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A prospective study of the natural history of femoropopliteal artery stenosis using duplex ultrasound
Eur J VascSurg 7, 444 447 (1993)
A Prospective Study of the Natural History of Femoropopliteal Artery Stenosis Using Duplex Ultrasound M. R. W h y m a n 1, C. V. Ruckley 2 and F. G. R. Fowkes 1 1Wolfson Unit for Prevention of Peripheral Vascular Diseases, Medical School, Teviot Place, Edinburgh EH8 9AG and 2Department of Surgery, Royal Infirmary of Edinburgh, Lauriston Place, Edinburgh, UK A foreknowledge of impending femoral artery occlusion might allow prevention of a deterioration in ischaemia by percutaneous transluminal angioplasty (PTA) in the preocclusive stage. A window of opportunity exists beforea femoral stenosis progresses to occlusion and its treatment by PTA becomes more difficult, riskier and associated with lower patency. The size of this window is unknown. This is the first report of a study of the natural history offemoral stenoses using Duplex ultrasound. The aims of the study were to determine in patients with symptomatic peripheral arterial disease: (1) the incidence of progression from stenosis to occlusion; (2) the change in severity of stenosis over time; and (3) the relationship between severity of stenosis and progression to occlusion. The velocity ratio (VR) of 43 femoral artery stenoses in 38 patients was measured by Duplex ultrasound. The examination was repeated after a median of 28 (range 5-76) weeks. Nine/43 (21%) stenoses progressed to occlusion within a median of 13 weeks. Little overall change in VR was demonstrable if occlusion did not occur. Only stenoses with a VR >3 progressed to occlusion in the study period. The data suggest that severe stenoses progress more rapidly than moderate, with a relatively brief window of opportunity. If PTA is to be undertaken, stenoses with a VR greater than 3 should be treated with minimum delay. Key Words: Femoropopliteal; Stenosis; Duplex ultrasound.
Introduction In the lower limb atheromatous disease has a predilection for the superficial femoral artery.1 Symptoms of peripheral arterial disease (PAD) correlate poorly with the nature and extent of disease in the artery. 2 However, it is generally accepted that if a major progression in the severity of disease occurs in an individual, there is an accompanying deterioration in the severity of ischaemia. A foreknowledge of impending arterial occlusion (and possible deterioration of ischaemia) might allow its prevention by active intervention in the preocclusive stage. This could, in principle, be achieved by percutaneous transluminal angioplasty (PTA), which has been shown to have a high primary success in treating femoral stenoses, 3 a low risk, 4 and an acceptable patency. 5"6 The procedure is more difficult and associated with lower patency rates when occlusions are recanalised and dilated, particularly w h e n the occlusion is long. 7 Please address all correspondenceto: M. R. Whyman,Department of Surgery, RoyalInfirmaryof Edinburgh, LauristonPlace, Edinburgh, Scotland. 0950-821X/93/070444+04 $08.00/0© 1993Grune & StrattonLtd.
There is, therefore, a " w i n d o w of opportunity", within which treatment is technically easier and safer. The size of this window has not been ascertained, that is, the length of time between the finding of a stenosis and its progression to occlusion is unknown. Many stenoses which cause claudication might progress to occlusion soon after the patient first presents to the clinician, and there is therefore a need for a short-term prospective study. Serial arteriography has been used to study the natural history of PAD. In studies by Warren et al., 8 Coran and Warren, 9 and Kuthan, 1° it was shown that progression of lesions was infrequently matched by a deterioration in symptoms. Around 52% of lesions (stenoses and occlusions combined) progressed within 2.5 years, 1° and 74% progressed within 5 years. 9 In vitro studies have demonstrated that there is an excellent agreement between Duplex ultrasound velocity ratio (VR) measurement and degree of stenosis. 11 The reproducibility of measurements is high, particularly with the lesser degrees of stenosis, and may be as good as the agreement between two radiologists' readings of the same stenosis shown on con-
Femoropopliteal Artery Stenosis
ventional arteriography. Duplex has the added advantage of being non-invasive. It should, therefore, be an ideal tool for natural history studies, but to date this is the first published report of the natural history of femoral artery stenosis using Duplex ultrasound. The aims of this study were to determine in patients with symptomatic PAD: (1) The incidence of progression from stenosis to occlusion. (2) The change in severity of stenosis over time. (3) The relationship between severity of stenosis and progression to occlusion.
