Assessment of BMI and Venous Thromboembolism Rates in Patients on Standard Chemoprophylaxis Regimens After Undergoing Free Tissue Transfer to the Head and Neck

Key Points

Question

Is body mass index (BMI) associated with venous thromboembolism (VTE) or hematoma rates in patients receiving fixed-dose enoxaparin 30 mg twice daily after free tissue transfer to the head and neck region?

Findings

In this cohort study of 765 patients treated with standard chemoprophylaxis regimens after free tissue transfer to the head and neck, BMI and obesity were both associated with increased rates of VTE.

Meaning

These findings may suggest insufficient VTE prophylaxis in this group and a potential indication for weight-based dosing.

This cohort study assesses whether there is an association between body mass index and postoperative venous thromboembolism and hematoma rates in patients treated with prophylactic enoxaparin 30 mg twice daily.

Abstract

Importance

Venous thromboembolism (VTE) is a severe complication after free tissue transfer to the head and neck (H&N). Enoxaparin 30 mg twice daily (BID) is a common regimen for chemoprophylaxis. However, differences in enoxaparin metabolism based on body weight may influence its efficacy and safety profile.

Objective

To assess the association between BMI and postoperative VTE and hematoma rates in patients treated with prophylactic enoxaparin 30 mg BID.

Design, Setting, and Participants

This was a retrospective review of a prospectively collected cohort from 2012 to 2022. Postoperative VTE, hematoma, and free flap pedicle thrombosis were recorded within 30 days of index surgery. The setting was a tertiary academic referral center. Participants included patients undergoing H&N reconstruction with free flaps that received fixed-dose subcutaneous enoxaparin 30 mg BID postoperatively. Statistical analysis was conducted from April to May 2022.

Main Outcomes and Measures

Outcomes include incidence of VTE, hematoma, and flap pedicle thrombosis events within 30 days of the surgery. Univariate and multivariable regression models were used to evaluate associations between BMI and other patient factors with these outcomes.

Results

Among the 765 patients included, 262 (34.24%) were female; mean (SD) age was 60.85 (12.64) years; and mean (SD) BMI was 26.36 (6.29). The rates of VTE and hematoma in the cohort were 3.92% (30 patients) and 5.09% (39 patients), respectively. After adjusting for patient factors, BMI was the only factor associated with VTE (OR, 1.07; 95% CI, 1.015-1.129). Obesity (BMI >30) was associated with increased odds of VTE (OR, 2.782; 95% CI, 1.197-6.564). Hematoma was not associated with BMI (OR, 0.988; 95% CI, 0.937-1.041). Caprini score of at least 9 was not associated with VTE (OR, 1.259; 95% CI, 0.428-3.701).

Conclusions and Relevance

This cohort study found that obesity was associated with an increased risk of VTE in patients after microvascular H&N reconstruction and while on standard postoperative chemoprophylaxis regimens. This association may suggest insufficient VTE prophylaxis in this group and a potential indication for weight-based dosing.

Introduction

Venous thromboembolism (VTE), composed of deep vein thrombosis (DVT) and pulmonary embolism (PE), is responsible for 5% to 10% of all inpatient deaths in the US.^1,2,3 Patients undergoing microsurgical reconstruction in the head and neck region (H&N) with free tissue transfer (FTT) are at especially increased risk for VTE due to the prolonged surgery time, cancer diagnosis, and immobilization after the reconstruction.^4,5,6,7,8 Previous studies have shown VTE rates in this population ranging from 1.4% to 5.8%.^5,9 However, VTE risk has been reported as high as 13% to 26.3% in the absence of antithrombotic chemoprophylaxis.^2,5,9

Perioperative and postoperative VTE chemoprophylaxis has become standard of care after major H&N surgery. These measures include mechanical prophylaxis (compression stockings and sequential compression devices) as well as chemoprophylaxis (enoxaparin, heparin, or aspirin).^5,10,11 Published guidelines stress the need for chemoprophylaxis in patients undergoing H&N, but no agreement has been reached on the best chemical agent, the dosage, and the length of anticoagulation in patients undergoing microsurgical reconstruction in the H&N area.^{4,5,12,13,14,15,16,17} Low molecular weight heparin (enoxaparin) is widely used for VTE chemoprophylaxis in surgical patients because of its predictable pharmacodynamics and the low risk for heparin-induced thrombocytopenia.^{5,6,15,17,18,19}

Even though VTE is considered a preventable postoperative complication, breakthrough VTE happens in high-risk groups even in the presence of chemoprophylaxis^3,5,10,16 with an incidence of between 2% and 10% in this high-risk patient group.^{4,11,20,21,22} Variances in body weight between patients can affect the efficacy of chemoprophylaxis, impacting the risk of VTE and hematoma formation.²³ Similarly, patients who are underweight may have a higher risk for bleeding events due to supratherapeutic anti-Xa activity when taking enoxaparin.^18,21,24,25 Appropriate chemoprophylactic dosing for patients undergoing H&N reconstruction is paramount, as both hematoma and thrombosis pose risks to patient morbidity, mortality, and flap viability.

