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(Chest. 2005;128:2190-2194.)
© 2005 American College of Chest Physicians

Barometric Pressure and the Incidence of Pulmonary Embolism*

Mehmet Meral, MD; Arzu Mirici, MD; Sahin Aslan, MD; Metin Akgun, MD; Hasan Kaynar, MD; Leyla Saglam, MD and Metin Gorguner, MD

* From the Departments of Pulmonary Medicine (Drs. Meral, Mirici, Akgun, Kaynar, Saglam, and Gorguner) and Emergency Medicine (Dr. Aslan), Faculty of Medicine, Ataturk University, Erzurum, Turkey.

Correspondence to: Mehmet Meral, MD, Department of Pulmonary Medicine, Faculty of Medicine, Ataturk University, Erzurum 25240 Turkey; e-mail: mmeral{at}atauni.edu.tr


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Objectives: Reports in the literature suggest that weather changes may play a role in venous thrombotic disease. An increase in patients with pulmonary embolism (PE) during the spring season led us to investigate the relationship between atmospheric pressure (AP) and the incidence of PE, as diagnosed in most of the patients by helical CT angiography, and in the minority of patients by conventional pulmonary angiography and lung scanning.

Methods: We retrospectively investigated the charts of 91 consecutive patients with a diagnosis of PE who were evaluated by the Department of Pulmonary Medicine between August 2000 and September 2004. We documented AP changes as recorded by the Erzurum Provincial Department of Meteorology. Of the 91 patients, the diagnosis of PE was made by helical CT angiography in 84 patients, isotope lung scan in 5 patients, and conventional pulmonary angiography in 2 patients.

Results: More patients presented in the spring months (March, n = 15; April, n = 10; and May, n = 12) than during other seasons (p < 0.001). The frequency of PE was inversely related to general average AP (r = – 0.70; p < 0.01). When the average seasonal AP was correlated with the incidence of PE, however, the relationship was found to not be statistically significant (r = – 0.66; p = 0.34). There was no correlation between the severity of PE or mortality and AP.

Conclusions: The incidence of PE was significantly higher in the spring months, when AP was low. A regional study to capture all PE patients will need to be done to confirm our findings. Other meteorologic factors should be investigated regarding their effect on thromboembolic disease.

Key Words: atmospheric pressure • helical CT • predisposing factor • pulmonary embolism


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Certain meteorologic conditions may correlate with an increase in pulmonary embolism (PE) through a variety of mechanism. In 1940, De Takats et al1 reported the incidence of PE to be higher during the spring months and during periods of low atmospheric pressure (AP), while Newton,2 in 1951, reported the incidence of postoperative phlebothrombosis to be higher during the passage of weather fronts. These studies, done without standard diagnostic testing for PE, were more recently supported by a 1992 study3 that used isotope lung scanning for PE diagnosis. More recent studies45 using modern diagnostic techniques found the incidence of PE to be higher in the winter compared to other seasons, while Stein et al6 found no increase in venous thromboembolism (VTE) frequency in any region of the United States in any particular season. Also, fatal PE has been reported to have a peak incidence in the winter months.78 In all these studies, however, it is unclear if the increase in PE is related to AP or to other factors. In healthy male volunteers, hypobaria to 76 kPa air pressure transiently increases the markers of activated coagulation (eg, concentration of prothrombin fragments 1 and 2, thrombin-antithrombin complex, activity of factor VIIa).9

Although isotope lung scanning is reported to have high negative and positive predictive values in patients with low and high clinical pretest probabilities, respectively, only 34% of patients with PE are within the categories.10 This means that the diagnosis of PE can definitely be confirmed or excluded in only a minority of typical patients by isotope lung scanning.11 A normal lung scan result does not fully exclude the diagnosis of PE, while a high-probability scan result may have a false-positive result for the diagnosis of PE.12 Helical CT angiography is slowly replacing isotope lung scanning for the diagnosis of PE because of its higher specificity and interobserver agreement, especially in patients with preexisting cardiopulmonary disease.131415 Van Strijen et al16 found false-negative helical CT rates as low as 0.4%, and sensitivity as high as 99.6%, in a group of 130 patients with suspected PE. Gottsater et al17 concluded that a negative helical CT finding in patients with suspected PE might exclude clinically significant PE with a high-enough degree of accuracy to obviate the need for further diagnostic investigations and anticoagulation in the vast majority of patients.

