1991, The British Journal of Radiology, 64, 785-791
Duplex ultrasound of the common femoral vein in pregnancy and puerperium By M. J. Duddy, FRCR and \ J . M. McHugo, MRCP, FRCR The Birmingham Maternity Hospital, Queen Elizabeth Medical Centre, Edgbaston, Birmingham B15 2TG, UK {Received November 1990 and in revised form March 1991)
Keywords: Thrombosis, Ultrasound, Doppler, Pregnancy
Abstract. The common femoral veins of 34 apparently normal women were examined by ultrasound during or just after pregnancy. The cross sectional areas of these veins were measured at rest in the supine position and during a Valsalva manoeuvre. The veins were also assessed by pulsed Doppler ultrasound. We believe that Doppler examination is superior to calibre response assessment during the Valsalva manoeuvre in excluding an isolated iliac occlusion in women in late pregnancy and in early puerperium.
Fatal pulmonary embolus occurs in 1:100 000 pregnancies in this country and it vies with hypertensive disease of pregnancy as the commonest cause of maternal death. The most recent Report on Confidential Enquiries into Maternal Deaths in England and Wales (Turnbull et al, 1989) calls for earlier detection of thrombus in the veins of the pelvis and legs, and states that reduced flow in the femoral vein detected by noninvasive methods is an indication for anticoagulant therapy. ' The incidence of deep vein thrombosis (DVT) is around 0.6 per 1000 pregnancies in both the antenatal and postnatal periods. (Bergqvist et al, 1983a; Kierkegaard, 1983; Polak & O'Leary, 1988). DVT in pregnancy more frequently originates in the iliofemoral segment rather than in the calf veins (Bergqvist et al, 1983a; Bergqvist & Hedner, 1983b). This is distinct from the non-obstetric DVT which tends to originate distally. Furthermore, of those thromboses which result in fatal pulmonary embolus, over half are confined to the iliac veins (Turnbull et al, 1989). This presents problems in diagnosis. The clinical features are atypical often with lower abdominal or pelvic pain and oedema. Unlike the distal veins, the iliac veins are inaccessible to compression ultrasound. Cuff impedance plethysmography and phleborheography are not widely practised in the UK and false positive results have been reported during pregnancy (Clarke-Pearson & Jelovsek, 1981; Nicholas el al, 1985). Ultrasound Frede and Ruthberg (1988) used the gravid uterus of 25 weeks' gestation as an acoustic window to image the iliac veins directly (Fig. 1) and demonstrate a thrombo* Author for correspondence. Vol. 64, No. 765
Figure 1. Sonogram of the iliac vessels through a 16 week gestation gravid uterus as an acoustic window. CIV = common iliac vein, CIA = common iliac artery, IIA = internal iliac artery, EIA = external iliac artery.
sis. This method relies on imaging echogenic thrombus and would not detect fresh thrombus which is often isoechoic with blood. Furthermore, isolated iliac DVT is more frequent in late pregnancy when the iliac veins are often outside the focal zone of most probes. The increase in calibre of the common femoral vein (CFV) during the Valsalva manoeuvre has been used to assess the patency of the iliofemoral segment (Effeney et al, 1984). A 50% or more increase in antero-posterior (AP) diameter is quoted as a criterion for patency and a 10% or less increase as a criterion for occlusion. This indirect method of assessing the iliac veins has been used by other authors (Appleman et al, 1987; O'Leary & Kane, 1988; Zwiebel, 1988) and its use has been 785
M. J. Duddy and J. M. McHugo
Figure 2. Transverse sonogram of the common femoral vessels (a) at rest and (b) during compression with the transducer. There is complete collapse of the CFV during compression (arrows).
advocated in suspected DVT during pregnancy (Polak & O'Leary, 1988). Pulsed Doppler sampling of the CFV offers a further indirect method of assessing iliac vein patency (O'Leary & Kane, 1983; Thiele, 1984; Zwiebel, 1988). The Doppler signal from the CFV should be spontaneous, phasic and unidirectional. Lack of spontaneous flow implies occlusion at, proximal to or distal to the CFV. Lack of respiratory phasic variation of the signal (caused by pressure changes in response to the respiratory cycle) implies occlusion proximal to the CFV. Retrograde flow in the CFV implies valvular incompetence. Venous flow may be augmented by compression of veins distal to the sample site or by the release of a compression proximal to the sample site (Thiele, 1984).