In this study, we assess the association between BMI and the incidence of VTE, hematoma, and pedicle thrombosis in patients undergoing major H&N surgery with FTT reconstruction. We hypothesized that there is no association between BMI and outcomes of the study.

Methods

We conducted a cohort study as a single-center, retrospective review of a prospectively collected cohort at the University of Pittsburgh Medical Center in Pittsburgh, Pennsylvania. The study was approved by the University of Pittsburgh institutional review board. The approval granted an exception from informed consent of the participants because the research involved no more than minimal risk to the participants, the waiver will not adversely affect the rights and welfare of the patients, and the research could not practicably be carried out without waiver of alteration. The study complies with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.

Data Collection

Medical records of all patients who underwent microsurgical reconstruction with free tissue transfer to the H&N area from January 2012 through March 2022 were reviewed. Demographical data (age, sex, etiology), smoking history, history of VTE, BMI, Caprini score, antithrombotic chemoprophylaxis regimen, VTE events, hematoma events, and flap pedicle thrombosis events were collected. Free flaps were categorized by flap type and presence or absence of osseous component. Moreover, a medical record–based Charlson Comorbidity Index (CCI) score was calculated for each participant. Based on this score, the overall comorbid health status of each patient was classified into 4 categories: none with a CCI score of 0, mild with CCI scores of 1 to 2, moderate with CCI scores of 3 to 4, and severe with CCI scores at least 5.

All patients received mechanical thromboembolism prophylaxis with sequential compression devices initiated in the operating room, 120 mg rectal acetylsalicylic acid immediately postoperatively, and 81 mg for the first 5 postoperative days per institutional protocol. As per our head-and-neck postoperative protocol, all patients were moved out of bed to at minimum a chair on postoperative day 1, unless the patient was medically unstable for mobilization. Patients were helped out of bed with assistance on postoperative day 2 and beyond and encouraged to ambulate at least once a day.

Inclusion Criteria and Exclusion Criteria

We included all patients who obtained a fixed dose of enoxaparin 30 mg twice daily (BID) starting from postoperative day 1 and continued throughout the hospitalization. We excluded all patients who obtained the enoxaparin later than postoperative day 1 as well as those who were treated with heparin drip intraoperatively for any reason (eg, arterial or venous thrombosis of the free flap pedicle). Moreover, patients who received a chemoprophylaxis regimen other than enoxaparin 30 mg BID were excluded. We did not exclude patients whose chemoprophylaxis was changed during the hospitalization due to a VTE event.

The VTE rate was defined by summing rates of pulmonary embolism (PE) and deep-vein thrombosis (DVT). However, several PE or DVT events for 1 patient only counted once in our model. Hematoma rate was restricted to bleeding events requiring operating room intervention. Free flap pedicle thrombosis was defined as arterial or venous thrombosis requiring surgical exploration of the anastomosis. All outcomes were noted within 30 days of the index surgery.

According to the Centers for Disease Control and Prevention (CDC), patients can be classified into 4 categories based on their BMI: underweight (<18.5), healthy (18.5 to <25), overweight (25 to <30), and obese (≥30).

Patients who met the inclusion criteria of our study were sorted based on a meaningful BMI cutoff of 30 into 2 groups (BMI ≥30 [obese] and BMI <30 [nonobese]), according to the CDC definition of obesity. This cutoff point was previously used in several surgical populations including H&N microsurgical population (Figure).^26,27,28,29

Figure. Flowchart of the Patients Included in the Study and the Analyses.

BMI indicates body mass index; H&N, head and neck; FTT, free tissue transfer.

