was obtained with a vascular clamp distal to the flow probe. Pressure-recording catheters (PE 160, ClayAdams, Inc., New York, N. Y.) were placed in the right atrium through an external jugular vein and were inserted into the saph-
From the Division of Surgery, Walter Reed Army Institute of Research, Washington, D.C. 20012. Received for publication May 18, 1972, Address for reprints: Creighton B. Wright, Major, MC, Division of Surgery, Walter Reed Army Institute of Research, Washington, D. C. 20012.
• In conducting the research described in this report, the investigators adhered to the "Guide for Laboratory Animal Facilities and Care," as promulgated by the Committee on the Guide for Laboratory Animal Facilities and Care of the Institute of Laboratory Animal Resources, National Academy of Sciences-National Research Council.
answered: (1) Does one anastomosis, as opposed to two, in the venous system alter hemodynamics in the extremity? (2) What degree of obstruction in the venous system can be tolerated without alteration in arterial blood flow? Methods*
195
The Journal of
1 96
Wright and Swan
Thoracic and Cardiovascular Surgery
enous system through one of its branches. They were then connected to transducers (Statham Instruments, Inc., Hato Rey, Puerto Rico) calibrated at the start of each experiment with a mercury manometer. The data were collected simultaneously on a multichannel recorder (Sanborn Co., Waltham, Mass.). Following a 30 minute postoperative interval, control measurements were made of femoral arterial blood flow, central venous pressure, and peripheral venous pressure. A 2 em. segment of the femoral vein was then excised and replaced with the jugular venous graft with the use of a continuous 6-0 braided polyester suture (DeKnatel, Inc., Queens Village, N. Y.). Femoral arterial flow and pressures were recorded during the vascular anastomosis and for a 60 minute interval after release of the vascular clamps. Pressure recordings were then obtained across the anastomoses by means of the peripheral venous pressure catheter. In a single experiment, a primary anastomosis of the divided femoral vein was performed in the previously described fashion, and the same parameters were measured. In a second, single experiment, after insertion of the vein graft, progressive increments of venous obstruction were produced by means of a snare in the midportion of the graft, and the effects upon peripheral pressure as well as femoral arterial blood flow were recorded. The results were expressed in terms of the mean ± the standard error, and their statistical significance was assessed with the t test for paired data.> Results During the control period, femoral arterial blood flow was 176 ± 14 ml. per minute. Saphenous venous pressure was 3.0 ± 0.1, and central venous pressure was 2.2 ± 0.2 mm. Hg. After occlusion of the femoral vein, femoral arterial blood flow fell significantly (p < 0.001) to 60 ± 4 ml. per minute, within minutes, and remained at this low level until venous occlusion was terminated. During the same interval,
peripheral venous pressure increased significantly (p < 0.001) to 25.4 ± 4.5 mm. Hg and remained elevated during venous occlusion. At the end of the period of venous occlusion, peripheral venous pressure had decreased to 16.6 ± 2.8 mm. Hg. This value was significantly (p < 0.01) below the initial high value for venous pressure but was still significantly (p < 0.001) above control. During the same time interval, central venous pressure remained unchanged. Release of the vascular clamps after completion of the autogenous vein graft restored femoral arterial blood flow to the control value within minutes, and flow remained unchanged for the duration of the 1 hour observation period. Peripheral venous pressure also returned to control levels within minutes after release of femoral venous occlusion. Pressure recordings across the venous anastomoses revealed no demonstrable gradient at either site. These results are summarized in Fig. 1. In a single study, the primary anastomosis resulted in similar changes in arterial blood flow and peripheral venous pressure after release of the vascular clamps. There was no pressure gradient across the suture lines. When the effects of progressive occlusion of the autogenous vein graft were expressed in terms of peripheral venous pressure increments plotted against decrements in femoral arterial flow, an asymptotic relationship was observed (Fig. 2). Doubling the peripheral venous pressure reduced femoral arterial blood flow by 30 per cent. When pressure was increased fourfold, flow was halved. However, increasing peripheral venous pressure to a value six times control reduced femoral arterial flow to only 30 per cent of the control value.
