Positional variation in hemodynamic and morphologic parameters of the iliofemoral venous segment in healthy volunteers

Abstract

Objective

Iliofemoral venous stenting has become the standard of care for patients with quality-of-life-impairing manifestations of chronic iliac venous obstruction. The initial diagnosis of chronic iliac venous obstruction is through the use of duplex ultrasound (DUS). Here, set diameters are used for each segment (common femoral vein [CFV], external iliac vein [EIV], and common iliac vein [CIV]) to determine if a patient has venous stenosis or not. However, there is concern that DUS measurements obtained in the supine position (standard approach) are different from those when the person is standing, thereby raising the question as to the appropriate positioning for such a study. This study evaluates this possibility through DUS examination of healthy adults.

Methods

Sixteen limbs (eight healthy volunteers) without any symptoms or signs of chronic venous disease underwent DUS of their right and left CFV, EIV, and CIV segments in the supine and standing positions. Measurements included peak velocity (PV), time-averaged velocity (TAV), and the smallest vein diameter in each segment. Measurements were done for each person across six cohorts: CFV, EIV, CIV, CFV + EIV, EIV + CIV, and CFV + EIV + CIV. Normality of paired differences was assessed using the Shapiro-Wilk test. Variables demonstrating non-normal distributions were analyzed using the Wilcoxon signed-rank test, whereas normally distributed variables were compared using paired t-tests. Effect size was quantified using Cohen’s d_z. P value ≤.05 was considered significant.

Results

In the CFV, the PV and TAV were both greater in the supine than in the standing position (P < .05), whereas the diameter was smaller in the supine position (mean difference, 1.1 mm; P = .05). In the EIV, a similar finding was observed; however, statistical significance was noted only for PV (P = .02) and TAV (P = .02). In the CIV, no statistically significant change in PV, TAV, or diameter was noted when supine was compared with the standing position. When EIV and CIV segments were combined, a statistically significant decrease in the PV (0.07 cm/s, P = .02) and TAV (0.02 cm/s, P = .03) was noted in the standing compared with supine. An increase in diameter (mean difference, 0.2 mm; P = .39) was noted going from supine to standing position. Effect size analysis (Cohen’s d_z) demonstrated a moderate to large effect size for PV and TAV and a small effect size for diameter.

Conclusions

Venous duplex-derived flow velocities in the iliac vein in healthy volunteers demonstrate a moderate to large, statistically significant decrease from supine to standing position without a significant change in the vein diameter. This raises the question about the relevance of any increase in vein diameter from supine to standing, which has become an important contemporary issue. Further study is warranted.

Background

Duplex ultrasound (DUS) is central to the noninvasive evaluation of iliofemoral venous pathology because it provides both anatomic and hemodynamic information, including venous diameter, compressibility, flow phasicity, augmentation response, and reflux or obstruction patterns. Unlike arteries, venous dimensions are highly dynamic. Measured diameters vary with intraluminal pressure, respiration, Valsalva maneuver, transducer pressure, hydration, body habitus, and, importantly, patient position. Normal larger veins may change caliber with deep inspiration or Valsalva because of changes in intra-abdominal pressure. Early duplex work by Moneta et al¹ demonstrated that the mean maximal common femoral vein (CFV) diameter increased 92%, whereas the peak flow velocity decreased from 41 ± 10 cm/s to 13 ± 5 cm/s when normal subjects (n = 12) were tilted from -10° (head down) to +30° (head up). The authors noted a linear inverse relationship between mean peak velocity (PV) and mean corrected diameter (r = -0.99). Czyzewska et al,² in their study on 205 healthy volunteers focusing on just CFV cross-sectional area on DUS, noted that the area is the largest in the frog leg position. Engelhorn et al,³ in their study on 50 adult female asymptomatic volunteers, did not find a significant difference in compression of the left common iliac vein (CIV) in the standing position compared with the supine position. The impact of position on the diagnosis of stenosis in the symptomatic patient with pelvic venous disease has been raised by a recent study⁴ that noted that >50% stenosis in the left CIV dropped from 100% in the supine position to 32% in the standing position. Some of these data have resulted in an increasingly prevalent practice among interventionalists who have expertise in the treatment of chronic iliac venous obstruction (CIVO) to scan patients with suspected CIVO in the supine and standing positions. The rationale behind this approach is that with standing, the vein flops open and the stenosis diagnosed in the supine position disappears. The current study, by evaluating the entire iliofemoral venous segment (CFV, external iliac vein [EIV], and CIV) through assessment of velocities and diameters, attempts to shed light on the impact of position on venous hemodynamics and morphology.