The Caprini score is a widely used risk assessment model for estimating VTE risk after surgery and can be calculated by assigning each risk factor for VTE with point(s) proportional to the relative risk of VTE.^16,30,31,32 Each patient is classified into 1 of 6 groups based on the cumulative score, and each group corresponds to a specific VTE risk and recommended chemoprophylaxis measures. Individuals with a Caprini score of at least 9 are considered the highest risk (10.7%) for developing VTE postoperatively.^31,33,34 This high-risk group has been studied in several previous studies in the general otolaryngology³⁵ and surgical population.³³ Consequently, we performed a post hoc analysis for patients with a Caprini score of at least 9 as this subgroup is believed to have a 20-fold increase in risk for VTE, when compared to all patients with a Caprini score of less than 9.^16,35,36 The BMI was defined as the closest BMI value to the date of surgery noted in the preoperative medical record, and Caprini score was calculated using the 2005 Caprini risk assessment model (CRAM).³⁷

Statistical Analysis

Patient baseline demographics and clinical characteristics were summarized overall and by outcome status using means and SDs for continuous variables and frequencies and percentages for categorical variables. Variables considered in this study were chosen a priori as candidate risk factors for VTE, hematoma, and pedicle thrombosis based on clinical relevance. First, univariate logistic regression models were fitted to assess the unadjusted association of BMI and other factors with VTE, hematoma, and pedicle thrombosis. Odds ratios (ORs), corresponding 95% confidence intervals (CIs), and appropriate effect size measures were used to quantify the magnitude, precision, and strength of estimated associations. Next, multivariable logistic regression was performed to evaluate associations between BMI and outcomes while controlling for other potentially confounding patient factors. All variables in (Table 1) were considered for inclusion in the multivariate regression model. The model was constructed using automated forward variable selection with a threshold of P < .20 for inclusion in the model. As the primary risk factor of interest, BMI was retained in every step of model selection regardless of significance. All analyses assumed a type 1 error rate of .05 with no adjustments made for multiplicity (the threshold for statistical significance was P < .05). All analyses were conducted using the SPSS statistical software version 19 (IBM Corp) from April to May 2022.

Table 1. The Rate of Development of VTE, Hematoma, or Pedicle Thrombosis as a Function of Baseline Demographic and Clinical Features.

Variable	Total (n = 765)	VTE postoperative (n = 30)		Hematoma (n = 39)		Free flap pedicle thrombosis (n = 50)
		No. (%)	OR (95% CI)	No. (%)	OR (95% CI)	No. (%)	OR (95% CI)
Sex
Female	262	10 (3.8)	1.043 (0.481-2.263)	10 (3.8)	1.541 (0.739-3.214)	19 (7.3)	0.839 (0.464-1.517)
Male	503	20 (4.0)		29 (5.8)		31 (6.2)
History of tobacco use
No	208	10 (4.8)	1.043 (0.481-2.263)	10 (4.8)	1.087 (0.520-2.272)	12 (5.8)	1.195 (0.612-2.335)
Yes	557	20 (3.6)		29 (5.2)		38 (6.8)
Overall, burden of comorbid ailments
None	25	1 (<0.1)	[Reference]	0	NA	1 (4.0)	[Reference]
Mild	105	2 (2.0)	0.466 (0.042-10-260)	5 (4.8)	[Reference]	6 (5.7)	1.454 (0.167-12.656)
Moderate	320	16 (5.0)	1.263 (0.241-23.267)	20 (6.3)	1.333 (0.487-3.645)	25 (7.8)	2.033 (0.264-15.667)
Severe	315	11 (3.5)	0.868 (0.158-16.201)	14 (4.4)	0.930 (0.326-2.647)	18 (5.7)	1.454 (0.186-11.368)
FTT because of cancer
No	82	3 (3.7)	1.083 (0.321-3.654)	2 (2.4)	2.291 (0.541-9.686)	5 (6.1)	1.086 (0.418-2.819)
Yes	683	27 (4)		37 (5.4)		45 (6.6)
FTT contains osseous component
No	530	17 (3.2)	1.767 (0.843-3.7)	25 (4.7)	1.279 (0.652-2.508)	22 (4.2)	3.123 (1.746-5.585)
Yes	235	13 (5.5)		14 (6.0)		28 (11.9)
Personal history of VTE
No	727	29 (4.0)	0.650 (0.086-4.907)	36 (5.0)	1.645 (0.482-5.604)	50 (6.9)	NA
Yes	38	1 (3.3)		3 (7.9)		0
Family history of VTE
No	726	28 (3.9)	1.347 (0.309-5.873)	38 (5.2)	0.476 (0.063-3.564)	47 (6.5)	1.203 (0.357-4.054)
Yes	39	2 (5.1)		1 (2.6)		3 (7.7)
Obesity
BMI <30	575	17 (3.0)	2.782 (1.197-6.564)	32 (5.6)	0.649 (0.281-1.495)	40 (7.0)	0.743 (0.364-1.516)
BMI ≥30	190	13 (6.8)		7 (3.7)		10 (5.3)
Caprini score
Caprini score <9	681	26 (3.8)	1.259 (0.428-3.701)	35 (5.1)	0.922 (0.319-2.664)	47 (6.9)	0.499 (0.152-1.641)
Caprini score ≥9	84	4 (4.8)		4 (4.8)		3 (3.6)