Discussion The results of this study confirm previous observations- 14 that occlusion of the canine femoral vein causes a marked reduction in the flow of blood through the ipsilateral femoral artery. This reduction in arterial
Volume 65
Venous repair in canine hind limb
Number 2 February. 1973
FEMORAL VEIN OCCLUSION
200
c
E
<,
---'
E
1-
VEIN GRAFT OPENED
1
/--1-
1 -
~
o_I u,
>a::
197
100
-
1
w ~
a::
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a::
o
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o
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t_,
30 C'
I
E E 20
u.i
a::
::J
(J) (J)
w
10
.
a::
CL
0
,. SAPHENOUS VEIN • SUPERIOR VENA CAVA
.-~.--
,-----,
......:._~---=,
o
30
60 TIME,
90
min
Fig. 1. The effects of occlusion of the femoral vein on femoral arterial blood flow, central venous pressure, and peripheral venous pressure are compared before and after placement of an autogenous vein graft in the femoral vein. Each point in the graph represents the mean ± the standard error of five experiments in 5 animals.
flow is sustained for up to 4 hours of venous occlusion and is accompanied by a marked rise in peripheral venous pressure." The increase in venous pressure raises peripheral resistance, which accounts for the fall in arterial flow. This sequence of events would presumably jeopardize the success of arterial reconstructive procedures and provides experimental support for the contention that combined arterial and venous injuries require as careful attention to venous reconstruction as to arterial repair." Clearly, ligation of the femoral vein is contraindicated in concomitant arteriovenous injuries in which venous reconstruction is technically feasible. Because of the extreme elasticity of the vein, a defect there is less amenable to treatment by primary anastomosis than is one in the related artery; thus the surgeon faces the probability of two suture lines in most
venous repair procedures. The present study provides evidence that, hemodynamically, there is no difference between performance of a primary anastomosis as opposed to insertion of an autogenous vein graft in restoring continuity of a divided femoral vein. Except for the time saved, a single anastomosis does not appear more advantageous than several suture lines. Under certain circumstances the surgeon may elect to ligate an injured femoral vein because of other priorities, such as injuries to the same patient or other patients, or because of the lack of a suitable venous autograft. In those cases in which assistance in the decision-making process is desirable, a measurement of peripheral venous pressure can be of prognostic significance. If venous ligation does not significantly increase peripheral venous pressure, then sufficient
The Journal of
1 98
Wright and Swan
Thoracic and Cardiovascular Surgery
e
E 200
<,
E
s ~
IJ..
...J
~
w
100
~
a:
...J
iio ~ W
IJ..
o
10
20
PERIPHERAL VENOUS PRESSURE, mm Hg
Fig. 2. The effects of progressively increasing degrees of constriction of the venous system at the site of the autogenous vein graft upon blood flow through the femoral artery.
collateral venous drainage has already provided for a satisfactory reduction in peripheral resistance and femoral arterial flow may not be seriously compromised. The technique of measuring peripheral venous pressure is simple and readily accomplished by insertion of a small catheter into a branch of the femoral vein distal to the point of ligation. The catheter is connected to a sterile, saline-filled manometer. Peripheral venous pressure is normally in the neighborhood of 50 mm. of water for supine man. The same technique can provide a quantitative evaluation of the success of individual anastomotic suture lines, where questionable, during inspection of the repaired vein. As indicated in the present study, the critical gradient is 5 to 15 mm. Hg. Values above this range may cause a 50 per cent reduction in femoral arterial blood flow. Where possible, the surgeon should strive for anastomoses of grossly normal caliber with no pressure gradients. As Haimovici" has recently stressed, there is no substitute for meticulous technique in achieving success with venous anastomoses.
