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Anesthesia & Analgesia:
doi: 10.1213/ANE.0b013e3182860e3c
Patient Safety: Research Report

The Effects of the Trendelenburg Position and Intrathoracic Pressure on the Subclavian Cross-Sectional Area and Distance from the Subclavian Vein to Pleura in Anesthetized Patients

Kwon, Mi-Young MD*; Lee, Eun-Kyung PhD; Kang, Hye-Ju MD*; Kil, Ho-young MD*; Jang, Kee-Hoon MD*; Koo, Min-Seok MD, PhD*; Lee, Gunn-Hee MD*; Lee, Myung-Ae MD, PhD*; Kim, Tae-Yop MD, PhD

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From the *Department of Anesthesiology, National Medical Center, Seoul; Department of Statistics, Ewha Women’s University, Seoul; and Department of Anesthesiology, Konkuk University Medical Center, Research Institute of Biomedical Science, Konkuk University School of Medicine, Seoul, Korea.

Accepted for publication December 10, 2012.

Published ahead of print March 11, 2013

Supported by grants from the Konkuk University Medical Center, Research Institute of Biomedical Science, Konkuk University School of Medicine.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Tae-Yop Kim, MD, PhD, Department of Anesthesiology, Konkuk University Medical Center, Research Institute of Biomedical Science, Konkuk University School of Medicine, 4-12 Hwayang-dong, Gwangjin-gu, Seoul 143-729, Korea. Address e-mail to pondkim@unitel.co.kr.

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Abstract

BACKGROUND: The effects of maneuvers to increase intrathoracic pressure and of Trendelenburg position on the cross-sectional area (CSA) of the subclavian vein (SCV) and the relationship between the SCV and adjacent structures have not been investigated.

METHODS: In ultrasonography-guided SCV catheterization (N = 30), the CSA of the SCV and the distance between the SCV and pleura (DSCV-pleura) were determined during 10-second airway opening, and 10-second positive inspiratory hold with 20 cm H2O in the supine position (S-0, and S-20) and the 10° Trendelenburg position (T-0, and T-20). In addition to a statistical significance of P < 0.05, CSA and DSCV-pleura differences of ≥15% were defined as clinically relevant changes.

RESULTS: CSA (mean [95% confidence interval]) in S-20, T-0, and T-20 (1.02 [0.95–1.14] cm2, 1.04 [0.95–1.15] cm2, and 1.14 [1.04–1.24] cm2, respectively) was significantly larger than a CSA in S-0 (0.93 [0.86–1.00] cm2, all P < 0.001). However, only the increase of CSA in T-20 vs S-0 (0.21 cm2, 23.2%) was clinically meaningful (≥15%). The number of patients who showed CSA increase ≥15% was more in S-0 to T-20 (57%) compared with those in S-0 to S-20 (23%) and S-0 to T-0 (27%). DSCV-pleura measurements (mean) in S-20 and T-20 (0.61 and 0.60 cm) were significantly shorter than those in S-0 (0.70 cm, all P < 0.001), but the reductions of DSCV-pleura were not clinically meaningful (≥15%).

CONCLUSIONS: The combined application of inspiratory hold and Trendelenburg position provided a greater and more relevant degree of CSA increase without compromising DSCV-pleura, which may facilitate SCV catheterization. Further investigations are needed to determine whether these results affect the success rate of catheterization and the risk of procedural injury.

The infraclavicular approach for percutaneous cannulation of the subclavian vein (SCV) was introduced in the early 1950s and is now frequently used for inserting the central venous catheter to administer medication or fluid and obtain cardiovascular measurements. However, considering the frequency of serious complications, including pneumothorax (1.5%–2.0%),1,2 placing patients in a position that facilitates successful SCV cannulation may be required. Several studies have identified the size and location of the SCV using computed tomography, magnetic resonance imaging scanning, and ultrasonography.3–8 These studies speculate that the Trendelenburg position,3–5 leg elevation,6,8 and ipsilateral shoulder brought caudally without shoulder roll3,4,7 are beneficial maneuvers to facilitate successful SCV cannulation. The benefits of turning the patient’s head toward the neutral or contralateral side, however, remain controversial.3,4