Abbreviations: BMI, body mass index; FTT, free tissue transfer; OR, odds ratio; VTE, venous thromboembolism.

Values are rounded to the nearest 0.1 or the tenths place.

Results

Description of the Study Population

Among the 976 patients reviewed for this study, 765 patients met the inclusion criteria. Of those included, 262 (34.24%) were female; mean (SD) age was 60.85 (12.64) years; and mean (SD) BMI was 26.36 (6.29]). The incidence rates were 3.92% (30 of 765) for VTE, 5.09% (39 of 765) for hematoma, and 6.53% (50 of 765) for flap pedicle thrombosis. Out of the whole cohort, 190 patients (24.84%) were obese, 84 patients (10.98%) were deemed to be at the highest risk for VTE (Caprini score ≥9) and 235 (30.72%) obtained a FTT containing an osseous component.

Venous Thromboembolism

In univariate analyses, Caprini score (OR, 1.095; 95% CI, 0.890-1.311) and age (OR, 0.891; 95% CI, 0.97-1.027) as continuous variables were not associated with VTE. Unadjusted data for categorical clinical factors and the development of VTE are shown in (Table 1). BMI as a continuous variable was associated with VTE (OR, 1.060; 95% CI, 1.004-1.115). When comparing the mean BMI for those who developed VTE with those who did not, the effect size of the difference (d = 0.44) was found to exceed Cohen convention for a small effect (d = 0.2).

To provide a clinically meaningful context for obese patients and facilitate the interpretation of BMI as a risk factor, we divided the cohort based on a BMI cutoff of 30 into 2 groups: patients considered obese with BMI of at least 30 (190 of 765 [24.84%]) and patients considered nonobese with BMI less than 30 (575 of 765 [75.16%]). There was an unadjusted association between the development of VTE and being obese (OR, 2.782; 95% CI, 1.197-6.564).

The rate of VTE among patients with a Caprini score of at least 9, the highest-risk VTE group, was 4.8% (4 of 84), and the rate in the other patients was 3.8% (26 of 681) (difference, 0.9%; 95% CI, −2.4% to 7.9%; OR, 1.259; 95% CI, 0.428-3.701]. Although the rate of VTE was higher in the highest Caprini risk group, no association between Caprini score and VTE can be made due to the width of the confidence interval. No other factors were univariately associated with VTE in our cohort (Table 1). In the final multivariable model for VTE, obesity was retained in the model regardless of significance, as it is the risk factor of primary interest. The only other variable that passed the threshold of P < .20 to enter the multivariable regression model among all variables in (Table 1) was presence of osseous component within the free flap. The association between BMI of at least 30 and VTE persisted even after controlling for the presence of osseous component within the free flap (adjusted OR, 2.481; 95% CI, 1.178-5.223) (Table 2).

Table 2. Multivariable Logistic Regression Analysis for Risks of VTE.

Variable	Adjusted odds ratio (95% CI)
FTT contains no osseous component	[Reference]
FTT contains osseous component	1.764 (0.836-3.718)
BMI <30	[Reference]
BMI ≥30	2.481 (1.178-5.223)

Abbreviations: BMI, body mass index; FTT, free tissue transfer; VTE, venous thromboembolism.

