Limited Influence of Flail Chest in Patients With Blunt Thoracic Trauma – A Matched-pair Analysis

Discussion

The purpose of this study was to analyze the independent influence of flail chest on morbidity and mortality in multiply injured patients with relevant blunt thoracic trauma. Although flail chest was associated with a more frequent presence of lung contusion and pneumo-/hematothorax, no independent influence on post-traumatic course of treatment, including occurrence of ARDS as well as mortality, was found.

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Figure 1.

Flow diagram of study patients.

This might, to some extent, be in contrast to the clinical reality. From daily experience, patients with a flail chest injury do have a more complicated clinical course. However, there are two major concerns. For one thing, there are several different definitions of flail chest and for another, flail chest could be a surrogate parameter of the severity of organ-related injury.

Addressing the first concern in depth, flail chest is more a collective term rather than a firm definition. The most accepted definition is most probably a segmental fracture of three or more consecutive ribs. This is consistent with the AIS. The Association for the Advancement of Automotive Medicine defines flail chest as three or more adjacent ribs, each fractured in more than one location (e.g. posterolateral and anterolateral) to create a free-floating segment, which may or may not result in paradoxical chest movement (22). Two major studies carried out by Dehghan et al. (11) and Schulz-Drost et al. (12) used the AIS coding to identify patients with flail chest. Other studies regarding flail chest either did not specify the definition applied (10) or used an umbrella term which left a wide margin of interpretation (9, 18). Borman et al. (9), as well as Kilic et al. (18) state that “flail occurs when the integrity of the thoracic rigid cage is compromised and a segment, its size depending on the number of fractured ribs or costal cartilages, loses its continuity with the rest of the thoracic cage and moves paradoxically during spontaneous respiration”. This represents a clinically driven definition and is partially in contrast to the anatomic AIS definition. Paradoxical chest movement is a sine qua non in the one definition and optional in the other. Thus, comparability of patients is severely limited. It seems conclusive to clinically identify patients at particular risk. The use of the clinical definition therefore results in a more seriously ill patient group. Not surprisingly, these patients have a poorer outcome compared to clinically more stable ones. As yet there are no data about inter-and intra-rater reliability. Thus, comparability and generalizability of the findings are very limited. To exaggerate, the gain of knowledge is that seriously ill patients are more ill than healthier patients. Therefore, if the clinical definition of flail chest is unreliable, it might be better to use the anatomic definition. Major advantages are its objectivity and reproducibility. These advantages, however, are counteracted by the inherent problems of multicentric registry data. Hundreds and even thousands of different users enter data. In theory, all users have to apply the same definition of flail chest. In practice, there is no validation of the data. In the German Trauma Registry, there is only a consistency check. Entries are only spot-checked if documentation of diagnoses in the clinical information system is consistent with the documentation in the registry. The extent to which documented diagnoses are actually appropriate according to the AIS definition is simply not checked. Therefore, data validity of multicentric registries is questionable to some extent. Dehghan et al. identified 3,467 patients (1%) with flail chest during a 3-year period using the US/Canadian National Trauma Data Bank (11). Schulz-Drost et al. identified 3,492 patients (16.1%) with flail chest during a 5-year period using the German Trauma Registry (12). Both authors used the same AIS-based definition. The substantial difference in the incidence might be explained, to some extent, by different inclusion criteria. The US/Canadian National Trauma Data Bank includes every hospitalized patient with a relevant injury, whereas the German Trauma Registry includes patients admitted to the operating theatre as well as Intensive Care Unit following Emergency Room treatment. Dehghan et al. (11) included patients with an ISS of 9 or higher and Schulz-Drost et al. (12) with an ISS of 16 or higher. Against this background, incidence of flail chest in the study performed by Schulz-Drost et al. (16.1%) (12) seems reasonable but is rather high compared to our own data (7.5%). This is even more surprisingly since our own data are part of the German Trauma Registry, and demographic as well as baseline parameters are comparable for both patient populations. Therefore, the question arises again whether data validity exists. A major advantage of our study is its data validity. A CT scan was performed in all the patients. The diagnoses were verified by a consultant with special qualification in diagnostic radiography. In summary, the lack of a standardized definition and the uncertain data validity compromise the significance and generalizability of many studies.

