Predicting outcomes after blunt chest wall trauma: development and external validation of a new prognostic model

Blunt chest-wall trauma accounted for over 15% of all trauma admissions to Emergency departments (EDs) worldwide [1]. Reported mortality ranges between 4 and 60%, however, no current national guidelines exist to assist in the management of this patient group unless the patient has severe, immediate life-threatening injuries [2]. The difficulties in the management of the blunt chest-wall trauma patient are becoming increasingly well recognised in the literature [3, 4]. The blunt chest-wall trauma patient commonly presents to the ED initially with no respiratory difficulties, but can develop respiratory complications approximately 48 to 72 hours later [5, 6]. Clinical symptoms are not considered an accurate predictor of outcome following non-life threatening blunt chest-wall trauma [7]. Decisions on the appropriate level of care required by the patient following discharge from the ED are therefore difficult, which is further compounded by the lack of current national guidelines. A number of well-documented risk factors for morbidity and mortality exist for blunt chest-wall trauma, including patient age, pre-existing disease, number of ribs fractured and the onset of pneumonia during the recovery phase [2, 8].

A prognostic model enables the clinician to use combinations of predictor values to estimate a probability that a specified outcome will occur [9]. The resulting model may be used to divide the patient into categories of risk or predict probabilities of a pre-specified outcome [9]. A number of models exist for blunt chest trauma, however, most are designed for use with patients with multiple injuries and very few have been externally validated or presented in a clinically practical way [3]. For the purpose of this study, blunt chest-wall trauma was defined as blunt chest injury resulting in chest wall contusion or rib fractures, with or without non-immediate life-threatening lung injury [2]. We have developed and validated a prognostic model for the development of complications following blunt chest-wall trauma. Using the results of the prognostic model, we have also developed a simple risk score for use in the clinical setting which can assist the clinician in the management of the blunt chest-wall trauma patient.

There were two distinct phases to the overall study; the development and the validation phases. Published guidelines for prognostic model development were followed throughout the completion of this work [9‐12]. These guidelines outlined the stages of model development and the appropriate statistical analysis that should be undertaken at each phase [9‐12].

We have developed and validated a prognostic model for predicting the development of complications following blunt chest-wall trauma. The final model has excellent discrimination suggesting that the clinician can confidently assess whether the patient with the higher risk prediction using the model will develop complications following blunt chest-wall trauma, compared to the patients with low risk predictions who will not develop complications. As expected, the validation model demonstrated poorer calibration and numerous authors have offered explanations for this result in a validation sample [22, 23]. It is likely that the poor calibration was due to the significantly lower rates of the development of complications in the validation sample.

Common practice is simply to reject an original prediction model as a result of decreased predictive performance in the validation sample. A new prediction model is then developed and as a consequence the original dataset is neglected and clinicians are faced with numerous possible prediction models, very few of which have been externally validated for use in new samples [22]. Research now suggests that the model should be adjusted in order to improve its performance on the new population and this adjusted model is then based on both the original and validation data, further strengthening its stability and generalisability [9]. The model in this validation study was therefore updated using a previously described method known as recalibration [19, 23]. By simply adjusting the intercept for the original model, the validation model calibration was improved.

Patient age, number of rib fractures, chronic lung disease, pre-injury anticoagulants and oxygen saturation levels were the significant risk factors for development of complications following blunt chest-wall trauma. Patient age, number of rib fractures and chronic lung disease have been reported as significant risk factors for poor outcomes in a number of recent studies and possible explanations for these factors have been previously discussed [2, 16, 24]. Pre-injury anticoagulant use and oxygen saturation levels have only been reported as risk factors for the development for complications following blunt chest-wall trauma in a previous study by Battle et al. [2] and therefore, further research into these risk factors would be beneficial [16].