of femoral arterial flow. Ligation of the femoral vein, primary anastomosis, and interposition of an autogenous vein graft were compared. Femoral arterial flow was measured with an electromagnetic flowmeter; arterial, central venous, and peripheral venous pressures were recorded. Occlusion of the femoral vein resulted in a marked and sustained reduction in femoral arterial flow, accompanied by a marked increase in peripheral venous pressure. There was no difference between one and two suture lines in terms of the hemodynamics of femoral vein reconstruction. Primary anastomosis of the divided vein or insertion of an autogenous vein graft caused femoral arterial flow and peripheral venous pressure to return to control within minutes after release of the vascular clamps. These findings provide experimental evidence in support of the contention that concomitant arterial and venous injuries of an extremity require venous reconstruction to protect arterial reconstructive procedures. A satisfactory venous repair is one without ~" pressure gradient across any suture line. The technical assistance of SP5 Ernest Alix is gratefully acknowledged. The data were analyzed statistically with the assistance of Mr. Douglas Tang, Chief, Department of Biostatistics and Mathematics, Walter Reed Army Institute of Research, Washington, D. C. REFERENCES
2
3
4
Summary
The effects of femoral venous surgery upon femoral arterial blood flow were studied in anesthetized dogs to define that procedure most advantageous to restoration
5
Barcia, P. J., Nelson, T. G., and Whelan, T. J.: The Importance of Venous Occlusion in Arterial Repair Failure: An Experimental Study, Ann. Surg. 175: 223, 1972. Chandler, J. "G., and Knapp, R. W.: Early Definitive Treatment of Vascular Injuries in the Vietnam Conflict, J. A. M. A. 202: 960, 1967. Cohen, A, Baldwin, 1. N., and Grant, R. N.: Problems in the Management of Battlefield Vascular Injuries, Amer. J. Surg. 118: 526, 1969. Dart, C. H., Johnson, G., Peters, R. M., and Womack, N. A.: Hemodynamic Effects of Femoral Venous Occlusion Before and After an Acute Arteriovenous Fistula, Ann. Surg. 164: 190, 1966. Greenstein, A. J., and Mannick, J. A.: Effect of Restricted Outflow on the Early Patency of Arterial Grafts, Surg. Forum 17: 140, 1966.
Volume 65 Number 2
Venous repair in canine hind limb
19 9
Februory, 1973
6 Haimovici, H., Hoffert, P, W., and Zinicola, N.: An Experimental and Clinical Evaluation of Grafts in the Venous System: Collective Review, Surg. Gynecol. Obstet. 131: 1173, 1970. 7 Hughes, C. W.: The Primary Repair of Wounds of Major Arteries, Amer. J. Surg. 141: 297, 1955. 8 Hughes, C. W.: Arterial Repair During the Korean War, Ann. Surg. 147: 555, 1958. 9 Husni, C. A.: Venous Reconstruction in Postphlebitic Disease, Circulation 43: 147, 1971 (Suppl, I).
10 Rich, N. M., Baugh, J. H., and Hughes, C. W.:
11 12 13 14
Popliteal Artery Injuries in Vietnam, Amer. J. Surg. 118: 531, 1969. Rich, N. M., Hughes, C. W., and Baugh, J. H.: Management of Venous Injuries, Ann. Surg. 171: 724, 1970. Snedecor, G. W., and Cochran, W. G.: Statistical Methods, ed. 6, Ames, Iowa, 1967, Iowa State University Press. Spencer, F. C., and Grewe, R. V.: The Management of Arterial Injuries in Battle Casualties, Ann. Surg. 141: 304, 1955. Wright, C. B., and Swan, K. G.: Hemodynamics of Venous Occlusion in the Canine Hindlimb, Surgery 73: 141, 1973.