However, despite definitional uncertainties, the more interesting question seems to be whether flail chest has an independent influence on morbidity and mortality or is just a surrogate parameter of the severity of organ-related injury. Velmahos et al. identified flail chest and extrathoracic injuries as independent risk factors for significant respiratory complications (14). Kilic et al. reported that injury severity was a major risk factor for mortality from flail chest trauma. However, this study seems to have serious shortcomings. The reported mean ISS of patients with traumatic brain injury and flail chest was 75, which is equivalent to a non-survivable injury. Nevertheless, mortality was 19%. Patients with flail chest without traumatic brain injury had a mean ISS of 55.7; the corresponding mortality was 0% (18). Based on our professional experience and with regard to the structure and validity of the ISS, these findings are inexplicable. Borman et al. reported that accompanying extra-thoracic injuries, especially traumatic brain injury, increased mortality. Since there was no control group, it is not clear whether this finding is only due to the increased ISS. The authors did not make a statement on the independent role of flail chest compared with other thoracic injuries (9). Huber et al. demonstrated that extra-thoracic factors (age over 65 years, blood transfusion, shock, severe extra-thoracic injury) prolong ventilation. This also applies to intrathoracic injuries. Nearly every type of injury to the lung parenchyma, hemo- and severe pneumothorax, and major thoracic vessel and cardiac injuries prolonged ventilation with a correlation of injury severity to the expected prolongation (29). Dehghan et al. also identified traumatic brain injury as a major risk factor of adverse outcome in patients with flail chest (11). In line with our own results, Huber et al. demonstrated unilateral flail chest not to be a predictor of increased mortality; only bilateral flail chest was significantly associated with elevated mortality (29). Since there were no patients with bilateral flail chest in our own study population, we are not able to make an objective statement concerning this matter. Schulz-Drost et al. reported a more frequent occurrence of multiple organ dysfunction syndrome and septic complications in patients with flail chest. Mortality was increased accordingly. Bilateral flail chest, as expected, had the highest mortality (12). However, injury severity was increased in the flail chest group (35.1±14.2 vs. 28.3±11.3) and the authors omitted analysis of the impact of flail chest injuries independent of increased injury severity (12). In summary, our own results support the assumption that flail chest is a surrogate parameter of increased severity of thoracic injury rather than an independent predictor of adverse outcome.

However, there are certain issues that are poorly analyzed or discussed. The impact of lung contusion and laceration is hardly understood yet. Huber et al. demonstrated a linear correlation of injury severity to the expected prolongation of ventilation for lung contusion as well as lung laceration (29). But these findings are inconsistent. Dehghan et al. reported a significant increase in the rate of pulmonary complications, even though the magnitude of the differences was low and clinical relevance was not existent (11). From a clinical point of view, it seems obvious that parenchymatous injuries to the lung affect ventilation and the risk of ARDS. But it is difficult to prove. What matters is the extent of lung contusion. However, this depends on the period from the accident to the first CT. The shorter the period, the greater the risk that the real extent of contusion is masked. To our knowledge, no study has yet focused on the correlation between volumetric extent of lung contusion and pulmonary complications. Another relevant issue is the possible bias caused by patients who died before reaching the hospital or in the Emergency Room. Kleber et al. reported that tension pneumothorax was one of the leading causes of trauma-related preventable deaths in the pre-hospital setting (30). Various studies and our own results show that pneumothorax is more common in patients with flail chest (11, 12, 29). Since every study excluded patients who died pre-hospital or in the Emergency Room, it is unclear, to what extent this biases the result. It is conceivable that patients with especially severe thoracic injuries die before reaching hospital and only patients with a rather benign form of flail chest arrive still alive at the hospital. Due to this selection bias, the outcome differences may be levelled.

This limitation also applies to our retrospective study. However, data quality is high and definition of flail chest in comparison to serial rib fractures is well-defined. Nevertheless, there are some limitations one should take into consideration when interpreting the data. Therapy of blunt thoracic trauma has continuously changed during the study period. Alternating position and prone position replaced kinetic therapy. Respiratory therapy also changed. However, there are no convincing data that demonstrate advantages of one approach yet. There are other limitations that might bias our results. Eligibility criteria were well-defined; therefore, numerous patients were excluded. Furthermore, this study analyzed patients with blunt thoracic trauma only. Due to totally different dynamics of blunt and penetrating trauma, a transfer of results is not possible. Therefore, we cannot give any objective recommendations regarding penetrating trauma. However, our results can be generalized to high income countries with comparable therapy according to actual guidelines.

In conclusion, we demonstrated that there was no independent impact of flail chest on post-traumatic mortality and morbidity in multiply injured patients with blunt thoracic trauma. Therefore, it seems inappropriate to give recommendations on operative therapy based on the anatomic diagnosis of flail chest alone, although this is of particular interest. Future studies should analyze which particular injuries in blunt thoracic trauma independently worsen the outcome and necessitate an operative therapy. Special attention needs to be paid to the extent of lung contusion and in particular fracture patterns that hamper ventilation. A set definition of flail chest and scientifically sound knowledge about its impact on multiply injured patients provides the basis for this and helps to draw conclusions about the potential benefits of an operative therapy.