(a)
Although compression proximal to the CFV may not be possible directly, the Valsalva manoeuvre (by increasing intra-abdominal pressure) has a similar effect. There is complete cessation of flow followed by augmentation as normal respiration is resumed, provided the iliac veins are patent. Iliac occlusion abolishes this Valsalva response of the Doppler signal. " Augmentation by distal compression is abolished by occlusion distal to the sample site, but according to Thiele (1984) remains intact in the presence of a proximal occlusion. O'Leary & Kane (1988), on the other hand, suggest that loss of augmentation at the CFV with calf compression may indicate an iliac vein occlusion. Pulsatileflowin the extremities is said always to be abnormal and usually caused by congestive heart failure (Zwiebel, 1988).
(b)
Figure 3. Transverse sonogram of the CFV (a) at rest and (b) during a Valsalva manoeuvre. D t = major axis, D 2 = minor axis, A = cross sectional area.
786
The British Journal of Radiology, September 1991
Duplex ultrasound of the CFV in pregnancy
(b)
Figure 4. Pulsed Doppler scan of the CFV showing spontaneous flow, (a) Flow is phasic with the respiratory cycle (arrows), (b) Pulsatile flow with the cardiac cycle (arrows) is superimposed on respiratory phasic variation (open arrows) in a patient with no history or signs of heart disease.
The aim of this study was to determine the effect of the enlarged uterus in late pregnancy and early puerperium on the Doppler characteristics of the CFV and on the Doppler and calibre response of the CFV to the Valsalva manoeuvre. Patients and method
Ultrasound assessment was made of 68 CFVs in 34 women. 10 were in early pregnancy (8 to 9 weeks' gestation). 19 women were in late pregnancy (28 to 40 weeks' gestation) one of whom was carrying twins at 37 weeks. Five women were between 2 and 10 days postpartum, two of whom had been delivered by caesarian section. All the women were normal volunteers with no clinical evidence of DVT. None of the women were examined more than once. Real-time ultrasound was performed using an electronically focused 5 MHz sector transducer with 5 MHz pulsed Doppler capability (Acuson, Mountain View, California). The lowest possible high pass filter (125 Hz)
was selected. The women were examined in the supine position. The CFVs were imaged transversely and longitudinally and assessed for their compressibility and the presence of intraluminal echoes (Fig. 2). The veins were examined at the level just proximal to the bifurcation of the common iliac artery. Assuming an elliptical cross section, the major and minor axes were measured and the cross sectional area calculated. The area is produced automatically by our machine, but can be calculated using the equation: area = j x major axis x minor axis. Area measurements were performed at rest and during a Valsalva manoeuvre (Fig. 3). Pulsed Doppler assessment was made during quiet respiration to elicit respiratory phasic variation of the signal (Fig. 4a). The women were then asked to perform the Valsalva manoeuvre and the Doppler signal was assessed to determine whether blood flow was blocked (i.e. reduced below the level of the high pass filter), merely reduced but still detectable, or whether flow was
Figure 5. Pulsed Doppler scan showing the CFV flow response to a Valsalva manoeuvre, (a) Complete block is achieved. There is augmentation of flow as normal breathing is resumed (arrow), (b) Reversed flow is achieved implying incompetence of venous valves, (c) Increased antegrade flow during a Valsalva manoeuvre at 39 weeks' gestation. This would seem to be a spurious response to an incorrectly performed Valsalva. Vol. 64, No. 765
787
M. J. Duddy and J. M. McHugo
Figure 6. Pulsed Doppler scan showing normal augmentation of CFV flow during calf compression.
reversed (Fig. 5). As normal respiration was resumed the presence of post Valsalva accentuation of the signal (proximal augmentation) was recorded. Finally, the presence of accentuation of the signal due to digital compression of the calf (distal augmentation) was assessed (Fig. 6). Seven CFVs in four women in their third trimester of pregnancy were also examined in the opposite decubitus positions, i.e. the vein under examination being uppermost. A /-test on a logarithmic transformation of the data was used to compare the means for two-sided p values. Results
Of the 68 CFVs examined, all were easily compressible and there would have been no difficulty in excluding CFV thrombosis. No intraluminal echoes were found in the veins examined in the first trimester group. Moving echoes were seen in 16/38 (42%) of veins in the third trimester and in 2/10 (20%) of veins in the postnatal group. Static intraluminal echoes were found in none of the veins examined.
