Our results showed that fatal PE was relatively uncommon after major trauma (1.6%), but it accounted for about 11.9% of all deaths. We estimated that about 50% of the deaths from fatal PE were preventable. Severity of injuries, co-morbidities, and BMI were important risk factors for fatal PE.
The incidence of fatal PE in our cohort was comparable with other reports.3,12,13
Although fatal PE was relatively uncommon, it accounted for a significant proportion of deaths (11.9%). Our results showed that many patients who died from fatal PE did not have very severe injuries; the predicted risks of mortality were much lower than those who died from other causes (Table ). The estimated attributable mortality of PE suggested that about half of the deaths from fatal PE were potentially preventable. UFH prophylaxis, although only used in less than half of the patients with fatal PE, was not very effective in preventing fatal PE in our cohort, a finding similar to some other reports.3
The American College of Chest Physicians has recently strongly recommended LMWH prophylaxis (Grade 1A) for venous thromboembolism prophylaxis after major trauma. Our data support this recommendation because UFH was not very effective in preventing fatal PE and the medium time to fatal PE after the injury (18 days) was long after the injury when LMWH may be safely commenced.
Because the absolute incidence of fatal PE was relatively low among all trauma patients (1.6%) and the attributable mortality of PE was about 50%, the crude mortality of the whole cohort would only reduce slightly from 13.8% to 13.0% even if all PE were prevented by IVC filters. These results suggested that the estimated number of IVC filters needed to prevent one fatal PE was relatively large (mean 125, 95% CI 100–167). Even if IVC filters were used only for patients who survived the first 3 days after the injury (n
=900), the number needed to prevent one fatal PE was still large (mean 113, 95% CI 90–154). The non-linear relationship between the severity of injuries and fatal PE suggested that a very selective approach to use IVC filters to prevent fatal PE after major trauma was needed. When the severity of injuries was either very mild or extreme, patients were more likely to survive without fatal PE or die directly from the injuries, respectively (Fig. b
). Therefore, an expensive and invasive preventive therapy such as IVC filter is likely to be most cost-effective if it is used for those with severe injuries that are compatible with survival (e.g. Injury Severity Score between 20 and 45) or practically those who survive the first few days after their severe injuries but have ongoing contraindications to initiation of LMWH. In patients with severe head injury who have contraindications to LMWH but are likely to survive with a favourable neurological outcome (e.g. predicted risk <70%),21
an early placement of an IVC filter may be the most effective measure to prevent fatal PE.
Our results showed that BMI and Charlson's co-morbidity index were important risk factors for fatal PE after major trauma. These findings were consistent with the literature on PE in patients without trauma. Various abnormalities of haemostasis have been reported in obesity, including increased platelet aggregability, circulating mircoparticles, and reduced fibrinolysis, resulting in increased risk of venous thromboembolism.22
Co-morbidity may increase the risk of fatal PE by either an increased risk of venous thromboembolism (e.g. chronic obstructive airway disease)23–25
or a reduced haemodynamic tolerance to a submassive PE (e.g. heart failure).25
Trauma patients with co-morbidities and an increased BMI may benefit most from early initiation of LMWH or placement of an IVC filter if they have contraindications to LMWH.
This study has some limitations. Firstly, fatal PE was relatively uncommon in our cohort of patients and hence the results were imprecise. Secondly, this was a single-centre study and our results may not be applicable to other centres. Because fatal PE is relatively rare after major trauma, a multicentre study will significantly increase the power of the study and external validity. A prospective multicentre observational study will, however, need a significant amount of funding to allow collection of comprehensive data from a large number of trauma patients, and mandatory post-mortem examinations for all deaths after major trauma may not be feasible at all study centres. Thirdly, because of the retrospective nature of the study, we could not ascertain the use of pneumatic compression devices in our patients and this may have confounded our results.26
Finally, our study period was relatively long; medical practice such as the early use of CT pulmonary angiography or early mobilization after damage control surgery might have changed the pattern of fatal PE during the study period.
In spite of these limitations, this study has some strengths when compared with previous studies on PE after major trauma. Firstly, we used autopsy-proven PE as a study endpoint instead of clinical diagnosis which tends to underestimate the true incidence of fatal PE.9,13
Even in a recent report on PE after major trauma, an autopsy was used to confirm fatal PE in only 23% of the deaths.16
Secondly, we have included a comprehensive assessment of risk factors for fatal PE. Some of these risk factors for fatal PE, such as BMI and co-morbidity, have not been thoroughly assessed in patients after major trauma.9
Finally, we have modelled the non-linear relationships between these risk factors and fatal PE; these results may be useful for clinicians to choose appropriate patients for an IVC filter or design a randomized controlled study to assess the cost-effectiveness of IVC filters. In the most recent audit of our trauma patients in 2007 and 2008, LMWH prophylaxis was still not used, but 40 IVC filters (7.4%), 18 CT pulmonary angiography (3.4%), and 11 duplex-Doppler ultrasonography (2%) were used for a total of 538 trauma patients. Of these 538 trauma patients, a total of eight patients had CT pulmonary angiography-confirmed PE (1.5%), another seven patients had duplex-Doppler-confirmed deep vein thrombosis (1.3%), and one patient, with a BMI of 40, died from fatal PE (0.2%). These recent data together with the results of the present study suggest that a combination of early diagnosis and the selective use of IVC filters may be useful in reducing the incidence of fatal PE.
In summary, fatal PE appeared to be a potential preventable cause of late mortality after major trauma. Severity of injuries, co-morbidity, and BMI were important risk factors for fatal PE. More research is needed to assess whether the selective use of IVC filter is efficacious and cost-effective for patients with these risk factors and contraindications to LMWH prophylaxis.