Incidence of deep venous thrombosis and stratification of risk groups in a university hospital vascular surgery unit

Background: There is a knowledge gap with relation to the true incidence of deep vein thrombosis among patients undergoing vascular surgery procedures in Brazil. This study is designed to support the implementation of a surveillance system to control the quality of venous thromboembolism prophylaxis in our country. Investigations in specific institutions have determined the true incidence of deep vein thrombosis and identified risk groups, to enable measures to be taken to ensure adequate prophylaxis and treatment to prevent the condition. Objective: To study the incidence of deep venous thrombosis in patients admitted to hospital for non-venous vascular surgery procedures and stratify them into risk groups. Method: This was a cross-sectional observational study that evaluated 202 patients from a university hospital vascular surgery clinic between March 2011 and July 2012. The incidence of deep venous thrombosis was determined using vascular ultrasound examinations and the Caprini scale. Results: The mean incidence of deep venous thrombosis in vascular surgery patients was 8.5%. The frequency distribution of patients by venous thromboembolism risk groups was as follows: 8.4% were considered low risk, 17.3% moderate risk, 29.7% high risk and 44.6% were classified as very high risk. Conclusion: The incidence of deep venous thrombosis in vascular surgery patients was 8.5%, which is similar to figures reported in the international literature. Most vascular surgery patients were stratified into the high and very high risk for deep venous thrombosis groups.


INTRODUCTION
Deep venous thrombosis is the greatest cause of in-hospital deaths worldwide and, paradoxically, the most avoidable. 1Among elderly patients, annual rates of mortality due to deep venous thrombosis and pulmonary thromboembolism are 21% and 39% respectively. 2In 2010, it was estimated that there are 900,000 cases of thromboembolism per year in the United States and that one third of these lead to death.Additionally, around 4% of the survivors exhibit pulmonary hypertension with significant restrictions to daily activities and quality of life. 3t has been estimated that 25% to 50% of patients with deep venous thrombosis will develop postthrombotic syndrome, which can cause considerable reductions in quality of life. 4n view of the above, measures should be taken to reduce the numbers of deaths and complications caused by venous thromboembolism, through identification of risk factors and prescription of appropriate prophylaxis.
Caprini has proposed a more individualized assessment of these risk factors.The model is widely used in research and serves as a reference for appropriate prophylaxis, particularly for high-risk patients. 5he University of Michigan Health System adopted this model and used it to assess 8,216 patients to establish the best forms of prophylaxis, and the results of that study are considered the best evidence in support of the validity of the Caprini scale. 6n Brazil, Deheinzelin et al. 7 used both the Caprini and the American College of Chest Physicians models for identification of risk factors to assess the quality of prophylaxis against deep venous thrombosis given to patients in four hospitals in São Paulo.
In each institution, these investigations revealed the true incidence of deep venous thrombosis and enabled identification of risk groups, which in turn made it possible to adopt measures based on the best scientific evidence in order to prevent this severe condition.
In vascular surgery in particular, the incidence of deep venous thrombosis is unclear and figures reported in the literature vary.Those studies that do exist are not recent and were based on small samples.

OBJECTIVE
To determine the incidence of deep venous thrombosis and stratify its risk factors in patients admitted to a University Hospital for treatment by the vascular surgery service.
Additionally, the hospital gave permission for the study to be conducted on its premises.

Patients
A total of 202 patients admitted for treatment by the vascular surgery service between March 2011 and July 2012 were assessed.

Risk factors for deep venous thrombosis
Patients' risk factors were assessed using the model developed by Caprini et al. 8 (Tables 1 and 2).

Prophylaxis for venous thromboembolism
The seventh and eighth editions of the American College of Chest Physicians' guidelines were used to define the appropriateness of prophylaxis protocols in this study. 1,9

Lower limb venous ultrasonography
All patients were examined using lower limb vascular ultrasonography.Patients who aroused clinical suspicion of venous thrombosis while in hospital were sent for vascular ultrasonography on the day the condition was suspected and asymptomatic patients were examined on the day they were discharged from hospital.Both lower limbs were examined, as recommended by the Intersocietal Commission for the Accreditation of Vascular Laboratories (2008).

Statistical analysis
The results of this research were analyzed using SPSS (the Statistical Package for the Social Sciences) 20.0, IBM, United States.

Logistic regression was used to analyze the incidence of venous thrombosis in terms of diagnosis at admission and to determine associations between incidence of deep venous thrombosis and risk factor groups.
The chi-square test was used to identify associations between incidence of venous thrombosis and demographic data.

Demographic data
The sex distribution of the sample was 75 (37.1%)females and 127 (62.9%) males.

Risk factors
Table 5 lists the frequency distribution for risk groups.It was observed that 74.3% of the patients were classified as high or very high risk.
Deep venous thrombosis was not detected in any of the low risk vascular surgery patients.The incidence was 17.6% (three patients) in the moderate risk group.Deep venous thrombosis incidence rates were 23.5% (four patients) and 58.8% (10 patients) in the high and very high risk groups respectively.