20 GESTATION (weeks)
o 10 20 PUBfMUM (days)
Figure 7. Resting CFV cross sectional area plotted against gestational age and number of days post-partum. 788
20 GESTATION Iweeks)
•h
0 10 20 PUEPERUH (days)
Figure 8. Percentage increase in CFV cross sectional area during a Valsalva manoeuvre plotted against gestational age and number of days post-partum.
In Figure 7 the CFV cross sectional area is plotted against gestational age and number of days postpartum. In Figure 8 the percentage increase of CFV cross sectional area during a Valsalva manoeuvre is plotted similarly. The mean resting cross sectional areas and the mean percentage increase in areas during a Valsalva manoeuvre between the three groups and between the right and left sides are summarized in Table I. There was no significant difference between the sizes of the right and left CFVs in any of the three groups. The mean cross sectional area of the CFVs in the third trimester group (130.1 mm2) and in the postpartum group (91.0 mm2) were significantly greater (p < 0.001 and p < 0.001, respectively) than in the first trimester group (45.6 mm2). Similarly, the mean percentage increase in areas during a Valsalva manoeuvre in the third trimester (34.5%) and the post-partum groups (86.2%) were significantly lower (p < 0.001 and p < 0.001, respectively) than in the first trimester group (203.8%). Seven veins of women in the third trimester were also examined in the opposite decubitus position and the CFV assessed. This reduced the mean cross sectional area at rest from 173.0 mm2 (supine) to 92.1 mm2. The effect of this on the percentage increase in cross sectional area during a Valsalva manoeuvre is shown in Figure 9. The mean percentage increase (supine) at 18.3% was increased to 140.7% (decubitus). Spontaneous antegrade flow was elicited in all CFVs examined by pulsed Doppler. Phasic variation with respiration (Fig. 4a) was present in all veins in the first trimester and postnatal groups and in 92% of veins in the third trimester group,flowbeing continuous in three veins. Of these, there was a normal Doppler response to a Valsalva manoeuvre, i.e. complete block plus post Valsalva augmentation (Fig. 5a) in two and increased antegrade flow on the Valsalva in the third (Fig. 5c). Cardiac pulsation was superimposed on the respiratory The British Journal of Radiology, September 1991
Duplex ultrasound of the CFV in pregnancy Table I. Results of cross sectional area assessments of the common femoral veins Cross sectional area (mm 2 ) At rest
On a Valsalva manoeuvre
Percentage increase a Valsalva manoeuvre
Mean
(SD)
Mean
Mean
(SD)
First trimester « = 20
Right Left Total
45.9 45.4 45.6
(20.3) (12.0) (16.4)
135.2 130.4 132.8
192.2 215.4 203.8
(98.7) (121.7) (110.8)
Third trimester « = 38
Right Left Total
129.0 130.9 130.1
(42.1) (49.4) (45.5)
163.6 170.1 166.8
31.8 36.9 34.4
(28.8) (28.1) (28.6)
Post-partum n = 10
Right Left Total
89.8 92.2 91.0
(37.7) (31.1) (34.6)
171.4 164.0 167.7
96.2 76.2 86.2
(49.6) (41.8) (46.9)
undulation of the signal in the veins of two patients (Fig. 4b), one in the first and one in the third trimester. Neither of these women had a history or signs of heart disease. Retrograde flow on the Valsalva manoeuvre was detected in five veins (13%) in the third trimester group (Fig. 5b). A Valsalva response comprising complete block or retrograde flow plus post Valsalva augmentation was present in 90% in the first trimester, 50% in the third and 60% in the postnatal group. One woman at 39 weeks' gestation produced an increase in antegrade flow in both CFVs during the Valsalva manoeuvre (Fig. 5c). The same manoeuvre produced 58% and 28% increases in the cross sectional areas of the right and left CFVs, respectively. There was no augmentation on cessation of the Valsalva (only during the Valsalva). Flow in the right CFV at rest was non-phasic in this patient. Of the seven veins examined in the decubitus position, phasic variation was more pronounced than in the supine position in four and a normal Doppler Valsalva response was obtained in five veins where it had been normal in only two in the supine position. All patients had positive, normal distal augmentation (Fig. 6).