Methods

Study design

Single-center analysis of prospectively collected data. Sitero Institutional Board review approval was obtained for the study involving DUS evaluation of healthy volunteers.

Setting

The RANE center is a tertiary center for the management of venous and lymphatic disorders.

Participants

Volunteers without any symptoms or signs of chronic venous disease underwent DUS evaluation of their right and left iliofemoral venous segments.

Measurements

DUS was performed using the GE S8 ultrasound (GE Medical Systems) with a 9-MHz linear probe for the CFV and a 1 to 5-MHz curvilinear probe for the EIV, CIV, and the inferior vena cava. B-mode, B-flow, color flow Doppler, and pulse wave Doppler modalities were used during the scan. Measurements included PV, time-averaged velocity (TAV), and vein diameters of the CIV, EIV, and CFV segments in each limb of eight healthy volunteers (16 limbs) in the supine and standing positions. PV was defined as the maximum velocity recorded within the Doppler spectral waveform, whereas TAV was defined as the mean velocity calculated over the sampled waveform interval. All velocity measurements were recorded in centimeters per second (cm/s), whereas diameter was recorded in millimeters.

Statistical analysis

Statistical analysis was performed using SPSS, version 28 (IBM). Comparisons were made between supine and standing positions of the CIV, EIV, and CFV segments in each limb of eight healthy volunteers individually and in combination. Normality of paired differences was assessed using the Shapiro-Wilk test. Variables demonstrating non-normal distributions were analyzed using the Wilcoxon signed-rank test, whereas normally distributed variables were compared using paired t-tests. Effect size was quantified using Cohen’s d_z. P value ≤.05 was considered significant.

Results

There was a total of 16 limbs (eight volunteers). Of these, there were five women and three men. The median age of the group was 30 years, and the median body mass index was 24.

Hemodynamic impact

In the CFV, the PV and TAV were both greater in the supine than in the standing position (P < .05) (Table I). In the EIV, a similar finding was observed, with statistical significance noted for both PV (P = .02) and TAV (P = .02). In the CIV, no statistically significant change in PV or TAV was noted when supine was compared with the standing position. On combining the CFV and EIV segments (Table II), the PV and TAV both decreased from supine to standing position (P < .05). When EIV and CIV segments were combined, a statistically significant decrease in the PV (0.07 cm/s, P = .02) and TAV (0.02 cm/s, P = .03) was noted in the standing compared with supine. In the CFV + EIV + CIV cohort, findings similar to the CFV + EIV cohort were noted with a significant decrease in PV (0.08 cm/s, P < .001) and TAV (0.04 cm/s, P < .001) in the standing position (Table II).

Table I.

Result of analysis of individual venous segments

Individual segment analysis
	Mean/median∗ difference (supine - upright)	SD/IQR	P value	95% CI
CFV (n = 15 limbs)
PV (cm/s)	0.08∗	0.04-0.26	0.003	0.06-0.25
TAV (cm/s)	0.06	0.05	<.001	0.04-0.09
Diameter (mm)	−1.1	2.28	0.05	−2.33 to 0.20
EIV (n = 14 limbs)
PV (cm/s)	0.08	0.13	0.02	0.01-0.17
TAV (cm/s)	0.03	0.05	0.02	0.001-0.05
Diameter (mm)	−0.36	2.5	0.3	−1.8 to 1.09
CIV (n = 8 limbs)
PV (cm/s)	0.003	0.1	0.96	−0.15 to 0.16
TAV (cm/s)	0	0.02	1	−0.04 to 0.04
Diameter (mm)	0.3	1.3	0.72	−1.80 to 2.30

CFV, Common femoral vein; CI, confidence interval; CIV, common iliac vein; EIV, external iliac vein; IQR, interquartile range; PV, peak velocity; SD, standard deviation; TAV, time-averaged velocity.

