Blood Gaseous Changes in Patients Undergoing a Dangling Protocol after Lower Leg Free Flap Reconstruction

Abstract

Background

Patients undergoing free flap reconstruction of the lower extremity are typically started on a postoperative dangling protocol. This protocol gradually exposes the free flap to increasing gravitational forces. This study aimed to quantify the internal blood gaseous changes within the free flap over the course of a dangling protocol.

Methods

Three patients who underwent lower leg free flap reconstruction were included. Capillary blood gas samples were taken daily for six consecutive days (postoperative days 7–12) from the free flap before and after dangling, as well as from the contralateral, “healthy” leg. Mean blood gas curves were created for partial pressure of oxygen (pO₂), partial pressure of carbon dioxide (pCO₂), and potential hydrogen (pH) levels. A paired t-test was used to compare mean blood gas values.

Results

Baseline mean pO₂ was decreased (p = 0.002), pCO₂ was increased (p < 0.001), and pH levels were lowered (p = 0.021) at the site of the free flap, compared with the contralateral, “healthy” leg. Mean pre- and post-dangling pO₂ levels were not significantly different (p = 0.380). Mean pCO₂ levels significantly increased after dangling (p = 0.028). Over the course of the dangling protocol, mean pO₂, pCO₂, and pH curves remained considerably stable.

Conclusion

Blood samples taken from the free flap show that the mean pO₂ and pH levels are lower, and pCO₂ levels are higher compared with the contralateral, “healthy” leg. Furthermore, an increase in dangling duration does not cause significant changes in blood gas values. This raises the question of whether the current form of the dangling protocol accomplishes its intended goal of gradually challenging the flap.

Keywords

reconstruction - free flap - dangling - lower leg - blood gas

Free flap reconstruction is commonly used for the reconstruction of large and complex defects of the lower legs after oncological resection or trauma.[1] [2] Following surgery, patients are typically started on a dangling protocol to gradually expose the free tissue transplant to increasing gravitational forces. However, protocols vary widely in terms of starting point, frequency, and duration of dangling.[3] [4] [5] [6] [7] The heterogeneity of dangling protocols results from the absence of conclusive evidence regarding the optimal postoperative treatment strategy.

The vascular structures in a free tissue transplant lack the intrinsic mechanisms to correct for alterations in perfusion during the early postoperative phase.[8] In the case of lower extremity reconstructions, dangling of the leg results in the increased pooling of venous blood. During dangling, free flaps often turn blue due to venous pooling, which can cause concern to the patient and physician. Venous blood has a considerably lower partial pressure of oxygen (pO₂), potential hydrogen (pH), and a higher partial pressure of carbon dioxide (pCO₂) compared with arterial blood.[9] During dangling, one would expect that venous stasis within the flap would result in a pO₂ decrease, a pCO₂ increase, and subsequently, a pH decrease. Despite the fact that many studies have tried to objectify the internal gaseous environment within the free flap during the dangling process, none have taken blood samples from the free flap.[10] [11] Capillary blood gas samples taken from the free flap can provide better insights into the internal gaseous changes and the degree of “physiological stress” that the free flap experiences during dangling.

The aim of this blood gas sample study was to measure the internal gaseous changes within the free flap during the dangling protocol, to be able to objectify the changes that are witnessed during the dangling process. Findings can contribute to the objective knowledge on dangling after free flap lower extremity reconstruction and may help to improve postoperative protocols that are eloquent for best postoperative outcomes but also fast recovery.

Methods

Study Design and Participants

In this study, we prospectively collected blood gas samples from patients who underwent lower leg free flap reconstruction at the University Medical Center Utrecht (UMCU) between 2018 and 2023 and were enrolled in the “Dangle Study.”[12] Inclusion and exclusion criteria are detailed in [Table 1]. Patients started a dangling protocol on postoperative day (POD) 7 after an initial period of immobilization. The institution's dangling protocol is presented in [Table 2].