Materials and Methods Thirty-eight patients (27 male, 11 female), median age 67 (range 41-78) years with 43 stenoses, were studied. Patients were selected from those attending outpatients with claudication being investigated with a view to PTA, and from in-patients undergoing arteriography for rest-pain. In the early part of the study arteriography had already been performed and was used to identify isolated superficial femoral and popliteal artery stenoses. Duplex ultrasound was then used to quantify the degree of stenosis. Later in the study, Duplex ultrasound alone was used to identify and measure stenoses. An "isolated stenosis" was defined as an area of discrete arterial narrowing easily distinguishable from adjacent artery, whether the latter was regarded as diseased or normal. With the exception of one case of a stenosis proximal to an occlusion, all stenoses studied were the most significant lesions in the femoropopliteal segment in terms of degree of luminal narrowing. The Duplex ultrasound examination was carried out by a single observer (M.R.W.) with patients in the supine and prone positions to study femoral and popliteal arteries, respectively. A 5 MHz linear array transducer of an ATL UM9 (Advanced Laboratory Technology, Bothell, WA, U.S.A.) colour Duplex scanner was used in all cases. The ratio of intrastenosis to prestenosis peak systolic velocity after correction for Doppler angle was used to estimate the degree of narrowing (VR). A second Duplex examination of the same stenosis was performed by the same observer after a median of 28 (range 5-76) weeks. The range was large because in many cases the second examination had to be organised to coincide with an out-patient appointment, and to preempt planned interventions. Four patients were examined on a
445
third occasion. In these instances if the stenosis had still not progressed to occlusion the follow-up period was taken to be the interval between the first and third examinations. If the stenosis had progressed to occlusion, the follow-up period was taken to be the interval between the second and third examinations. Significant change of a stenosis was defined as an increase or decrease in the VR of ->20%. This value is derived from in vitro variability data 11 and is the minim u m variation in VR one would expect in vivo. Occlusion was diagnosed when a complete loss of both colour and Doppler spectral waveform occurred within the vessel lumen as seen on real-time imaging. In this series there were no cases where the diagnosis of occlusion was difficult or equivocal. For the purposes of data analysis an arbitrary classification of severity of stenosis was drawn up. Stenoses were divided into categories of VR <3, 3-6 and >6. Fisher's exact probability test was used to examine the difference between proportions of stenoses progressing to occlusion in each category.
Results The distribution of stenoses within each category is shown in Fig. 1. None of the stenoses with an initial VR <3 progressed to occlusion. One half (6/12) of those in the VR range 3-6 and one-third (3/9) of those in the VR range >6 occluded. The difference between the proportion of stenoses occluding in these two groups was not significant (p = 1.99). The change in severity of stenoses between the first and last Duplex
25 n = 22 2O
15
n = 12 ffiffJJf~
fffilliJ~ fiifffif~ fffffJff~
z
n=9
fi]]J~JJ
<3
I
3-6
>6
Initial VR
Fig. 1. Severity of stenoses at first Duplex scan and evolution according to a subsequent scan. (El) VR decrease, (1~)no change, (I) VR increase, ([] occlusion. Eur J VascSurg Vol 7, July1993
446
M.R. Whyman et aL
100
15
80
+
12 +
60
+
9
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o
o
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o o
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an o
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o
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o
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-60 -80 -
100 0
1'0
2'0
3'0 4'0 5'0 6'0 7'0 8 0 Interval (weeks) Fig. 2. Relative change in velocity ratio between first and last Duplex examinations by time for stenoses not progressing to occlusion. Dashed lines are limits representing no significant change in velocity ratio, i.e. initial VR _+20%.
examinations is illustrated in Figs 1 and 2. Figure 1 shows the data for all stenoses a n d indicates that w h e n the VR was < 6 on the first examination the m e a s u r e m e n t was rarely lower on the second occasion. With an initial VR >6, the second measurem e n t was often lower (if the vessel had not occluded). Figure 2 shows the m a g n i t u d e of change in VR for those stenoses w h i c h did not occlude. The m e a n change was a decrease in VR of 4.6% (range - 5 3 to +96%). Essentially, the increase in VR observed in the majority of m i n o r stenoses was balanced b y the decrease in VR observed in the more severe stenoses. For stenoses which p r o g r e s s e d to occlusion the relationship b e t w e e n the initial VR and time within which occlusion occurred (the interval b e t w e e n examinations) is s h o w n in Fig. 3. The data suggest a t e n d e n c y for stenoses with a high VR to progress to occlusion more rapidly than those with a lower VR. Progression occurred in these nine cases within a m e d i a n of 13 weeks. Following progression of a stenosis to occlusion one patient required major amputation, b u t this patient also had significant crural disease. N o n e of the other patients required reconstructive surgery as a result of such progression.