Table II.

Result of analysis of combined venous segments

Combined segment analysis
	Mean/median∗ difference (supine - upright)	SD/IQR	P value	95% CI
CFV + EIV (n = 29 limbs)
PV (cm/s)	0.09∗	0.04-0.17	<.001	0.06-0.165
TAV (cm/s)	0.05	0.05	<.001	0.03-0.07
Diameter (mm)	−0.72	2.4	0.06	−1.6 to 0.18
EIV + CIV (n = 22 limbs)
PV (cm/s)	0.07	0.13	0.02	0.01-0.13
TAV (cm/s)	0.02	0.04	0.03	0.00-0.04
Diameter (mm)	−0.17	2.5	0.39	−1.4 to 1.1
CFV + EIV + CIV (n = 33 limbs)
PV (cm/s)	0.08∗	0.01-0.15	<.001	0.05-0.145
TAV (cm/s)	0.04	0.05	<.001	0.02-0.06
Diameter (mm)	−0.58	2.4	0.09	−1.4 to 0.27

CFV, Common femoral vein; CI, confidence interval; CIV, common iliac vein; EIV, external iliac vein; IQR, interquartile range; PV, peak velocity; SD, standard deviation; TAV, time-averaged velocity.

Morphologic impact

In the CFV, the diameter was smaller in the supine position (mean difference, 1.1 mm; P = .05). In the EIV, a similar finding was noted with the mean diameter being 0.36 mm smaller, but this finding was not statistically significant (P = .3). In the CIV, the diameter in the supine position was 0.3 mm smaller (P = .72). On combining the CFV and EIV segments, the diameter increased by 0.72 mm (P = .06) from supine to standing position. When EIV and CIV segments were combined, a decrease in the diameters (mean difference, 0.17 mm) in the supine position was noted, but these changes did not reach statistical significance (P = .39). In the combined CFV + EIV + CIV cohort, the diameter was 0.58 mm smaller in the supine position (P = .09) (Table II).

Effect size

From a hemodynamic standpoint, a significant decrease in the PV and TAV was noted when going from supine to standing position in the CFV, EIV, CFV + EIV, EIV + CIV, and CFV + EIV + CIV groupings. The effect size was moderate for PV and large for TAV. With regard to vein diameter, the only statistically significant change in this volunteer group was in the CFV, where the diameter increased by 1.1 mm on standing. The effect size was small (Table III).

Table III.

Analysis of size effect through calculation of Cohen’s d_z

	Cohen’s dz	95% CI
CFV
PV (cm/s)	0.79	0.20-1.37
TAV (cm/s)	1.27	0.51-2.01
Diameter (mm)	−0.47	−1.01 to 0.09
EIV
PV (cm/s)	0.65	0.04-1.24
TAV (cm/s)	0.60	0.00-1.18
Diameter (mm)	−0.14	−0.68 to 0.40
CIV
PV (cm/s)	0.03	−1.00-1.03
TAV (cm/s)	0.00	−0.98-0.98
Diameter (mm)	0.20	−0.85 to 1.20
CFV + EIV
PV (cm/s)	0.72	0.30-1.13
TAV (cm/s)	0.91	0.45-1.35
Diameter (mm)	−0.31	−0.69 to 0.07
EIV + CIV
PV (cm/s)	0.54	0.01-1.04
TAV (cm/s)	0.50	−0.01-0.99
Diameter (mm)	−0.07	−0.55 to 0.41
CFV + EIV + CIV
PV (cm/s)	0.65	0.26-1.03
TAV (cm/s)	0.81	0.39-1.21
Diameter (mm)	−0.24	−0.60 to 0.12

CFV, Common femoral vein; CI, confidence interval; CIV, common iliac vein; EIV, external iliac vein; PV, peak velocity; TAV, time-averaged velocity.