Table 1 Inclusion and exclusion criteria

Inclusion criteria	● Male or female ● Aged between 16 and 99 years ● Free flap reconstruction of the lower leg
Exclusion criteria	● Under the age of 16 years ● Insufficient Dutch proficiency ● Patients unable to give informed consent ● Patients who are mentally incapacitated ● Reconstruction requiring two or more free flaps ● Patients who are undergoing a secondary free flap reconstruction due to partial or total free flap necrosis

Table 2 Dangling protocol at the University Medical Center Utrecht

Day	Dangling protocol
Surgery	Immobilization
Postoperative day 1
Postoperative day 2
Postoperative day 3
Postoperative day 4
Postoperative day 5
Postoperative day 6
Postoperative day 7	3 ×/day 5 minutes each time
Postoperative day 8	3 ×/day 10 minutes each time
Postoperative day 9	3 ×/day 20 minutes each time
Postoperative day 10	3 ×/day 30 minutes each time
Postoperative day 11	3 ×/day 45 minutes each time
Postoperative day 12	3 ×/day 60 minutes each time
Postoperative day 13 and onwards	Unrestricted mobilization

Study Procedures and Measurements

Capillary blood was taken daily by the senior author (D.D.K.) during one of the dangling sessions for a total period of six consecutive days (POD 7–12). A 23-gauge needle and the i-STAT Alinity point-of-care testing (POCT) device (Abbott Laboratories, IL) were used to collect a drop of blood from the site of the free flap, as well as from the contralateral, “healthy” leg, which served as the control group. Blood was taken two times per session from the center of the free flap: before and immediately after dangling of the lower leg. Blood from the contralateral, “healthy” leg was drawn only once before the start of the dangling session. We analyzed these samples for pO₂, pCO₂, and pH levels. Additionally, bicarbonate (HCO₃ ⁻) and hemoglobin levels were measured. The POCT protocol is detailed in [Supplementary Table S1].

Statistical Analysis

Mean capillary blood gas curves for pO₂, pCO₂, pH, HCO₃ ⁻, and hemoglobin were created. Each figure contains three curves: (1) before dangling, (2) after dangling, and (3) for the control leg. The y-axis reflects blood gas values. The x-axis ranges from POD 7, when dangling was introduced, until POD 12, after which unrestricted mobilization was allowed. Furthermore, blood gas curves for individual subjects were constructed and are presented in the [Supplementary Figs. S1] to [S3]. The normality of the data was assessed using the Shapiro–Wilk test. A paired t-test was used to investigate the difference in pre- and post-dangling blood gas values. The same paired t-test was used to assess differences in baseline blood gas values between the free flap (i.e., pre-dangling values) and the contralateral, “healthy” leg. Finally, least squares regression was used to analyze trends in the blood gas values which are presented as slopes with their corresponding p-values.

Results

Patient Population

Ten patients were enrolled in the “Dangle Study” at the UMCU and assigned to a postoperative dangling protocol. Six patients did not give their consent for daily blood gas analysis, resulting in a final inclusion of four patients. One patient had complaints of orthostatic hypotension during the first blood sample collection and therefore withdrew from this study. The baseline patient characteristics are shown in [Table 3].

Table 3 Baseline characteristics

Characteristic	Total (N = 3)
Age, years	24, 52, and 52
Sex (male/female)	2/1
Cardiovascular risk factors
Smoking	1
Diabetes	0
Hypertension	0
Anticoagulants	0
Lower leg defect
Side (right/left)	2/1
Trauma cause	3
Classification
Gustilo classification	IIIC, closed fracture, and unknown
Location
Lower one-third of lower leg	3
Lower leg reconstruction
Type of flap
Gracilis	2
ALT	1
Type of arterial anastomosis
ETS	2
ETE	1
Number of arterial anastomosis
1	3
Type of venous anastomosis
ETE	3
Number of venous anastomosis
1	2
> 1	1

Abbreviations: ALT, anterolateral thigh; ETE, end-to-end; ETS, end-to-side.

Blood Gas Curves

Mean blood gas curves for pO₂, pCO₂, and pH are shown in [Fig. 1].