Ligation has been the treatment of choice for wounds of major veins during wartime. Because of anatomic relationships, venous injuries are usually accompanied by arterial injuries." 8, 10 The surgeon usually directs his greatest concern, in terms of therapy, to reconstruction of the injured artery rather than the associated vein.v 3, 7, 10, 12 In a recent report of vascular wounds in Vietnam, 33 per cent of the injured veins were repaired, and the remainder were ligated." Reports which predate this analysis indicate that very few venous repairs were undertaken." 3, 7, 8, 12 It has been assumed that, because of extensive collateral circulation, ligation of major veins would be tolerated by the leg. In general, this is true; however, it has been demonstrated recently that ligation of the femoral vein causes a marked reduction in femoral arterial blood flow. This reduction in arterial blood flow would presumably jeopardize the patency rate of arterial reconstructive procedures.': 2, 4, 5 The present study was designed to assess the effects of autogenous vein grafts placed in the femoral venous system, as well as the effects of primary venous anastomoses, upon arterial blood flow. Two questions are to be
Adult mongrel dogs, with a mean weight of 22 kilograms, were anesthetized with pentobarbital (30 mg. per kilogram given intravenously). The femoral artery and vein were exposed in each inguinal region. After a segment of an external jugular vein was excised and flushed with heparinized saline, its branches were ligated. Femoral arterial blood flow was measured with an electromagnetic blood flowmeter (Biotronex Laboratory, Inc., Silver Spring, Md.), connected to a flowmeter transducer (In Vivo Metric Systems, Los Angeles, Calif.) , and calibrated in vitro and in vivo with whole blood. Zero blood flow was obtained with a vascular clamp distal to the flow probe. Pressure-recording catheters (PE 160, ClayAdams, Inc., New York, N. Y.) were placed in the right atrium through an external jugular vein and were inserted into the saph-
From the Division of Surgery, Walter Reed Army Institute of Research, Washington, D.C. 20012. Received for publication May 18, 1972, Address for reprints: Creighton B. Wright, Major, MC, Division of Surgery, Walter Reed Army Institute of Research, Washington, D. C. 20012.
• In conducting the research described in this report, the investigators adhered to the "Guide for Laboratory Animal Facilities and Care," as promulgated by the Committee on the Guide for Laboratory Animal Facilities and Care of the Institute of Laboratory Animal Resources, National Academy of Sciences-National Research Council.
answered: (1) Does one anastomosis, as opposed to two, in the venous system alter hemodynamics in the extremity? (2) What degree of obstruction in the venous system can be tolerated without alteration in arterial blood flow? Methods*
195
The Journal of
1 96
Wright and Swan
Thoracic and Cardiovascular Surgery
enous system through one of its branches. They were then connected to transducers (Statham Instruments, Inc., Hato Rey, Puerto Rico) calibrated at the start of each experiment with a mercury manometer. The data were collected simultaneously on a multichannel recorder (Sanborn Co., Waltham, Mass.). Following a 30 minute postoperative interval, control measurements were made of femoral arterial blood flow, central venous pressure, and peripheral venous pressure. A 2 em. segment of the femoral vein was then excised and replaced with the jugular venous graft with the use of a continuous 6-0 braided polyester suture (DeKnatel, Inc., Queens Village, N. Y.). Femoral arterial flow and pressures were recorded during the vascular anastomosis and for a 60 minute interval after release of the vascular clamps. Pressure recordings were then obtained across the anastomoses by means of the peripheral venous pressure catheter. In a single experiment, a primary anastomosis of the divided femoral vein was performed in the previously described fashion, and the same parameters were measured. In a second, single experiment, after insertion of the vein graft, progressive increments of venous obstruction were produced by means of a snare in the midportion of the graft, and the effects upon peripheral pressure as well as femoral arterial blood flow were recorded. The results were expressed in terms of the mean ± the standard error, and their statistical significance was assessed with the t test for paired data.> Results During the control period, femoral arterial blood flow was 176 ± 14 ml. per minute. Saphenous venous pressure was 3.0 ± 0.1, and central venous pressure was 2.2 ± 0.2 mm. Hg. After occlusion of the femoral vein, femoral arterial blood flow fell significantly (p < 0.001) to 60 ± 4 ml. per minute, within minutes, and remained at this low level until venous occlusion was terminated. During the same interval,
peripheral venous pressure increased significantly (p < 0.001) to 25.4 ± 4.5 mm. Hg and remained elevated during venous occlusion. At the end of the period of venous occlusion, peripheral venous pressure had decreased to 16.6 ± 2.8 mm. Hg. This value was significantly (p < 0.01) below the initial high value for venous pressure but was still significantly (p < 0.001) above control. During the same time interval, central venous pressure remained unchanged. Release of the vascular clamps after completion of the autogenous vein graft restored femoral arterial blood flow to the control value within minutes, and flow remained unchanged for the duration of the 1 hour observation period. Peripheral venous pressure also returned to control levels within minutes after release of femoral venous occlusion. Pressure recordings across the venous anastomoses revealed no demonstrable gradient at either site. These results are summarized in Fig. 1. In a single study, the primary anastomosis resulted in similar changes in arterial blood flow and peripheral venous pressure after release of the vascular clamps. There was no pressure gradient across the suture lines. When the effects of progressive occlusion of the autogenous vein graft were expressed in terms of peripheral venous pressure increments plotted against decrements in femoral arterial flow, an asymptotic relationship was observed (Fig. 2). Doubling the peripheral venous pressure reduced femoral arterial blood flow by 30 per cent. When pressure was increased fourfold, flow was halved. However, increasing peripheral venous pressure to a value six times control reduced femoral arterial flow to only 30 per cent of the control value.