Hematoma

In univariate analyses, hematoma was not associated with BMI as a continuous variable (OR, 0.988; 95% CI, 0.937-1.041). The incidence of hematoma was 5.8% (29 patients) among 503 male patients and 3.8% (10 patients) among 262 female patients (difference, 2.0%; 95% CI, 1.6% to 4.9%; OR, 1.5; 95% CI, 0.74 to 3.20); 5.4% (37 patients) for patients undergoing FTT because of cancer and 2.4% (2 patients) undergoing FTT not because of cancer (difference, 3.0%; 95% CI, −3.2% to 5.6%; OR, 2.30; 95% CI, 0.54 to 9.70); 7.9% (3 patients) with personal history of VTE and 5.0% (36 patients) without personal history of VTE (difference, 2.9; 95% CI, −2.5% to 16.0%; OR, 1.6, 95% CI, 0.48 to 5.6). The hematoma incidence rate was 2.6% (1 patient) among patients with a family history of VTE vs 5.2% (38 patients) among patients without (difference 2.6; 95% CI, −8.0 to 5.5; OR, 0.48; 95% CI, 0.063 to 3.56); 3.7% (7 patients) among patients with obesity (BMI ≥30) vs 5.6% (32 patients) among patients without obesity (difference, 1.9%; 95% CI, −2.2% to 4.8%; OR, 0.65; 95% CI, 0.28 to 1.50) (Table 1).

Free Flap Pedicle Thrombosis

There was no association between BMI as a continuous variable and the pedicle thrombosis in univariate regression (OR, 0.995; 95% CI, 0.950-1.042). Pedicle thrombosis was more likely in patients who received FTT with osseous component (11.9% [28 patients]) than not (4.2% [22 patients]) (difference, 7.7%; 95% CI, 3.7% to 13.0%; OR, 3.1; 95% CI, 1.7 to 5.6).

The rate of thrombosis was 0% (0 patients) among patients with a personal history of VTE vs 6.9% (50 patients) among patients without a personal history of VTE (difference, 6.9%; 95% CI, −2.4% to 9.0%; OR, 0.17; 95% CI, 0.01 to 2.80); and 3.6% (3 patients) among patients with Caprini score of at least 99 vs 6.9% (47 patients) among patients with Caprini score less than 9 (difference, 3.3%; 95% CI, −3.3% to 6.5%; OR, 0.5; 95% CI, 0.15 to 1.6). (Table 1).

Discussion

In this cohort study, we found that preoperative patient BMI was associated with VTE events after microsurgical H&N reconstructive surgery. This elevated risk for VTE in patients with obesity persists even after controlling for other factors associated with VTE. When compared with all other patients, patients with obesity were more than twice as likely to develop VTE. BMI was not associated with hematoma or free flap pedicle thrombosis, and a high Caprini score (≥9) was not associated with postoperative VTE. To our knowledge, this study is unique in that it investigates the association between BMI and VTE events after head and neck microvascular reconstructive surgery while controlling for fixed-dose enoxaparin in the postoperative setting. Although we reported higher incidence rates of hematoma in men, FTT because of cancer, and personal history of VTE, and lower reported rates in patients with a family history of VTE and obesity, these results were not statistically significant due to the small number of events and wide confidence intervals around the estimates. Our reported rates of pedicle thrombosis were higher in patients who received FTT with osseous component and lower in patients with a personal history of VTE and Caprini score of at least 9, but again, these results were not statistically significant because of the low number of events and wide confidence intervals.

Obesity (BMI ≥30) has been associated with increased operative time, increased mortality, poorer wound healing, and increased surgical site infection in various surgical and FTT populations, including the H&N population.^38,39,40,41 However, previous publications about patients with H&N cancer found elevated BMI to have better survival, less recurrence, and lower mortality.^41,42,43 The VTE rate in our population (3.92%) was similar to previously published studies.^{4,16,44,45,46} Though it is known that patients requiring microsurgical H&N surgery are at high risk of VTE, authors have not reached a consensus on the importance of elevated BMI in this risk spectrum. In a review of 134 patients undergoing H&N microvascular reconstruction, Thai et al⁹ reported a 29% increase in the risk of VTE associated with elevated BMI in patients with H&N cancer after controlling for several factors. They also noted that patients with cancer and previous history of VTE, blood transfusion, and older age had higher odds of acquiring VTE in the postoperative setting. In contrast, de la Garza et al²⁶ found that there was no increased risk of perioperative medical complications including VTE in patients with elevated BMI in a retrospective study on 582 H&N cases. In this study, the authors did not control for standardized postoperative chemoprophylaxis regimens, nor did they account for VTE as an independent outcome. Similar to our study, Crippen and colleagues⁴⁷ divided a cohort of over 2000 patients into 4 BMI categories and reviewed postoperative medical and surgical outcomes in patients undergoing head and neck reconstruction. Although DVT was included as part of a composite outcome measure, there was no difference in rates of medical complications when patients were stratified by BMI groups. In our cohort, VTE rates increased with rising BMI and demonstrated a clear elevated risk in obese patients when compared with all other patients.