To date, the relationship between PE and AP has not yet been confirmed by studies that use more advanced imaging studies for PE, such as helical CT. When we noticed an increase in patients admitted with PE in the spring, we decided to retrospectively investigate the relationship between AP and PE, in which most patients had the diagnosis of PE made using helical CT angiography.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Patients
We retrospectively reviewed the charts of patients admitted to the Department of Pulmonary Medicine at our tertiary university medical center with a diagnosis of PE (made by helical CT angiography, conventional angiography, or isotope lung scan) from August 2000 until September 2004. Patients with predisposing factors for PE, such as a recent traffic accident or surgery within the previous year, or recent short-term immobilization, were excluded from the study. Of the 91 PE patients who met the inclusion criteria, the diagnosis of PE was made by helical CT angiography in 84 patients, isotope lung scan in 5 patients, and conventional pulmonary angiography in 2 patients (performed because of interobserver disagreement on the helical CT interpretation).

Helical CT Angiography
Helical CT angiography of the pulmonary circulation was obtained with a single-slice helical CT scanner (X-Vision Gx; Toshiba; Nasu, Japan). CT examinations included thin-collimated angiograms, obtained with 2- to 3-mm collimation. Other technical parameters included 5 mm/s table speed, 120 kilovolt peak, 250 mA, 1-s scan time, and a pitch of 1.7 to 2.0. The z-axis coverage initiated at level of the subsegmental vessels of the upper lobes down to the lower lobes. CT scans were reconstructed at lung and mediastinal windows settings. Contrast medium consisted 120 to 140 mL of iohexol (300 mg of iodine per milliliter; Omnipaque; Nycomed Imaging; Cork, Ireland) was injected at a rate of 3 to 5 mL/s using a power injector by an 18- to 20-gauge Venflon cannula (Eastern Medikit Limited; Gurgaon, Haryana, India) inserted in an antecubital vein. The start delay was 12 to 15 s for patients with normal hemodynamic status and 18 to 20 s for patients with suspected pulmonary hypertension and right-heart failure. Two blinded senior radiologists who were blinded to clinical data and each other interpreted all helical CT angiograms. For the diagnosis of acute PE, the following criteria were used: (1) a central homogeneous low-attenuation area (filling defect) in the vascular lumen outlined by contrast material, (2) an eccentric filling defect that projects into the lumen with an acute angle between the filling defect and the vessel wall, and (3) a low-attenuation homogeneous area not surrounded by contrast material in a normal-sized or enlarged pulmonary artery (complete obstruction).18

Meteorologic Data
Daily atmospheric pressure data (in millibars [mb]) from the same time frame were obtained from our provincial meteorologic institute, also located in our city, which lies at an altitude of 1,800 m (5,900 feet). Because it was impossible to determine the exact time of PE onset in most patients, the AP value attributed to each patient was the arithmetic mean AP of the 3 days preceding the clinical presentation of PE.3

Statistical Analysis
The Pearson {chi}2 test was used to compare baseline characteristics and to compare the number of patients presenting in each month and season. The Pearson correlation was used to determine the correlation between the average AP values and PE frequency for each month and season. The Pearson correlation was also used to determine the correlation between the average AP values and severity of PE for each season. A p value < 0.05 was considered to indicate statistical significance. Data were analyzed using statistical software (SPSS version 9.0; SPSS; Chicago, IL).


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Demographic characteristics of the patients and atmospheric pressures of each season are summarized in Tables 1 and 2 , respectively. Demographic characteristics and associated predisposing factors did not differ significantly in patients presenting in different seasons. The number of PE patients was higher in the spring (n = 47) than in the other seasons (p < 0.001) [Table 2, Fig 1 ]. The number of PE patients was significantly higher in March (n = 15), April (n = 10), and May (n = 12) than in others months (p < 0.05) [Table 2, Fig 2 ]. The average AP (± SD) was lower in the spring than in the other seasons (820 ± 5 mb, 821 ± 2 mb, 826 ± 2 mb, and 821 ± 7 mb in the spring, summer, autumn, and winter, respectively). However, only the difference between spring and autumn was significant (p < 0.001). Although there was a negative correlation between PE frequency and average AP values of the seasons, it was not significant (r = – 0.66; p = 0.34).


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  		Table 1.. Demographics Characteristics and Risk Factors for PE in Patients According to Season*

 

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  		Table 2.. Number of PE Patients and AP According to Season for 4 Years (2000–2004)

 


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  		Figure 1.. The number of patients is highest in the spring (p < 0.001) and lowest in the autumn, matching to decreased and increased AP levels, respectively. However, the correlation between the number of patients and AP is not significant.