The results of this study have demonstrated that risk can be easily and accurately stratified from simple demographic and clinical variables on initial assessment of the blunt chest-wall trauma patient in the ED. The risk factors are all currently routinely measured in the ED and do not require expensive, time-consuming or complicated technology to investigate. This is one of the most important factors in the success of prognostic model development according to previous research [9]. The clinician would simply collect routine data, total the scores for each risk factor, then obtain the corresponding probability of the development of complications. A more accurate decision can be made by the clinician on whether the patient is safe for discharge home directly from the ED, or whether the patient requires admission to hospital. Not only could this reduce the development of complications in blunt chest-wall trauma patients through close observation and early aggressive prophylactic treatment in the admitted patient, but also reduce unnecessary admission of patients unlikely to develop complications.

The overall results of this study suggest that the final validation model could be safely and effectively used in the clinical setting in England and Wales for assisting in the management of blunt chest-wall trauma patients. This is the first prognostic model that has been developed and externally validated in a prospective multi-centre study for use with blunt chest-wall trauma patients. The model can be used with the less severely injured patient who on presentation to the ED is not suffering any overt signs of respiratory distress, but will potentially go on to develop severe life-threatening pulmonary complications. Research has demonstrated that careful observation and early aggressive therapy can limit these complications, therefore identification of the high-risk patient is imperative for optimal management [25]. It is inevitable, however, that the final decision on patient management must be individualised and many factors that cannot be translated into a statistical model must be considered. The overall purpose of the prognostic model is simply to guide clinical decision-making, not replace it.

This study has a number of strengths and limitations. External validation using a prospective multi-centre trial is considered the most robust validation technique ensuring generalisability of the study’s results [9]. Current methodological recommendations for clinical prediction research, as outlined by Bouwmeester et al. [17] have been followed in the design and completion of the prognostic model for use with blunt chest-wall trauma patients. These recommendations included sample size and selection, clear definitions of risk factors and outcomes under investigation, handling of missing data, reporting of both univariable and multivariable results and calculation of model performance measures. The final model was also recalibrated as recommended by recent research [19]. As a result the reliability and applicability is sufficient that the model could be safely and effectively used in the clinical setting. The external validation results also confirm the clinical usefulness of the model in blunt chest-wall trauma management throughout England and Wales. It is important to emphasise, however, that the validation model c-statistic is a very unusual result and should be interpreted with caution. It is more common for the c-statistic to decrease in the validation study, rather than to increase as we found.

One of the limitations of this study is the loss of patients to follow up. Due to limited resources, it was not considered feasible to investigate the patients’ follow up once they had left hospital care. Any use of primary care for complications that developed following hospital discharge would not have been included in the study results. The data collection was not fully blinded as recommended by Bouwmeester et al. [17], however, the clinicians collecting the data in the validation study were blinded to which of the risk factors and outcomes were being used in the final analysis. Another limitation of the validation study concerns the timing of the data collection. For example, the patient’s oxygen saturation levels may have varied according to the time in which they were recorded. If the data were collected before analgesia was given in the ED, then the results may have been worse than if the patient had received analgesia and could breathe more easily. As a result of these limitations, the results of this study should be considered with caution.

There was a significantly lower rate of complications in the validation sample than the development sample. This could be explained by the protocol for management of the blunt chest-trauma patient in the different hospitals. For example, in the hospital where the original model was developed, patients are routinely admitted to the ICU if they need invasive analgesia such as an epidural, as this is where epidural patients are currently managed. In order to quantify the number of rib fractures sustained by the patient, a chest radiograph or computed tomography (CT) scan and its subjective interpretation is required. Due to the inherent difficulties in identification of rib fractures on chest radiographs, the clinician is advised to record the number of rib fractures identified on imaging, or suspected clinically following physical examination of the patient [26].

The use of a composite outcome measure could result in one element dominating the total outcome measure. This was not the case in this study, however, as apart from the low mortality rate, there was a relatively even number of the different types of reported complications.

Blunt chest-wall trauma patients are often difficult to manage in the ED, due to the frequent onset of delayed complications. We have developed and validated a prognostic model, which can assist the clinician in the management of the blunt chest-wall trauma patient. The model has demonstrated excellent prediction capabilities and can be safely used in EDs managing this patient group.