Overall incidence of deep venous thrombosis
The total incidence of deep venous thrombosis in vascular surgery patients was 8.5% (17 patients).

DISCUSSION
In order to be in a position to reduce the incidence of thromboembolic events, it is necessary to determine the true incidence of venous thrombosis and stratify its risk factors, classifying them by risk.
In the present study, venous ultrasonography was employed as a trustworthy method for diagnosis of deep venous thrombosis in view of the fact that it has practically replaced venography because of its high sensitivity and specificity, accessibility and good cost-benefit profile. 10,11odacre et al. 12 conducted a meta-analysis in which they concluded that ultrasonography offers sensitivity of 96.4% for diagnosis of proximal deep venous thrombosis and 75.2% for distal venous thrombosis, with specificity of 94.3%.
Venous ultrasonography also enables investigation of deep venous thrombosis differential diagnoses.

Incidence of deep venous thrombosis
The objective of the present study was to determine the true incidence of deep venous thrombosis among patients admitted for non-venous vascular surgery, since the incidence rate of deep venous thrombosis is unclear in this specialty and reports in the literature vary.Those studies that do exist are not recent and were based on small samples, as described below.This is probably the largest sample of patients studied in Brazil to date.
Prospective studies of vascular surgery patients who had not been given any type of prophylaxis found deep venous thrombosis incidence rates of 21% (18 out  of 86 patients) using venography and 15% (15 out of 98 patients) using vascular ultrasonography. 13,14n assessment of 142 patients treated with a variety of vascular surgical procedures and given pharmaceutical and mechanical prophylaxis found a deep venous thrombosis incidence of 10%. 15owever, the various different types of vascular procedures involved should also be taken into consideration because surgical procedures to the aorta tend to be associated with high venous thromboembolism rates.Hollyoak et al. 16 observed a 12% incidence of thromboembolism (six out of 52 patients) among patients treated using surgical procedures to the aorta and a 9% incidence (five out of 54 patients) for infrainguinal revascularization procedures.
This difference in incidence of venous thrombosis when procedures involve the aorta was also detected by Farkas et al. 17 Their randomized study assessed patients who had been given pharmaceutical prophylaxis and treated with lower limb revascularization and surgical procedures involving the aorta.Deep venous thrombosis incidence rates were 3.4% in the infrainguinal revascularization group, 7% for aortofemoral revascularization procedures and 8% for aortic aneurysm repair.
Other studies have also shown that incidence rates of deep venous thrombosis tend to be higher among patients treated with surgical procedures involving the aorta, varying from 18% to 30%. 18,19he incidence of deep venous thrombosis among the vascular surgery patients studied here (8.5%) was similar to the studies cited above.This analysis showed that the type of vascular surgery procedure is of relevance and can possibly influence the deep venous thrombosis incidence rate.Just 3% were admitted for aortic conditions.In contrast, in 39% the reason for admission was peripheral arterial disease (critical ischemia).
Another interesting feature of this study is that it is not limited to analyzing the variations related to diagnosis at admission.The study also illustrates the paradigm shift that has taken place in treatment of aortic conditions and peripheral arterial diseases in these patients.The majority of revascularization treatments were conducted using endovascular techniques, whether to treat atherosclerotic disease in aortic and iliac segments, or for infrainguinal segments.Conventional surgical treatment of abdominal aortic aneurysms has been practically replaced by endovascular repair.This feature means that the results of this study provide pioneer data on the incidence of deep venous thrombosis with relation to use of endoluminal treatment.
The results of this study show that there is an association between deep venous thrombosis and acute arterial ischemia (p=0.01).This correlation is not reported in the extant literature.However, it is known that acute arterial ischemia of an extremity is a clinical emergency, with mortality rates of 15% to 20%, amputation rates greater than 25%, fasciotomy rates of around 25% and a greater than 20% rate of acute renal failure. 20This serious clinical picture associated with comorbidities means that such patients are considered at high or very high risk for venous thrombosis.As is stated in the literature, patients with acute arterial ischemia should be investigated for deep venous thrombosis. 21This association, detected here, merits further research.

CONCLUSIONS
The incidence of deep venous thrombosis was 8.5% between March 2011 and July 2012.None of the low risk patients had deep venous thrombosis.The incidence rates of deep venous thrombosis in the moderate, high and very high risk groups were 17.6% (three patients), 23.5% (four patients) and 58.8% (10 patients) respectively.

Table 2 .
Classification of patients into groups by risk factors.

Table 3 .
Diagnoses on admission for vascular surgery.

Table 4 .
Associations between deep venous thrombosis and admission diagnoses.

Table 5 .
Frequencies of deep venous thrombosis risk groups in vascular surgery patients.