Discussion The results of this study confirm previous observations- 14 that occlusion of the canine femoral vein causes a marked reduction in the flow of blood through the ipsilateral femoral artery. This reduction in arterial
Volume 65
Venous repair in canine hind limb
Number 2 February. 1973
FEMORAL VEIN OCCLUSION
200
c
E
<,
---'
E
1-
VEIN GRAFT OPENED
1
/--1-
1 -
~
o_I u,
>a::
197
100
-
1
w ~
a::
I" _ _I - - - I
a::
o
:2E w
o
u,
t_,
30 C'
I
E E 20
u.i
a::
::J
(J) (J)
w
10
.
a::
CL
0
,. SAPHENOUS VEIN • SUPERIOR VENA CAVA
.-~.--
,-----,
......:._~---=,
o
30
60 TIME,
90
min
Fig. 1. The effects of occlusion of the femoral vein on femoral arterial blood flow, central venous pressure, and peripheral venous pressure are compared before and after placement of an autogenous vein graft in the femoral vein. Each point in the graph represents the mean ± the standard error of five experiments in 5 animals.
flow is sustained for up to 4 hours of venous occlusion and is accompanied by a marked rise in peripheral venous pressure." The increase in venous pressure raises peripheral resistance, which accounts for the fall in arterial flow. This sequence of events would presumably jeopardize the success of arterial reconstructive procedures and provides experimental support for the contention that combined arterial and venous injuries require as careful attention to venous reconstruction as to arterial repair." Clearly, ligation of the femoral vein is contraindicated in concomitant arteriovenous injuries in which venous reconstruction is technically feasible. Because of the extreme elasticity of the vein, a defect there is less amenable to treatment by primary anastomosis than is one in the related artery; thus the surgeon faces the probability of two suture lines in most
venous repair procedures. The present study provides evidence that, hemodynamically, there is no difference between performance of a primary anastomosis as opposed to insertion of an autogenous vein graft in restoring continuity of a divided femoral vein. Except for the time saved, a single anastomosis does not appear more advantageous than several suture lines. Under certain circumstances the surgeon may elect to ligate an injured femoral vein because of other priorities, such as injuries to the same patient or other patients, or because of the lack of a suitable venous autograft. In those cases in which assistance in the decision-making process is desirable, a measurement of peripheral venous pressure can be of prognostic significance. If venous ligation does not significantly increase peripheral venous pressure, then sufficient
The Journal of
1 98
Wright and Swan
Thoracic and Cardiovascular Surgery
e
E 200
<,
E
s ~
IJ..
...J
~
w
100
~
a:
...J
iio ~ W
IJ..
o
10
20
PERIPHERAL VENOUS PRESSURE, mm Hg
Fig. 2. The effects of progressively increasing degrees of constriction of the venous system at the site of the autogenous vein graft upon blood flow through the femoral artery.
collateral venous drainage has already provided for a satisfactory reduction in peripheral resistance and femoral arterial flow may not be seriously compromised. The technique of measuring peripheral venous pressure is simple and readily accomplished by insertion of a small catheter into a branch of the femoral vein distal to the point of ligation. The catheter is connected to a sterile, saline-filled manometer. Peripheral venous pressure is normally in the neighborhood of 50 mm. of water for supine man. The same technique can provide a quantitative evaluation of the success of individual anastomotic suture lines, where questionable, during inspection of the repaired vein. As indicated in the present study, the critical gradient is 5 to 15 mm. Hg. Values above this range may cause a 50 per cent reduction in femoral arterial blood flow. Where possible, the surgeon should strive for anastomoses of grossly normal caliber with no pressure gradients. As Haimovici" has recently stressed, there is no substitute for meticulous technique in achieving success with venous anastomoses.