Although ultrasound guidance is frequently used to improve the success rate of central venous catheterization, several adjuvant maneuvers aimed at increasing the intrathoracic pressure, such as the Valsalva maneuver, hepatic compression, and positive intrathoracic pressure or positive end-expiratory pressure, have supplemented ultrasound guidance and increased the cross-sectional area (CSA) of the internal jugular vein (IJV) and promoted accurate and safe catheterization.9–12 In contrast, a few studies have demonstrated that the Trendelenburg position and leg elevation are not effective for producing a CSA increase of the SCV comparable with that of the IJV.6,8 This discrepancy is probably attributable to the different anatomical relationships of the SCV with adjacent fibrous structures surrounding the SCV and the increased intrathoracic pressure, which can paradoxically compress the SCV in the thoracic cage.8

Because the SCV and the needle’s path for SCV cannulation are located adjacent to the lung, pleura, and subclavian artery, maneuvers to increase the intrathoracic pressure may affect the SCV’s anatomical relationship by distending the lung or adversely compressing the SCV. However, despite the wide application of ultrasonography for central venous catheterization, insufficient research has investigated whether increasing the intrathoracic pressure as well as placing the patient in Trendelenburg position affect the CSA of the SCV or the SCV’s anatomical relationship with adjacent structures.

This study was based on the hypothesis that applying different postures and intrathoracic pressures would change the CSA of the SCV and the distances between the SCV and pleura (DSCV-pleura), which are important factors for facilitating SCV catheterization and decreasing the risk of procedural injury. Therefore, we analyzed the impact of the Trendelenburg position and application of positive intrathoracic pressure or their combination on the CSA of the SCV, the DSCV-pleura, and their changes (ΔCSA, ΔDSCV-pleura) during SCV catheterization in anesthetized patients.

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METHODS

Between September and November 2010, patients with ASA physical status I or II were screened at the National Medical Center, Seoul, Korea. Patients were excluded if they had a history of valvular heart disease, severe obstructive pulmonary disease, clavicle fracture, central venous catheterization, abnormal skin changes from previous surgery, or radiation at the site of catheterization. Additionally, subjects were excluded if the routine preoperative radiographic examination revealed bony thoracic cage deformities, including the clavicle or ribs, or pleural thickening related to previous empyema or pneumothorax. As part of the institutionally approved protocol, signed informed consent was obtained from each patient undergoing elective surgery.

After anesthesia induction and tracheal intubation, mechanical ventilation was initiated with a tidal volume of 10 mL/kg and a respiratory rate of 10–12/min to maintain normocapnia. An operator then captured 2-dimensional ultrasound images to determine the CSA of the right SCV and the shortest distance from the right SCV to pleura (DSCV-pleura) using ultrasonography (S-Nerve™; SonoSite Inc., Bothell, WA). The ultrasound transducer (bandwidth of 13–6 MHz, 23-mm broadband linear array, depth 6 cm) was placed at the medial third and middle third junction of the clavicle, corresponding to the percutaneous needle entry area for SCV catheterization. Light pressure was applied to the transducer to avoid SCV compression.

For assessment in the Trendelenburg position, the subject was placed flat (or supine) with the entire stretcher in a 10° head-down position. For assessment in full expiration, the ventilator was stopped with the pressure-relief valve open, and for the inspiratory hold, the airway pressure was maintained at 20 cm H2O by adjusting the pressure-relief valve in manual ventilator mode.

The CSA of the SCV and the DSCV-pleura were determined in each different posture and airway pressure condition as follows: volume-controlled ventilation, end-expiratory airway opening for 10 seconds, and inspiratory hold for 10 seconds with an airway pressure of 20 cm H2O in the supine position (S-control, S-0, and S-20). The same maneuvers were performed in the Trendelenburg position (T-control, T-0, and T-20). For these determinations, ultrasound images of the CSA and DSCV-pleura were obtained at least 3 seconds after instituting each maneuver for 10 seconds. A 30-second interval with controlled ventilation in the supine position was applied between each maneuver.

As a primary objective, the CSA and the DSCV-pleura were determined and multiple comparisons (S-0 vs S-20, S-0 vs T-0, S-0 vs T-20, S-20 vs T-0, S-20 vs T-20, and T-0 vs T-20) were made to identify a maneuver providing relevant changes in those variables that suggested facilitated SCV catheterization or increased risk of pleural injury. Considering the relatively limited distending potential due to its location surrounded by adjacent structures and paradoxical compression by the increased intrathoracic pressure, a 15% difference in the CSA was defined as a relevant change, and the same criterion was applied for evaluation of DSCV-pleura. The incidences (percentage of patients) of producing a relevant change in the CSA and DSCV-pleura (ΔCSA and ΔDSCV-pleura of ≥15%) by positive inspiratory hold (S-0 to S-20), Trendelenburg position (S-0 to T-0), or their combination (S-0 to T-20) were compared with χ2 test.