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Duplex ultrasound of the common femoral vein in pregnancy and puerperium By M. J. Duddy, FRCR and \ J . M. McHugo, MRCP, FRCR The Birmingham Maternity Hospital, Queen Elizabeth Medical Centre, Edgbaston, Birmingham B15 2TG, UK {Received November 1990 and in revised form March 1991)
Keywords: Thrombosis, Ultrasound, Doppler, Pregnancy
Abstract. The common femoral veins of 34 apparently normal women were examined by ultrasound during or just after pregnancy. The cross sectional areas of these veins were measured at rest in the supine position and during a Valsalva manoeuvre. The veins were also assessed by pulsed Doppler ultrasound. We believe that Doppler examination is superior to calibre response assessment during the Valsalva manoeuvre in excluding an isolated iliac occlusion in women in late pregnancy and in early puerperium.
Fatal pulmonary embolus occurs in 1:100 000 pregnancies in this country and it vies with hypertensive disease of pregnancy as the commonest cause of maternal death. The most recent Report on Confidential Enquiries into Maternal Deaths in England and Wales (Turnbull et al, 1989) calls for earlier detection of thrombus in the veins of the pelvis and legs, and states that reduced flow in the femoral vein detected by noninvasive methods is an indication for anticoagulant therapy. ' The incidence of deep vein thrombosis (DVT) is around 0.6 per 1000 pregnancies in both the antenatal and postnatal periods. (Bergqvist et al, 1983a; Kierkegaard, 1983; Polak & O'Leary, 1988). DVT in pregnancy more frequently originates in the iliofemoral segment rather than in the calf veins (Bergqvist et al, 1983a; Bergqvist & Hedner, 1983b). This is distinct from the non-obstetric DVT which tends to originate distally. Furthermore, of those thromboses which result in fatal pulmonary embolus, over half are confined to the iliac veins (Turnbull et al, 1989). This presents problems in diagnosis. The clinical features are atypical often with lower abdominal or pelvic pain and oedema. Unlike the distal veins, the iliac veins are inaccessible to compression ultrasound. Cuff impedance plethysmography and phleborheography are not widely practised in the UK and false positive results have been reported during pregnancy (Clarke-Pearson & Jelovsek, 1981; Nicholas el al, 1985). Ultrasound Frede and Ruthberg (1988) used the gravid uterus of 25 weeks' gestation as an acoustic window to image the iliac veins directly (Fig. 1) and demonstrate a thrombo* Author for correspondence. Vol. 64, No. 765
Figure 1. Sonogram of the iliac vessels through a 16 week gestation gravid uterus as an acoustic window. CIV = common iliac vein, CIA = common iliac artery, IIA = internal iliac artery, EIA = external iliac artery.
sis. This method relies on imaging echogenic thrombus and would not detect fresh thrombus which is often isoechoic with blood. Furthermore, isolated iliac DVT is more frequent in late pregnancy when the iliac veins are often outside the focal zone of most probes. The increase in calibre of the common femoral vein (CFV) during the Valsalva manoeuvre has been used to assess the patency of the iliofemoral segment (Effeney et al, 1984). A 50% or more increase in antero-posterior (AP) diameter is quoted as a criterion for patency and a 10% or less increase as a criterion for occlusion. This indirect method of assessing the iliac veins has been used by other authors (Appleman et al, 1987; O'Leary & Kane, 1988; Zwiebel, 1988) and its use has been 785
M. J. Duddy and J. M. McHugo
Figure 2. Transverse sonogram of the common femoral vessels (a) at rest and (b) during compression with the transducer. There is complete collapse of the CFV during compression (arrows).
advocated in suspected DVT during pregnancy (Polak & O'Leary, 1988). Pulsed Doppler sampling of the CFV offers a further indirect method of assessing iliac vein patency (O'Leary & Kane, 1983; Thiele, 1984; Zwiebel, 1988). The Doppler signal from the CFV should be spontaneous, phasic and unidirectional. Lack of spontaneous flow implies occlusion at, proximal to or distal to the CFV. Lack of respiratory phasic variation of the signal (caused by pressure changes in response to the respiratory cycle) implies occlusion proximal to the CFV. Retrograde flow in the CFV implies valvular incompetence. Venous flow may be augmented by compression of veins distal to the sample site or by the release of a compression proximal to the sample site (Thiele, 1984).