Eur J Vasc Surg Vol 7, July 1993
0 --
1'0 - 2'0 3r0 40 Interval between scans (weeks) Fig. 3. Relationship between initial velocity ratio and interval between first and second Duplex scans for stenoses progressing to occlusion. Discussion
A n i m p o r t a n t risk factor for progression from stenosis to occlusion might be the degree of a t h e r o m a t o u s n a r r o w i n g of an artery. 12 O n e might expect the m o s t a d v a n c e d pathological changes and the most dist u r b e d h a e m o d y n a m i c s to be associated with rapid progression. M a n y cardiovascular risk factors apart from severity of stenosis also n e e d to be c o n s i d e r e d in exploring the natural history of femoral lesions, but this was b e y o n d the scope of this study. A l t h o u g h a clear relationship b e t w e e n severity of stenosis and time to occlusion has not b e e n d e m o n strated in this study, there is a suggestion that severe stenoses progress more rapidly t h a n moderate. A larger s t u d y using a life-table analysis m e t h o d w o u l d be necessary to d e m o n s t r a t e the relationship m o r e clearly. H o w e v e r , a VR of > 3 appears to be a significant factor in the progression of femoral artery stenosis. According to in vitro studies such stenoses h a v e a r e d u c t i o n in luminal diameter of a r o u n d 50-55%. Femoral stenoses of this degree might not invariably obliterate the arterial l u m e n b y a gradual process, b u t in some cases might occlude s u d d e n l y d u e to an intraplaque e v e n t such as h a e m o r r h a g e , necrosis or dissection. There was no significant difference in the incidence of occlusion b e t w e e n m o d e r a t e and severe stenoses (arbitrarily defined as VR 3 - 6 and >6, respectively). H o w e v e r , the follow-up interval was not the same for the g r o u p s (medians of 46 and 28 weeks, respectively) and therefore a truly valid comparison c a n n o t be made. Nevertheless, if intervention such as PTA is to be u n d e r t a k e n , it s h o u l d take place as soon as possible after detecting a stenosis w h o s e VR is >3, or the w i n d o w of o p p o r t u n i t y for relatively safe, effective and easy PTA m a y be lost.
Femoropopliteal Artery Stenosis
For the stenoses which did not occlude there was not a significant overall change in magnitude of VR. However, the VR of the less severe stenoses tended to increase, whereas the VR of the more severe stenoses tended to decrease. One important question is-does this decrease in VR amount to regression? This is exceedingly unlikely. Although it has been suggested from clinical studies that regression of atheroma can occur in femoral arteries 13-16 and reversal of atheroma has been demonstrated in vitro, 17 it is unlikely that the pathological features present in a severe atheromatous plaque reverse significantly during its natural history. One explanation is that the variation in velocity ratio estimation by Duplex ultrasound in this range of stenoses is more significant than suggested by previous in vitro studies. The observed decrease in VR of many stenoses might therefore have occurred within the normal range of variation. It is certainly true to say that the measurement of a severe stenosis is not always easy and that peak systolic velocity can vary considerably in a single stenosis from one intrastenotic region to another. Another possible explanation for the velocity ratio being lower on the second reading is that the pre-stenosis velocity might have increased because of an increase in the severity of disease in this region. This is a largely unavoidable problem when using velocity ratio as an index of stenosis severity. In conclusion: (1) Progression from stenosis to occlusion of the femoral artery occurred in 9/43 (21%) cases within a median of 13 weeks. (2) Little overall change in VR was demonstrable if occlusion did not occur. Sudden plaque events as opposed to gradual enlargement may therefore be responsible for many cases of femoral artery occlusion. (3) Only stenoses with a VR >3 progressed to occlusion in the study period. The data suggest that severe stenoses progress more rapidly than moderate resulting in a relatively brief window of opportunity. If PTA is to be undertaken, stenoses with a VR >3 should be treated with minimum delay.