Preliminary retrospective CSA data (0.90 ± 0.19 cm2) were used to calculate the study power: at least 29 patients were required to identify a ΔCSA comparable to a 15% difference (0.135 cm2) with 80% power and type I error <0.05.

Repeated-measures analysis of variance (ANOVA) was used to analyze variables of CSA and DSCV-pleura, followed by a post hoc analysis with the Bonferroni method for multiple comparisons. Bonferroni corrected P value of <0.008 was considered to be statistically significant for 6 comparisons (0.05/6 = 0.008). To determine normality and sphericity of variables of CSA and DSCV-pleura for the repeated-measure ANOVA, the Shapiro-Wilk test and Mauchly test were performed, respectively. The variables of DSCV-pleura passed the normality test (P = 0.206) and Mauchly test (P = 0.065). However, the variables of CSA did not pass the Shapiro-Wilk test (P = 0.003) and Mauchly test (P < 0.001): the normal assumptions for the repeated-measure ANOVA were applied after applying the additional q-q plot and Greenhouse-Geisser correction for variables of CSA.

SAS version 9.1 (SAS Institute Inc., Cary, NC) was used for all statistical analyses.

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RESULTS

Thirty patients participated in this study; Table 1 lists their characteristics. All patients had normal body habitus with similar body mass index. The values of CSA of the SCV at S-control and T-control were 0.93 ± 0.17 cm2 and 1.03 ± 0.22 cm2, respectively, but those of DSCV-pleura at S-control and T-control were not measured because of the fluctuations in their values during the ventilatory movement.

Table 1
Table 1
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CSA values in S-0 and T-0 were not different from those in S-control and T-control, respectively (P = 0.984 and P = 0.504 in paired t test). Table 2 lists the CSA and DSCV-pleura in S-0, S-20, T-0, and T-20 and the changes of their mean values by each maneuver (S-0 to S-20, S-0 to T-0, and S-0 to T-20). Multiple comparisons of the CSAs at S-0, S-20, T-0, and T-20 revealed statistical differences in S-0 vs S-20, S-0 vs T-0, S-0 vs T-20, S-20 vs T-20, and T-0 vs T-20 (all P < 0.001): the CSA difference by the combined application of positive inspiratory hold and Trendelenburg position reached the degree defined as relevant (ΔCSA ≥15%), whereas those by applying positive inspiratory hold alone and applying the Trendelenburg position alone did not. Multiple comparisons of the DSCV-pleura in S-0, S-20, T-0, and T-20 revealed statistical differences in S-0 vs S-20, S-0 vs T-20, T-0 vs S-20, and T-0 vs T-20 (all P < 0.001); however, all DSCV-pleura differences by applying positive inspiratory hold in the supine position, applying the Trendelenburg position alone, and the combined application of positive inspiratory hold and the Trendelenburg position did not reach the degree defined as relevant ([INCREMENT]DSCV-pleura ≥15%). The incidence of the relevant CSA change ([INCREMENT]CSA ≥15%) in S-0 to T-20 was the highest (17/30, 57%) compared with those in S-0 to S-20 (7/30, 24%) and those in S-0 to T-0 (8/30, 27%), as shown in Figure 1.

Table 2
Table 2
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Figure 1
Figure 1
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DISCUSSION

We evaluated the effects of increased intrathoracic pressure (20 cm H2O) or Trendelenburg position on the CSA and DSCV-pleura during SCV catheterization and general anesthesia, and determined whether their changes were clinically relevant (defined as ΔCSA and ΔDSCV-pleura ≥15% vs S-0). Applying positive intrathoracic pressure alone or Trendelenburg position alone provided a statistically increased CSA of the SCV, but this increase did not meet our defined threshold for a relevant degree (ΔCSA of ≥15%). Only the combined application of these 2 maneuvers yielded a relevant increase in the CSA (ΔCSA 23.2% vs S-0). No maneuvers provided a relevant change of DSCV-pleura (ΔDSCV-pleura ≥15%) despite their statistically significant changes in some conditions.

This primary outcome revealed that only the combined maneuver provides a relevant degree of CSA increase, suggesting a possible facilitation of SCV catheterization without increasing the risk of pleural injury. The significantly higher incidence of providing the relevant CSA increase by the combined maneuvers (57%), compared with those by each maneuver alone (24%–27%), supports this conclusion.