Numerous other studies have identified other high-risk clinical factors associated with VTE in surgical populations including age, personal history of VTE, prolonged surgery, and malignancy.⁹ VTE risk stratification using Caprini index has been well studied in surgical populations including in otolaryngology surgical patients.⁴⁸ Individuals with a Caprini score of at least 9 are considered the highest risk (10.7%) for developing VTE postoperatively.^31,33,34 A similar Caprini score cutoff has been suggested in a retrospective review of inpatient otolaryngology surgery patients.^4,48 In our study, VTE incidence in patients with Caprini score of at least 9 did not significantly differ from that of all other patients. Similarly, Thai et al⁹ reported the inadequacy of Caprini score in stratifying the risk for VTE in their retrospective review of 134 microsurgical H&N reconstructions. Based on these findings, Caprini score risk assessment may need further validation in the H&N microsurgical population.

Postoperative hematoma is of substantial clinical importance in patients undergoing microsurgical H&N reconstruction as it may result in a return to the OR, need for transfusions, free flap pedicle compression, and, potentially, death. Previous studies reported a range for hematoma incidence between 4.7% and 17.5% in patients undergoing H&N reconstruction.^45,49,50

The protective effect of elevated BMI against bleeding has been previously described.⁴⁷ In our cohort, BMI was not associated with bleeding events. However, the predisposition to bleeding in underweight patients was previously described.⁴⁷ This susceptibility could be attributed to the malnourished status preoperatively from poor oral intake and cancer-related cachexia. Moreover, low caloric intake and weight loss have been shown to decrease platelet activity and to promote bleeding events.^47,51 Another possible explanation for the increased hematoma rate in underweight patients could be overanticoagulation.

Currently, there are no consensus guidelines for chemoprophylaxis in the H&N population. A recent survey of 74 high-volume head and neck reconstructive surgeons demonstrated that nearly all surgeons utilized prophylactic anticoagulation in the form of either subcutaneous heparin or enoxaparin. Microvascular surgeons noted their concern for both bleeding and thrombotic complications despite regular use of chemoprophylaxis.⁵ Our standard postoperative anticoagulation regimen is 30 mg enoxaparin BID, but other surgeons may utilize different dosages, different medications, and, sometimes, no anticoagulation at all. Although standardized dosing of enoxaparin is recommended for chemoprophylaxis, patients at the extremes of weight may have the potential for either underdosing or overdosing. Published CHEST guidelines state that “for thromboprophylaxis on fixed-dose enoxaparin, there is a strong negative correlation between total body weight and anti-factor Xa levels in obese patients.”^{8,21,24,52,53,54}

Patients with H&N cancer may be at especially high risk for diminished efficacy of enoxaparin. Our institutional data comparing VTE rates in patients who received breast-free tissue transfer and H&N free tissue transfer noted significantly lower anti-factor Xa levels in the head and neck population even though all patient received prophylactic enoxaparin.⁵⁵

Weight-based enoxaparin dosing is another approach that showed good efficiency in patients with elevated BMI in trauma, surgical colorectal, hospitalized, and critically ill populations.^{18,23,24,56,57} However, this dosing method may increase hematoma rates, making the selection of the optimal chemoprophylaxis difficult.²⁵

Consequently, prospective studies regarding optimal prevention of venous thromboembolism in the H&N reconstructive microsurgical population are needed before consensus can be reached on appropriate chemoprophylaxis. Future investigations may focus on enoxaparin dosage adjustment for patients with obesity and consideration of monitoring of antifactor Xa levels in the postoperative period.

Limitations

This study had some limitations. This is a single-center experience with institutional paradigms for postoperative thromboprophylaxis which may not represent the practices of other reconstructive surgeons.

Multi-institutional experience with more patients and more outcome events will clarify the association of BMI and other factors with VTE, hematoma, and pedicle thrombosis rates. Additionally, the true VTE, hematoma, and pedicle thrombosis rates may be underestimated, as we only included events in the 30-day postoperative period in our analysis. Lastly, while patient demographics were collected prospectively, some patient outcomes were collected retrospectively, which may introduce bias.

Conclusions

In patients undergoing head and neck microvascular reconstruction on fixed-dose chemoprophylaxis, elevated BMI was associated with an increased risk of VTE after head and neck reconstruction with free tissue transfer. This association may suggest insufficient VTE prophylaxis in high BMI groups and a potential indication for weight-based dosing. A higher Caprini score was not associated with VTE events, which may raise concerns about its utility in this specific population. Further studies are needed to identify the appropriate VTE chemoprophylaxis in this population.