(a)
Although compression proximal to the CFV may not be possible directly, the Valsalva manoeuvre (by increasing intra-abdominal pressure) has a similar effect. There is complete cessation of flow followed by augmentation as normal respiration is resumed, provided the iliac veins are patent. Iliac occlusion abolishes this Valsalva response of the Doppler signal. " Augmentation by distal compression is abolished by occlusion distal to the sample site, but according to Thiele (1984) remains intact in the presence of a proximal occlusion. O'Leary & Kane (1988), on the other hand, suggest that loss of augmentation at the CFV with calf compression may indicate an iliac vein occlusion. Pulsatileflowin the extremities is said always to be abnormal and usually caused by congestive heart failure (Zwiebel, 1988).
(b)
Figure 3. Transverse sonogram of the CFV (a) at rest and (b) during a Valsalva manoeuvre. D t = major axis, D 2 = minor axis, A = cross sectional area.
786
The British Journal of Radiology, September 1991
Duplex ultrasound of the CFV in pregnancy
(b)
Figure 4. Pulsed Doppler scan of the CFV showing spontaneous flow, (a) Flow is phasic with the respiratory cycle (arrows), (b) Pulsatile flow with the cardiac cycle (arrows) is superimposed on respiratory phasic variation (open arrows) in a patient with no history or signs of heart disease.
The aim of this study was to determine the effect of the enlarged uterus in late pregnancy and early puerperium on the Doppler characteristics of the CFV and on the Doppler and calibre response of the CFV to the Valsalva manoeuvre. Patients and method
Ultrasound assessment was made of 68 CFVs in 34 women. 10 were in early pregnancy (8 to 9 weeks' gestation). 19 women were in late pregnancy (28 to 40 weeks' gestation) one of whom was carrying twins at 37 weeks. Five women were between 2 and 10 days postpartum, two of whom had been delivered by caesarian section. All the women were normal volunteers with no clinical evidence of DVT. None of the women were examined more than once. Real-time ultrasound was performed using an electronically focused 5 MHz sector transducer with 5 MHz pulsed Doppler capability (Acuson, Mountain View, California). The lowest possible high pass filter (125 Hz)
was selected. The women were examined in the supine position. The CFVs were imaged transversely and longitudinally and assessed for their compressibility and the presence of intraluminal echoes (Fig. 2). The veins were examined at the level just proximal to the bifurcation of the common iliac artery. Assuming an elliptical cross section, the major and minor axes were measured and the cross sectional area calculated. The area is produced automatically by our machine, but can be calculated using the equation: area = j x major axis x minor axis. Area measurements were performed at rest and during a Valsalva manoeuvre (Fig. 3). Pulsed Doppler assessment was made during quiet respiration to elicit respiratory phasic variation of the signal (Fig. 4a). The women were then asked to perform the Valsalva manoeuvre and the Doppler signal was assessed to determine whether blood flow was blocked (i.e. reduced below the level of the high pass filter), merely reduced but still detectable, or whether flow was
Figure 5. Pulsed Doppler scan showing the CFV flow response to a Valsalva manoeuvre, (a) Complete block is achieved. There is augmentation of flow as normal breathing is resumed (arrow), (b) Reversed flow is achieved implying incompetence of venous valves, (c) Increased antegrade flow during a Valsalva manoeuvre at 39 weeks' gestation. This would seem to be a spurious response to an incorrectly performed Valsalva. Vol. 64, No. 765
787
M. J. Duddy and J. M. McHugo
Figure 6. Pulsed Doppler scan showing normal augmentation of CFV flow during calf compression.
reversed (Fig. 5). As normal respiration was resumed the presence of post Valsalva accentuation of the signal (proximal augmentation) was recorded. Finally, the presence of accentuation of the signal due to digital compression of the calf (distal augmentation) was assessed (Fig. 6). Seven CFVs in four women in their third trimester of pregnancy were also examined in the opposite decubitus positions, i.e. the vein under examination being uppermost. A /-test on a logarithmic transformation of the data was used to compare the means for two-sided p values. Results
Of the 68 CFVs examined, all were easily compressible and there would have been no difficulty in excluding CFV thrombosis. No intraluminal echoes were found in the veins examined in the first trimester group. Moving echoes were seen in 16/38 (42%) of veins in the third trimester and in 2/10 (20%) of veins in the postnatal group. Static intraluminal echoes were found in none of the veins examined.
