In a large cohort of patients in Stockholm County of inhabitant 2.2 million we found that a final diagnosis of PE was associated with an almost 3-fold increased risk of all-cause mortality, and a 50% increased risk of cardiovascular death compared to patients without VTE. Patients with DVT had an increased adjusted risk of all-cause mortality, but no increased risk of cardiovascular death. Cancer-related mortality was increased both in DVT and PE patients.
Most prior studies on mortality and causes of death in patients with VTE are conducted before the introduction of DOAC as a treatment of VTE. In two small cohort studies the 1-year all-cause mortality was 22% and 24% in patients with a first time VTE (15, 16). A large prospective cohort study published in 2013 showed a 3-month all-cause mortality of 8% in patients with VTE (17). A metanalysis including 10 trials and 35,029 patients had similar case fatality rate (16%) among patients with VTE that were treated with DOACs as we report in our study (18). In Sweden most of the community-acquired DVT-cases are diagnosed as outpatient facilities (often situated or connected to a hospital ED) and few at general practitioner’s centers, while PE is most often diagnosed at EDs and in hospitalized patients. Considering that our study population is solely comprised of patients presenting to the ED, the severity of disease in patients diagnosed with VTE may differ from that in other study settings.
The observation of a higher risk of cancer-related death in the DVT and PE-group compared to the control group was expected. Our results are consistent with the results from a prior large cohort study in which cancer was the most frequent cause of death in VTE patients within 30 days and at 1 year (10). In our study the number of cancer-related deaths exceeded that of patients with known cancer disease at baseline within the VTE-group which suggests that a considerable proportion of patients were diagnosed with cancer after the VTE-diagnosis. An earlier study showed that the survival of patients diagnosed with cancer at the time of the VTE-diagnosis is poorer than that of matched control patients with the same type and stage of cancer (19).
We found that the adjusted risk of cardiovascular death was 50% increased in patients with PE compared to patients in the control group, whether this is attributed to acute death from PE or from cardiovascular events such as acute heart failure, MI, or stroke, is unknown. The number of PE-related deaths are generally few, and approximately contribute to less than 0.4% of all deaths in Sweden (20). Due to a steady decline in autopsy rates in Sweden (21) the cause of death in many of the patients with PE is not certain. A prior study found an in-hospital mortality rate of only 1,1% in patients presenting with PE in the ED (22), which might suggest that acute death from PE could not fully explain the increased risk of cardiovascular death among patients with PE observed in our study. A previous small scale study reported that 40% of mortality is related to cardiovascular disease after PE, with a mean follow-up time of 3.8 years (23). An increased risk of arterial cardiovascular events in patients with unprovoked VTE was previously reported in a meta-analysis (24). If this can be assigned to similar risk factors for VTE and atherothrombotic disease, or if VTE could act as a trigger for atherothrombotic disease is unknown. Another meta-analysis showed a positive association between the presence of major risk factors of atherothrombotic disease, especially hypertriglyceridemia, and the risk of a VTE event (25). In our study the increased risk for cardiovascular death remained after adjustments for common risk factors of atherosclerosis, MI, and stroke such as hypertension, diabetes, and previous MI.
Age-standardized PE-related mortality has been decreasing in Europe since 2000 (26) and according to the Swedish healthcare annual statistic report a trend in reduced DVT-related mortality has been observed in the last few years (20). We did not have data before the introduction of DOAC, which limited our ability to investigate a potential change in VTE related mortality associated with current treatment strategies.
We do acknowledge that the observed increased all-cause mortality risk in VTE-patients was reflected in all categories of cause-specific death. A more detailed categorization of CV death would have been more informative; however, the absolute numbers of outcomes were relatively small which limited the ability to explore more details on cause-specific deaths. Nonetheless, in contrast to the expected excessive risk of cancer-related death found in the exposed group, we believe that the paralleled increased adjusted risk of CV death is a novel finding that would merit further attention. We believe that the later finding emphasizes that improvements in tailored preventative strategies for cardiovascular risk modification and monitoring are needed in these patients.
Strengths and limitations of the study
This is an observational cohort study, cautious interpretations of data in the context of any clinical setting should be considered, mainly because of the inherent risk with residual confounding that we could not control for. The study was conducted in a large cohort of patients with a relative long follow-up time which allowed us to calculate risk estimates the events with high precision. A strength of our study is that the source of our data, the Swedish registers, are known to be of high quality, including the Swedish Prescribed Drug Register (27, 28). The Cause-of-Death Register has excellent nationwide coverage with virtually no loss to follow-up (21). There are however reports of some degree of misclassification of VTE diagnoses in the Swedish NPR, specifically for DVT but also to some extent for PE (29).
We did not have data available from primary healthcare, as this is not included in the Swedish NPR. Consequently, misclassification of the exposure may have occurred, i.e. as some patients may have had a DVT diagnosed solely within primary care during the study period or even before our study started. Although we were able to control for the most important confounders in our statistical models, we do not have information regarding stage or type of cancer. It was not possible to conclude with certainty that the severity of cancers was similar between the DVT and PE group, mortality differences could be related to a worse prognosis of the cancer itself rather than the VTE. We acknowledge that both the differences in age, burden of cardiovascular comorbidities and prevalent cancer at baseline would all be contributing factors to the observed differences in outcomes between the groups. Competing risk events in the analysis of causes of death was not addressed in our analysis, however, we have adjusted for all these variables in our statistical models. In our belief the fully adjusted models sufficiently handle the confounding effect of these factors and provide adequate estimates on the association between exposure and the outcomes of interest. We consider that the findings in our study on Swedish data are generalizable to other countries with similar health care system.