447
References 1 WALDENR, ADAR R, RUBINSTEINZJ, BASSA. Distribution and symmetry of arteriosclerotic lesions of the lower extremities: an arteriographic study of 200 limbs. Cardiovasc Intervent Radiol 1985; 8: 180-182. 2 BAKERJD. Poststress Doppler ankle pressures. Arch Surg 1978; 113: 1171-1173. 3 JOHNSTONKW, RAE M, HOGG-JOHNSTONSA, et al. 5 Year results of a prospective study of percutaneous transluminal angioplasty. Ann Surg 1987; 206: 403-413. 4 BELLI AM, CUMBERLANDDC, KNOX AM, PROCTERAE, WELSH CL. The complication rate of percutaneous peripheral balloon angioplasty. Clin Rad 1990; 41: 380-383. 5 BECKERGJ, KATZENBT, DAKE MD. Noncoronary angioplasty. Radiology 1989; 170: 921-940. 6 JEANSWD, ARMSTRONGS, COLE SEA, HORROCKSM, BAIRDRN. Fate of patients undergoing transluminal angioplasty for lower limb ischaemia. Radiology 1990; 177: 559-564. 7 KREI'ELVM, VAN ANDELGJ, VAN ERP WFM, BRESLAUPJ. Percutaneous transluminal angioplasty of the femoropopliteal artery: initial and long term results. Radiology 1985; 156: 325-328. 8 WARRENR, GOMEZRL, MARSTONJAP, Cox JST. Femoropopliteal arteriosclerosis obliterans--arteriographic patterns and rates of progression. Surgery 1964; 55: 135-143. 9 CORAN AG, WARREN R. Arteriographic changes in femoropopliteal arteriosclerosis obliterans. A five-year follow up study. N Engl J Med 1966; 274: 643-647. 10 KUTHANF, BURKHALTERA, BAITSCHR, LUDIN H, WIDMERLK. Development of arterial disease in lower limbs--angiographic follow-up of 705 medical patients. Arch Surg 1971; 103: 545-547. 11 WHVMANMR, LENGGC, HOSKINSPR, et al. Accuracy and reproducibility of duplex sonography in a phantom model of femoral artery stenosis. J Vasc Surg, in press. 12 HALON DA, SAPOZNIKOVD, GOTSMANMS, LEWISBS. Can total coronary occlusions be predicted from a previous coronary arteriogram? Cathet Cardiovasc Diagn 1985; 11: 455-462. 13 DUFFIELDRGM, LEWIS, B, MILLER NE, JAMIESON CW, BRUNT JNH, COLCHESTERACF. Treatment of hyperlipidaemia retards progression of symptomatic femoral atherosclerosis. A randomised controlled trial. Lancet 1983; 2: 639-642. 14 OST CR, STENSEN S. Regression of peripheral atherosclerosis during therapy with high doses of nicotinic acid. Scand J Clin Lab Invest 1967; Suppl 99: 241-245. 15 BARNDTR JR, BLANKENHORNDH, CRAWFORDDW, BROOKSSH. Regression and progression of early femoral atherosclerosis in treated hyperlipoproteinaemic patients. Ann Intern Med 1977; 86: 139-146. 16 ERIKSONU, HELMIUSG, HEMMINGSSONA, RUHN G, OLSSONAG. Measurement of atherosclerosis by arteriography and microdensitometry: model and clinical investigations. In: SCHETTLER FG, GOTTO AM, MIDDELHOFFF, HABENICHTAJR, JURUTKAKR, eds. Atherosclerosis VI. Proceedings of the Sixth International Symposium on Atherosclerosis. Berlin: Springer-Verlag, 1983; 197. 17 ARMSTRONGML, HEISTADDD, MEGANMB, LOPEZJAG, HARRISON DG. Reversibility of atherosclerosis. Cardiovasc Clin 1990; 20: 113-126. Accepted 1 December 1992
Acknowledgements We are grateful to Mr P. Donnan, Department of Medical Statistics, Edinburgh University, for his advice.