20 GESTATION (weeks)
o 10 20 PUBfMUM (days)
Figure 7. Resting CFV cross sectional area plotted against gestational age and number of days post-partum. 788
20 GESTATION Iweeks)
•h
0 10 20 PUEPERUH (days)
Figure 8. Percentage increase in CFV cross sectional area during a Valsalva manoeuvre plotted against gestational age and number of days post-partum.
In Figure 7 the CFV cross sectional area is plotted against gestational age and number of days postpartum. In Figure 8 the percentage increase of CFV cross sectional area during a Valsalva manoeuvre is plotted similarly. The mean resting cross sectional areas and the mean percentage increase in areas during a Valsalva manoeuvre between the three groups and between the right and left sides are summarized in Table I. There was no significant difference between the sizes of the right and left CFVs in any of the three groups. The mean cross sectional area of the CFVs in the third trimester group (130.1 mm2) and in the postpartum group (91.0 mm2) were significantly greater (p < 0.001 and p < 0.001, respectively) than in the first trimester group (45.6 mm2). Similarly, the mean percentage increase in areas during a Valsalva manoeuvre in the third trimester (34.5%) and the post-partum groups (86.2%) were significantly lower (p < 0.001 and p < 0.001, respectively) than in the first trimester group (203.8%). Seven veins of women in the third trimester were also examined in the opposite decubitus position and the CFV assessed. This reduced the mean cross sectional area at rest from 173.0 mm2 (supine) to 92.1 mm2. The effect of this on the percentage increase in cross sectional area during a Valsalva manoeuvre is shown in Figure 9. The mean percentage increase (supine) at 18.3% was increased to 140.7% (decubitus). Spontaneous antegrade flow was elicited in all CFVs examined by pulsed Doppler. Phasic variation with respiration (Fig. 4a) was present in all veins in the first trimester and postnatal groups and in 92% of veins in the third trimester group,flowbeing continuous in three veins. Of these, there was a normal Doppler response to a Valsalva manoeuvre, i.e. complete block plus post Valsalva augmentation (Fig. 5a) in two and increased antegrade flow on the Valsalva in the third (Fig. 5c). Cardiac pulsation was superimposed on the respiratory The British Journal of Radiology, September 1991
Duplex ultrasound of the CFV in pregnancy Table I. Results of cross sectional area assessments of the common femoral veins Cross sectional area (mm 2 ) At rest
On a Valsalva manoeuvre
Percentage increase a Valsalva manoeuvre
Mean
(SD)
Mean
Mean
(SD)
First trimester « = 20
Right Left Total
45.9 45.4 45.6
(20.3) (12.0) (16.4)
135.2 130.4 132.8
192.2 215.4 203.8
(98.7) (121.7) (110.8)
Third trimester « = 38
Right Left Total
129.0 130.9 130.1
(42.1) (49.4) (45.5)
163.6 170.1 166.8
31.8 36.9 34.4
(28.8) (28.1) (28.6)
Post-partum n = 10
Right Left Total
89.8 92.2 91.0
(37.7) (31.1) (34.6)
171.4 164.0 167.7
96.2 76.2 86.2
(49.6) (41.8) (46.9)
undulation of the signal in the veins of two patients (Fig. 4b), one in the first and one in the third trimester. Neither of these women had a history or signs of heart disease. Retrograde flow on the Valsalva manoeuvre was detected in five veins (13%) in the third trimester group (Fig. 5b). A Valsalva response comprising complete block or retrograde flow plus post Valsalva augmentation was present in 90% in the first trimester, 50% in the third and 60% in the postnatal group. One woman at 39 weeks' gestation produced an increase in antegrade flow in both CFVs during the Valsalva manoeuvre (Fig. 5c). The same manoeuvre produced 58% and 28% increases in the cross sectional areas of the right and left CFVs, respectively. There was no augmentation on cessation of the Valsalva (only during the Valsalva). Flow in the right CFV at rest was non-phasic in this patient. Of the seven veins examined in the decubitus position, phasic variation was more pronounced than in the supine position in four and a normal Doppler Valsalva response was obtained in five veins where it had been normal in only two in the supine position. All patients had positive, normal distal augmentation (Fig. 6).
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