Fig. 1 Mean blood gas curves (pO₂, pCO₂, and pH). Blood collection occurred daily and started on the seventh day postoperative till twelve days postoperative. pCO₂, partial pressure of carbon dioxide; pH, potential hydrogen; pO₂, partial pressure of oxygen.

Mean pO₂ Curves

Average pO₂ levels appeared to be lower in the free flap compared with the contralateral, “healthy” leg (p = 0.002). When comparing pre- and post-dangling pO₂ levels, no significant changes were observed (p = 0.380). Despite the gradually increasing number of dangling periods, the mean pO₂ curves remained considerably stable (b = −1.01, p = 0.654) and never dropped below a pO₂ of 36.3 mm Hg.

Mean pCO₂ Curves

The average pCO₂ levels were consistently higher in the free flap (p < 0.001) and increased further after dangling (p = 0.028). Except for POD 12, no significant changes in mean pCO₂ levels were observed after dangling for an increasing amount of time (b = 0.48, p = 0.490).

Mean pH Curves

Average pH levels were lower in the free flap compared with the contralateral, “healthy” leg (p = 0.021). The “after dangling” pH curve seems to be an inverted curve of the “after dangling” pCO₂ curve. Only on POD 12 did we notice a drop in the pH to 7.33. During the rest of the dangling protocol, no significant changes in the mean pH levels were observed (b = − 0.01, p = 0.356).

Mean HCO₃ ⁻ and Hemoglobin Curves

The mean blood gas curves for HCO₃ ⁻ and hemoglobin are presented in [Supplementary Fig. S1]. HCO₃ ⁻ values were, on average, lower in the “healthy” contralateral leg than in the free flap (p = 0.049). The average hemoglobin levels were stable in the “healthy” contralateral leg. However, the free flap showed increased fluctuations in hemoglobin levels, particularly after dangling of the lower leg.

Blood Gas Curves Per Subject

The pO₂, pCO₂, and pH curves for each subject are depicted in [Supplementary Fig. S2]. Additionally, the HCO₃ ⁻ and hemoglobin curves are presented in [Supplementary Fig. S3].

Discussion

This study aimed to visualize the blood gas changes that occur postoperatively in a free tissue transplant to the lower extremity during the process of increasing gravitational forces, a so-called dangling protocol. Blood gas values reflect the underlying internal gaseous environment and are, therefore, a valuable indicator of “free flap health.” Our study shows that despite a dangling protocol, and therefore increasing periods of exposure to gravitational forces and subsequent clinical signs of venous stasis, the mean pO₂, pCO₂, and pH within the free flap remained stable between the start and end of the dangling protocol.

Blood Gaseous Changes

Our results show that during the postoperative phase: (1) the mean pO₂ decreased, (2) the mean pCO₂ increased, and (3) the mean pH levels lowered at the site of the free flap, compared with the contralateral, “healthy” leg. Following free flap surgery, the reconstructed site experiences microcirculatory challenges in establishing a new equilibrium between arterial inflow and venous outflow. Free flaps lack several mechanisms that compensate for abrupt changes in blood supply. As a result, perfusion is significantly different from healthy tissues.[11] The observed differences in blood gas levels are likely attributable to venostasis, which leads to increased mixing of deoxygenated with oxygenated blood.

Over weeks, vascular ingrowth leads to a restoration of basic innervation in the free flap.[12] The result is a new hemostatic state, and blood gas values are expected to normalize.

Dangling Procedure

Dangling of the lower leg results in an increase in gravitational forces and thereby an increase of venous pooling, resulting in a desaturation over time.[5] [10] [11] Based on this study, no significant differences between pre- and post-dangling pO₂ levels were observed. However, dangling resulted in a significant increase in the average pCO₂ levels, which can be attributed to the increased venostasis.

Kolbenschlag et al. (2014) described a steady increase in pre-dangling saturation levels throughout the training.[10] Patients in our study did not experience similar changes in pre-dangling pO₂ levels.