Eur J Vasc Surg Vol 7, July 1993
A Prospective Study of the Natural History of Femoropopliteal Artery Stenosis Using Duplex Ultrasound M. R. W h y m a n 1, C. V. Ruckley 2 and F. G. R. Fowkes 1 1Wolfson Unit for Prevention of Peripheral Vascular Diseases, Medical School, Teviot Place, Edinburgh EH8 9AG and 2Department of Surgery, Royal Infirmary of Edinburgh, Lauriston Place, Edinburgh, UK A foreknowledge of impending femoral artery occlusion might allow prevention of a deterioration in ischaemia by percutaneous transluminal angioplasty (PTA) in the preocclusive stage. A window of opportunity exists beforea femoral stenosis progresses to occlusion and its treatment by PTA becomes more difficult, riskier and associated with lower patency. The size of this window is unknown. This is the first report of a study of the natural history offemoral stenoses using Duplex ultrasound. The aims of the study were to determine in patients with symptomatic peripheral arterial disease: (1) the incidence of progression from stenosis to occlusion; (2) the change in severity of stenosis over time; and (3) the relationship between severity of stenosis and progression to occlusion. The velocity ratio (VR) of 43 femoral artery stenoses in 38 patients was measured by Duplex ultrasound. The examination was repeated after a median of 28 (range 5-76) weeks. Nine/43 (21%) stenoses progressed to occlusion within a median of 13 weeks. Little overall change in VR was demonstrable if occlusion did not occur. Only stenoses with a VR >3 progressed to occlusion in the study period. The data suggest that severe stenoses progress more rapidly than moderate, with a relatively brief window of opportunity. If PTA is to be undertaken, stenoses with a VR greater than 3 should be treated with minimum delay. Key Words: Femoropopliteal; Stenosis; Duplex ultrasound.
Introduction In the lower limb atheromatous disease has a predilection for the superficial femoral artery.1 Symptoms of peripheral arterial disease (PAD) correlate poorly with the nature and extent of disease in the artery. 2 However, it is generally accepted that if a major progression in the severity of disease occurs in an individual, there is an accompanying deterioration in the severity of ischaemia. A foreknowledge of impending arterial occlusion (and possible deterioration of ischaemia) might allow its prevention by active intervention in the preocclusive stage. This could, in principle, be achieved by percutaneous transluminal angioplasty (PTA), which has been shown to have a high primary success in treating femoral stenoses, 3 a low risk, 4 and an acceptable patency. 5"6 The procedure is more difficult and associated with lower patency rates when occlusions are recanalised and dilated, particularly w h e n the occlusion is long. 7 Please address all correspondenceto: M. R. Whyman,Department of Surgery, RoyalInfirmaryof Edinburgh, LauristonPlace, Edinburgh, Scotland. 0950-821X/93/070444+04 $08.00/0© 1993Grune & StrattonLtd.
There is, therefore, a " w i n d o w of opportunity", within which treatment is technically easier and safer. The size of this window has not been ascertained, that is, the length of time between the finding of a stenosis and its progression to occlusion is unknown. Many stenoses which cause claudication might progress to occlusion soon after the patient first presents to the clinician, and there is therefore a need for a short-term prospective study. Serial arteriography has been used to study the natural history of PAD. In studies by Warren et al., 8 Coran and Warren, 9 and Kuthan, 1° it was shown that progression of lesions was infrequently matched by a deterioration in symptoms. Around 52% of lesions (stenoses and occlusions combined) progressed within 2.5 years, 1° and 74% progressed within 5 years. 9 In vitro studies have demonstrated that there is an excellent agreement between Duplex ultrasound velocity ratio (VR) measurement and degree of stenosis. 11 The reproducibility of measurements is high, particularly with the lesser degrees of stenosis, and may be as good as the agreement between two radiologists' readings of the same stenosis shown on con-
Femoropopliteal Artery Stenosis
ventional arteriography. Duplex has the added advantage of being non-invasive. It should, therefore, be an ideal tool for natural history studies, but to date this is the first published report of the natural history of femoral artery stenosis using Duplex ultrasound. The aims of this study were to determine in patients with symptomatic PAD: (1) The incidence of progression from stenosis to occlusion. (2) The change in severity of stenosis over time. (3) The relationship between severity of stenosis and progression to occlusion.