Discussion

DUS represents the initial diagnostic tool for assessment of iliofemoral hemodynamics and morphology. It is easily available, and while there is an element of operator dependence, with well-trained and experienced sonographers, the intraoperator/interoperator differences are small and not of much significance.⁵ This leads to the important issue of variability noted when the DUS is done in the supine position compared with standing and the relevance of such variation. There is a contemporary school of thought that there is an increase in iliac vein diameter in patients with suspected CIVO when going from supine to standing/standing position, and that patients should be scanned in both positions.

Postural impact on venous hemodynamics

The reduction in PV and TAV observed in the standing position reflects normal physiologic adaptation to gravitational loading. Upon standing, venous blood volume shifts toward the dependent lower extremities, resulting in increased venous capacitance and pooling. As venous cross-sectional area in the lower extremity expands, flow velocity decreases. This is clearly noted in the CFV and has been demonstrated in prior studies.^1,6 The significant drop in the EIV or CIV velocities has not been reported previously. The absence of a significant finding in the CIV is likely the result of the smaller sample size of this cohort, given the inability to adequately visualize the CIV.

Morphologic adaptation to standing positioning

The most pronounced morphologic finding was the increase in CFV diameter during standing positioning. This change has again been demonstrated in the literature.^1,6 Although similar trends were observed within the EIV and CIV, statistical significance was reached only within the CFV. These findings are consistent with the highly compliant nature of the venous system and the increase in hydrostatic pressure that accompanies standing. The CFV appears particularly susceptible to these effects because of its location within the most gravity-dependent portion of the iliofemoral venous system.

Significance of postural variation

The present study demonstrates that body position has a significant influence on duplex ultrasound-derived iliofemoral venous hemodynamics, with the most pronounced effects observed in velocity-based parameters. Transition from the supine to standing position was associated with consistent reductions in PV and TAV, particularly within the CFV and EIV segments, where moderate-to-large effect sizes were observed. These findings are physiologically plausible and reflect gravitational redistribution of blood volume, increased venous capacitance, and reduced venous return in the standing position. Importantly, the magnitude of these changes was sufficiently large to influence interpretation of hemodynamic measurements, highlighting that venous flow velocities are not fixed physiologic values but are highly dependent on patient positioning at the time of examination.

In contrast, morphologic parameters demonstrated relative stability with postural change. Vein diameter exhibited only small effect sizes and no consistent statistically significant differences. This differential response suggests that postural variation primarily affects venous flow dynamics rather than venous morphology.

Clinically, these findings indicate that velocity-derived parameters may be more sensitive markers of physiologic and potentially pathologic alterations in venous return. The 20% to 30% drop in cardiac output noted when going from supine to standing^7,8 is due to a drop in velocity without a compensatory increase in the vein diameter to maintain flow. The observed dissociation between hemodynamic and morphologic responses also underscores the importance of interpreting venous duplex findings within the context of patient position.

Several limitations should be acknowledged. First, the study population consisted of healthy volunteers, and the findings may not be directly generalizable to patients with CIVO. Second, the sample size is small and likely has limited statistical power. Third, physiologic variables such as hydration status may have influenced measured parameters. Despite these limitations, the present study, by including a comprehensive evaluation of the CFV, EIV, and CIV segments, provides important evidence that iliac vein diameters in healthy volunteers are not significantly influenced by patient position.

Conclusions

In this study in healthy volunteers, patient position significantly influences iliofemoral venous DUS velocity measurements. Transition from the supine to standing position resulted in consistent reductions in PV and TAV, demonstrating moderate-to-large effect sizes and highlighting the sensitivity of venous flow dynamics to postural change. In contrast, venous diameter mostly exhibited comparatively small and statistically insignificant changes, suggesting that venous caliber is less affected by body position than flow velocity. This raises the question about the diagnostic relevance of any increase in vein diameter from supine to standing. Further study to determine such relevance, if any, is warranted.

Author Contributions

Conception and design: AJ

Analysis and interpretation: AJ

Data collection: AJ

Writing the article: AJ

Critical revision of the article: AJ

Final approval of the article: AJ

Statistical analysis: AJ

Obtained funding: Not applicable

Overall responsibility: AJ

Funding

None.

Disclosures

None.