Hemoglobin levels are predominantly elevated in the free flap when compared with the contralateral leg. Venous pooling increases the hydrostatic pressure, leading to increased extravasation of fluids, consequently resulting in edema and a rise in hemoglobin levels.[13]

Most importantly, it seems that the mean pO₂, pCO₂, and pH exhibit a considerable degree of stability throughout the training period. This would suggest that an increase in dangling duration does not lead to significant alterations in blood gas values.

Clinical Implications

A dangling protocol is designed to gradually challenge the free tissue transplant to increasing gravitational forces. The goal of dangling is to alter the physiology of the free flap by creating a hypoxic environment that drives angiogenesis and prevents venostasis with possible detrimental effects on the flap.[14] In this study, we found that mean pre- and post-dangling pO₂ levels are not significantly different. Additionally, average pO₂, pCO₂, and pH curves remain considerably stable throughout the training period. This raises questions about whether the current form of the dangling protocol accomplishes its intended goal of gradually challenging the flap. The absence of significant blood gas changes after dangling indicates that a dangling protocol does not significantly alter the physiology of the free flap. Based on our study, the flow within the free flap seems sufficient to prevent the gaseous changes from entering the realms of dangerous levels (during dangling after POD 7). We also acknowledge that a free flap does have early-stage ingrowth into the surrounding tissues between POD 7 and 12, which may help with this process. This study adds to the growing evidence questioning if a (late) dangling protocol is necessary at all.[15] [16]

Second, the blood gas values at the site of the free flap suggest the presence of venostasis in the postoperative period. Therefore, patients may benefit from compression therapy in the days following surgery to enhance venous return. During dangling of the lower extremity, there is an increase in the venous cross-sectional surface area and interstitial pressure.[4] The application of compression wrappings has been demonstrated to improve microcirculation during dangling and reduce edema formation and pain.[13] [17] It can be safely applied after lower extremity reconstruction as it does not affect the hemodynamics of the perforator.[18]

Strengths and Limitations

To our knowledge, this study is the first to describe the capillary blood gas changes that occur in a lower extremity free tissue transplant over the course of a dangling protocol. We collected both pre- and post-dangling blood gas values and simultaneously obtained blood from the contralateral leg, which served as a control.

Our study has several limitations. We used a POCT device to measure blood gas values. Of note is that the hemoglobin measurements after dangling on POD 7, 8, 11, and 12, for subject 2 are missing. For subject 3, data on the hemoglobin levels before dangling is missing on POD 8 to 11 and after dangling the lower leg for the whole postoperative study period. These data are missing due to the fact that the POCT machine did not have a sufficient amount of blood to analyze for these variables. Additionally, we included only three patients in this study, making it difficult to draw any definite conclusions.

Conclusion

Venous pooling commonly occurs during the postoperative phase of lower extremity reconstruction using a free tissue transfer. Dangling protocols are designed to let the free tissue transfer adapt to this venous stasis so that the flap is able to better endure these gravitational forces. Blood samples taken from the free tissue transfer in this study show that mean pO₂ and pH levels are lower, and pCO₂ levels are higher compared with the contralateral, “healthy” leg. Increased duration of dangling does not result in major changes in blood gas values, and the free tissue transfer seems to have the ability to stabilize the gaseous variables during dangling. This study adds to the growing body of evidence in questioning whether a dangling protocol in its present form accomplishes its intended goal of gradually challenging the free flap and holds any added value in clinical practice.

Conflict of Interest

None declared.

Previous Presentation

This research is original. An abstract was presented at the European Plastic Surgery Research Council in August 2024 (Brno, Czech).

Ethical Approval

The study was approved by the local institutional review board (Medisch Ethische Toetsings Commissie [METC], protocol no.: 17/920). All patients provided written informed consent before POD 5. Throughout the study, patients had the opportunity to withdraw at any given time.

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Publication History

Received: 14 January 2025

Accepted: 20 March 2025

Accepted Manuscript online:
07 April 2025

Article published online:
28 April 2025