Materials and Methods Thirty-eight patients (27 male, 11 female), median age 67 (range 41-78) years with 43 stenoses, were studied. Patients were selected from those attending outpatients with claudication being investigated with a view to PTA, and from in-patients undergoing arteriography for rest-pain. In the early part of the study arteriography had already been performed and was used to identify isolated superficial femoral and popliteal artery stenoses. Duplex ultrasound was then used to quantify the degree of stenosis. Later in the study, Duplex ultrasound alone was used to identify and measure stenoses. An "isolated stenosis" was defined as an area of discrete arterial narrowing easily distinguishable from adjacent artery, whether the latter was regarded as diseased or normal. With the exception of one case of a stenosis proximal to an occlusion, all stenoses studied were the most significant lesions in the femoropopliteal segment in terms of degree of luminal narrowing. The Duplex ultrasound examination was carried out by a single observer (M.R.W.) with patients in the supine and prone positions to study femoral and popliteal arteries, respectively. A 5 MHz linear array transducer of an ATL UM9 (Advanced Laboratory Technology, Bothell, WA, U.S.A.) colour Duplex scanner was used in all cases. The ratio of intrastenosis to prestenosis peak systolic velocity after correction for Doppler angle was used to estimate the degree of narrowing (VR). A second Duplex examination of the same stenosis was performed by the same observer after a median of 28 (range 5-76) weeks. The range was large because in many cases the second examination had to be organised to coincide with an out-patient appointment, and to preempt planned interventions. Four patients were examined on a
445
third occasion. In these instances if the stenosis had still not progressed to occlusion the follow-up period was taken to be the interval between the first and third examinations. If the stenosis had progressed to occlusion, the follow-up period was taken to be the interval between the second and third examinations. Significant change of a stenosis was defined as an increase or decrease in the VR of ->20%. This value is derived from in vitro variability data 11 and is the minim u m variation in VR one would expect in vivo. Occlusion was diagnosed when a complete loss of both colour and Doppler spectral waveform occurred within the vessel lumen as seen on real-time imaging. In this series there were no cases where the diagnosis of occlusion was difficult or equivocal. For the purposes of data analysis an arbitrary classification of severity of stenosis was drawn up. Stenoses were divided into categories of VR <3, 3-6 and >6. Fisher's exact probability test was used to examine the difference between proportions of stenoses progressing to occlusion in each category.
Results The distribution of stenoses within each category is shown in Fig. 1. None of the stenoses with an initial VR <3 progressed to occlusion. One half (6/12) of those in the VR range 3-6 and one-third (3/9) of those in the VR range >6 occluded. The difference between the proportion of stenoses occluding in these two groups was not significant (p = 1.99). The change in severity of stenoses between the first and last Duplex
25 n = 22 2O
15
n = 12 ffiffJJf~
fffilliJ~ fiifffif~ fffffJff~
z
n=9
fi]]J~JJ
<3
I
3-6
>6
Initial VR
Fig. 1. Severity of stenoses at first Duplex scan and evolution according to a subsequent scan. (El) VR decrease, (1~)no change, (I) VR increase, ([] occlusion. Eur J VascSurg Vol 7, July1993
446
M.R. Whyman et aL
100
15
80
+
12 +
60
+
9
4O
o
o
20
G
o o
G
++
an o
0 D
o
[]
a
+
+ + +
-20 o
o
[]
[]
-40
o
D o D
a
-60 -80 -
100 0
1'0
2'0
3'0 4'0 5'0 6'0 7'0 8 0 Interval (weeks) Fig. 2. Relative change in velocity ratio between first and last Duplex examinations by time for stenoses not progressing to occlusion. Dashed lines are limits representing no significant change in velocity ratio, i.e. initial VR _+20%.
examinations is illustrated in Figs 1 and 2. Figure 1 shows the data for all stenoses a n d indicates that w h e n the VR was < 6 on the first examination the m e a s u r e m e n t was rarely lower on the second occasion. With an initial VR >6, the second measurem e n t was often lower (if the vessel had not occluded). Figure 2 shows the m a g n i t u d e of change in VR for those stenoses w h i c h did not occlude. The m e a n change was a decrease in VR of 4.6% (range - 5 3 to +96%). Essentially, the increase in VR observed in the majority of m i n o r stenoses was balanced b y the decrease in VR observed in the more severe stenoses. For stenoses which p r o g r e s s e d to occlusion the relationship b e t w e e n the initial VR and time within which occlusion occurred (the interval b e t w e e n examinations) is s h o w n in Fig. 3. The data suggest a t e n d e n c y for stenoses with a high VR to progress to occlusion more rapidly than those with a lower VR. Progression occurred in these nine cases within a m e d i a n of 13 weeks. Following progression of a stenosis to occlusion one patient required major amputation, b u t this patient also had significant crural disease. N o n e of the other patients required reconstructive surgery as a result of such progression.