The Trendelenburg position shifts blood from the lower part of the body to the central veins and decreases the venous return from the upper part of the body to the heart by the difference in gravity.3,4 Although increasing intrathoracic pressure increases the CSA of the IJV by increasing the intravascular pressure and venous distension in the IJV,10 the effect of intrathoracic pressure on the CSA of the SCV has not been well studied. Our findings partially support those of previous studies, which speculated that each maneuver produces significant increases in the CSA of the IJV.6,9,10,13 However, each maneuver alone did not produce a significant increase in the CSA of the SCV. This result also suggests that additional filling of the intravascular lumen by proportioning blood to the central veins may be required to make the SCV more distensible and less compressible by overcoming the SCV’s relatively limited potential for distension in the thoracic outlet.8

Apart from infectious and thrombotic complications, SCV catheterization is associated with procedure-related injury of adjacent structures, which can result in serious complications such as arterial puncture, hematoma, hemothorax, or pneumothorax.1,14–16 Because movement from mechanical ventilation may contribute to procedural injury, the ventilator is generally turned off briefly to avoid injury of adjacent structures, especially during advancement of the needle for catheterization. Our findings of the statistical reduction in DSCV-pleura against the statistical increment of the CSA might suggest an increased risk of pleural injury. However, the degree of the reduction did not meet our defined threshold for clinical relevancy (ΔDSCV-pleura ≥15%) with regard to suggesting an increased risk of injury. Furthermore, considering the necessity of turning off the ventilator to reduce the risk of lung complications, it is important to note that stopping ventilation may lead to hypoxemia, especially in patients with severe cardiopulmonary disease. Because stopping the ventilator with the expiratory valve open generated no significant changes in the CSA of the SCV in both positions, central venous catheterization with the ventilator on could be used in selected patients at high risk of hypoxia or apnea.

Our study has several limitations. We did not analyze the clinical outcome of the relevant increase in CSA regarding the possible enhancement in the success rate of SCV catheterization. Additionally, all of the patients included in this analysis were Asian; their body sizes may be smaller than those of Caucasians, meaning that their CSA and DSCV-pleura could differ from other ethnic groups. Therefore, our study may not be applicable to all patients in terms of interpreting the larger CSA and greater degree of CSA change as the enhanced success rate of SCV catheterization. Further studies evaluating other ethnic groups and determining the success rate are required.

In conclusion, the combined application of positive inspiratory hold of 20 cm H2O and Trendelenburg position provided a relevant increase of the CSA of the SCV without relevant reduction of the [INCREMENT]DSCV-pleura in anesthetized patients. The results may suggest improved SCV catheterization without an increased risk of pleural injury by this maneuver, but further investigation is needed to determine the impact on the catheterization success rate or the risk of procedural injury.

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DISCLOSURES

Name: Mi-Young Kwon, MD.

Contribution: This author helped design the study, conduct the study, collect the data, analyze the data, and prepare the manuscript.

Attestation: Mi-Young Kwon approved the final manuscript, attested to the integrity of the original data and the analysis reported in this manuscript, and is the archival author.

Name: Eun-Kyung Lee, PhD.

Contribution: This author helped analyze the data and prepare the manuscript.

Attestation: Eun-Kyung Lee approved the final manuscript, and attested to the integrity of the original data and the analysis reported in this manuscript.

Name: Hye-Ju Kang, MD.

Contribution: This author helped conduct the study and collect the data.

Attestation: Hye-Ju Kang approved the final manuscript.

Name: Ho-young Kil, MD.

Contribution: This author helped conduct the study and collect the data.

Attestation: Ho-young Kil approved the final manuscript.

Name: Kee-Hoon Jang, MD.

Contribution: This author helped conduct the study and collect the data.

Attestation: Kee-Hoon Jang approved the final manuscript.

Name: Min-Seok Koo, MD, PhD.

Contribution: This author helped conduct the study and collect the data.

Attestation: Min-Seok Koo approved the final manuscript.

Name: Gunn-Hee Lee, MD.

Contribution: This author helped conduct the study and collect the data.

Attestation: Gunn-Hee Lee approved the final manuscript.

Name: Myung-Ae Lee, MD, PhD.

Contribution: This author helped conduct the study and collect the data.

Attestation: Myung-Ae Lee approved the final manuscript.

Name: Tae-Yop Kim, MD, PhD.

Contribution: This author helped design the study, analyze the data, and prepare the manuscript.

Attestation: Tae-Yop Kim approved the final manuscript, and attested to the integrity of the original data and the analysis reported in this manuscript.

This manuscript was handled by: Sorin J. Brull, MD, FCARCSI (Hon).

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REFERENCES

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