 


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  		Figure 2.. The number of patients is highest in March, April, and May (p < 0.05), and lowest in November, matching to decreased and increased AP levels, respectively. However, the correlation between the number of patients and AP is not significant.

 
There was a significant negative correlation between frequency of PE and average AP (r = – 0.70; p < 0.001) [Fig 3 ]. Although the SDs of AP were higher in February and December than in the other months, they was not associated with an increase in PE frequency (Fig 2). In addition, there was no correlation between average AP and mortality from PE (r = – 0.08; p = 0.47). Neither was there a significant correlation between severity of PE (massive or nonmassive) and AP (r = 0.05; p = 0.61) [Table 1].



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  		Figure 3.. There is a significant negative correlation between the number of patients and AP (r = – 0.70; p < 0.01).

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Perhaps the most important finding of our study was the increased number of patients with PE during periods of low AP (Fig 3). Increased PE frequency may be due to a predilection for thrombosis, caused by hyperprothrombinemia19 or spontaneous platelet aggregation20 that might have occurred during those periods. When looking at coagulation and oxygenation under simulated flight conditions, Bendz et al9 found a negative correlation between hypobaric hypoxia and coagulation parameters.

The other important finding of our study was the increased PE frequency in the spring, although there was no correlation between low AP in the spring and the number of patients. The increased PE frequency during the spring months might not only be due to low AP but also to other meteorologic factors occurring in the spring, such as positive and negative air ions, that may have physiologic effects that may result in VTE.21

Numerous studies have found a peak incidence of PE,4522 deep venous thrombosis,2223 and other thromboembolic events24 in the winter, not the spring, months. Green and Edwards,25 however, found a decrease in PE frequency in the winter, and an increase in the spring and autumn. Others626 state that the incidence of VTE does not change with seasonal variations. Because our city lies at a high altitude (1,800 m), our spring weather conditions are much like winter in the locations where other studies have been done. In the studies mentioned above, the relationship of meteorologic conditions to PE incidence was not well studied. Although we found no correlation between severity or mortality of PE and low AP, others7827 have reported an increase in the incidence of severity of PE and mortality during the winter months.

As seen in Table 1, the incidence of comorbid neurologic illness was slightly higher in the autumn and winter than in other seasons, although this increase was not statistically significant. Cooler weather may have a thrombotic effect, with resulting higher rates of the cerebral and coronary thrombosis in the winter.2829 Stolz et al30 found a biphasic distribution with the highest frequency of cerebral thrombosis in the summer and winter.

Although the present study was limited in scope, and lacked measurement of other meteorologic factors, our results, using CT angiography for the diagnosis of PE in most patients, were similar to those of larger, older studies. Scott et al3 used isotope lung scanning for PE diagnosis and also found a negative correlation between AP and thromboembolic disease. However, the diagnosis of PE can definitely be confirmed or excluded in only a minority of patients by isotope lung scanning.11 Although isotope lung scanning has high negative and positive predictive values when patients have low and high pretest probabilities, respectively, only 34% of patients suspected to have PE are within these categories.10 A normal lung scan result does not fully exclude the diagnosis of PE, and a high-probability scan result may have a false-positive result for the diagnosis of PE.12 Scott et al3 failed to mention coexisting diseases in their patients, such as acute or chronic cardiopulmonary disease or advanced age, in whom the interpretation of the isotope scans may be unreliable.3132 Isotope lung scanning is known to be less reliable when coexistent lung disease is present, and most of our patients had such conditions (Table 1). Because of this, we used a diagnostic test for PE that is less sensitive to misinterpretation in the presence of other lung disease.

Our data might be of use when designing larger, population-based studies. Determining the risk for PE, while tracking other meteorologic factors, will be important to determine before advancing to possible studies of prophylaxis in high-risk patients during periods of greatest susceptibility to PE development.

In conclusion, we found a higher incidence of PE in the spring months, during periods when the AP was lowest. Larger, population-based studies should be done to assess this association more closely, while also collecting data on other potentially confounding meteorologic factors.


   Acknowledgements
 
The authors thank Mecit Kantarci, MD, and Omer Onbas, MD, for their interpretation of helical CT angiograms; Dr. Omer Cevdet Bilgin for help with statistical analysis; Elif Yilmazel, MD, for help in collection of previous data; and John Fowler, MD, for editorial help.


   Footnotes
 
Abbreviations: AP = atmospheric pressure; mb = millibar; PE = pulmonary embolism; VTE = venous thromboembolism

Received for publication February 15, 2005. Accepted for publication April 11, 2005.


   References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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