Eur J Vasc Surg Vol 7, July 1993
0 --
1'0 - 2'0 3r0 40 Interval between scans (weeks) Fig. 3. Relationship between initial velocity ratio and interval between first and second Duplex scans for stenoses progressing to occlusion. Discussion
A n i m p o r t a n t risk factor for progression from stenosis to occlusion might be the degree of a t h e r o m a t o u s n a r r o w i n g of an artery. 12 O n e might expect the m o s t a d v a n c e d pathological changes and the most dist u r b e d h a e m o d y n a m i c s to be associated with rapid progression. M a n y cardiovascular risk factors apart from severity of stenosis also n e e d to be c o n s i d e r e d in exploring the natural history of femoral lesions, but this was b e y o n d the scope of this study. A l t h o u g h a clear relationship b e t w e e n severity of stenosis and time to occlusion has not b e e n d e m o n strated in this study, there is a suggestion that severe stenoses progress more rapidly t h a n moderate. A larger s t u d y using a life-table analysis m e t h o d w o u l d be necessary to d e m o n s t r a t e the relationship m o r e clearly. H o w e v e r , a VR of > 3 appears to be a significant factor in the progression of femoral artery stenosis. According to in vitro studies such stenoses h a v e a r e d u c t i o n in luminal diameter of a r o u n d 50-55%. Femoral stenoses of this degree might not invariably obliterate the arterial l u m e n b y a gradual process, b u t in some cases might occlude s u d d e n l y d u e to an intraplaque e v e n t such as h a e m o r r h a g e , necrosis or dissection. There was no significant difference in the incidence of occlusion b e t w e e n m o d e r a t e and severe stenoses (arbitrarily defined as VR 3 - 6 and >6, respectively). H o w e v e r , the follow-up interval was not the same for the g r o u p s (medians of 46 and 28 weeks, respectively) and therefore a truly valid comparison c a n n o t be made. Nevertheless, if intervention such as PTA is to be u n d e r t a k e n , it s h o u l d take place as soon as possible after detecting a stenosis w h o s e VR is >3, or the w i n d o w of o p p o r t u n i t y for relatively safe, effective and easy PTA m a y be lost.
Femoropopliteal Artery Stenosis
For the stenoses which did not occlude there was not a significant overall change in magnitude of VR. However, the VR of the less severe stenoses tended to increase, whereas the VR of the more severe stenoses tended to decrease. One important question is-does this decrease in VR amount to regression? This is exceedingly unlikely. Although it has been suggested from clinical studies that regression of atheroma can occur in femoral arteries 13-16 and reversal of atheroma has been demonstrated in vitro, 17 it is unlikely that the pathological features present in a severe atheromatous plaque reverse significantly during its natural history. One explanation is that the variation in velocity ratio estimation by Duplex ultrasound in this range of stenoses is more significant than suggested by previous in vitro studies. The observed decrease in VR of many stenoses might therefore have occurred within the normal range of variation. It is certainly true to say that the measurement of a severe stenosis is not always easy and that peak systolic velocity can vary considerably in a single stenosis from one intrastenotic region to another. Another possible explanation for the velocity ratio being lower on the second reading is that the pre-stenosis velocity might have increased because of an increase in the severity of disease in this region. This is a largely unavoidable problem when using velocity ratio as an index of stenosis severity. In conclusion: (1) Progression from stenosis to occlusion of the femoral artery occurred in 9/43 (21%) cases within a median of 13 weeks. (2) Little overall change in VR was demonstrable if occlusion did not occur. Sudden plaque events as opposed to gradual enlargement may therefore be responsible for many cases of femoral artery occlusion. (3) Only stenoses with a VR >3 progressed to occlusion in the study period. The data suggest that severe stenoses progress more rapidly than moderate resulting in a relatively brief window of opportunity. If PTA is to be undertaken, stenoses with a VR >3 should be treated with minimum delay.
447
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Acknowledgements We are grateful to Mr P. Donnan, Department of Medical Statistics, Edinburgh University, for his advice.
Eur J Vasc Surg Vol 7, July 1993