of femoral arterial flow. Ligation of the femoral vein, primary anastomosis, and interposition of an autogenous vein graft were compared. Femoral arterial flow was measured with an electromagnetic flowmeter; arterial, central venous, and peripheral venous pressures were recorded. Occlusion of the femoral vein resulted in a marked and sustained reduction in femoral arterial flow, accompanied by a marked increase in peripheral venous pressure. There was no difference between one and two suture lines in terms of the hemodynamics of femoral vein reconstruction. Primary anastomosis of the divided vein or insertion of an autogenous vein graft caused femoral arterial flow and peripheral venous pressure to return to control within minutes after release of the vascular clamps. These findings provide experimental evidence in support of the contention that concomitant arterial and venous injuries of an extremity require venous reconstruction to protect arterial reconstructive procedures. A satisfactory venous repair is one without ~" pressure gradient across any suture line. The technical assistance of SP5 Ernest Alix is gratefully acknowledged. The data were analyzed statistically with the assistance of Mr. Douglas Tang, Chief, Department of Biostatistics and Mathematics, Walter Reed Army Institute of Research, Washington, D. C. REFERENCES
2
3
4
Summary
The effects of femoral venous surgery upon femoral arterial blood flow were studied in anesthetized dogs to define that procedure most advantageous to restoration
5
Barcia, P. J., Nelson, T. G., and Whelan, T. J.: The Importance of Venous Occlusion in Arterial Repair Failure: An Experimental Study, Ann. Surg. 175: 223, 1972. Chandler, J. "G., and Knapp, R. W.: Early Definitive Treatment of Vascular Injuries in the Vietnam Conflict, J. A. M. A. 202: 960, 1967. Cohen, A, Baldwin, 1. N., and Grant, R. N.: Problems in the Management of Battlefield Vascular Injuries, Amer. J. Surg. 118: 526, 1969. Dart, C. H., Johnson, G., Peters, R. M., and Womack, N. A.: Hemodynamic Effects of Femoral Venous Occlusion Before and After an Acute Arteriovenous Fistula, Ann. Surg. 164: 190, 1966. Greenstein, A. J., and Mannick, J. A.: Effect of Restricted Outflow on the Early Patency of Arterial Grafts, Surg. Forum 17: 140, 1966.
Volume 65 Number 2
Venous repair in canine hind limb
19 9
Februory, 1973
6 Haimovici, H., Hoffert, P, W., and Zinicola, N.: An Experimental and Clinical Evaluation of Grafts in the Venous System: Collective Review, Surg. Gynecol. Obstet. 131: 1173, 1970. 7 Hughes, C. W.: The Primary Repair of Wounds of Major Arteries, Amer. J. Surg. 141: 297, 1955. 8 Hughes, C. W.: Arterial Repair During the Korean War, Ann. Surg. 147: 555, 1958. 9 Husni, C. A.: Venous Reconstruction in Postphlebitic Disease, Circulation 43: 147, 1971 (Suppl, I).
10 Rich, N. M., Baugh, J. H., and Hughes, C. W.:
11 12 13 14
Popliteal Artery Injuries in Vietnam, Amer. J. Surg. 118: 531, 1969. Rich, N. M., Hughes, C. W., and Baugh, J. H.: Management of Venous Injuries, Ann. Surg. 171: 724, 1970. Snedecor, G. W., and Cochran, W. G.: Statistical Methods, ed. 6, Ames, Iowa, 1967, Iowa State University Press. Spencer, F. C., and Grewe, R. V.: The Management of Arterial Injuries in Battle Casualties, Ann. Surg. 141: 304, 1955. Wright, C. B., and Swan, K. G.: Hemodynamics of Venous Occlusion in the Canine Hindlimb, Surgery 73: 141, 1973.