Predictors of mortality and validation of burn mortality prognostic scores in a Malaysian burns intensive care unit

The primary objective was to evaluate and identify risk factors which may predict outcome of in-patient mortality in a Malaysian burn’s population.

The secondary objective of this study was to validate the five well known burn mortality prognostic score (Baux score, Abbreviated Burn Severity Index (ABSI) score, Ryan score, Belgium Outcome Burn Injury (BOBI) score and revised Baux score) in predicting mortality within a Malaysian burns intensive care unit.

Prior to data collection, ethics approval was granted by the Medical Research and Ethics Committee of Malaysia, KKM.NIHSEC/P17–1904(5) in accordance with current guidelines on Good Clinical Practice, the Declaration of Helsinki, and subsequent relevant versions. The inclusion criteria were all burn patients aged ≥13-years-old, patients with major burns, and all patients with burns involving the face, hands, feet, genitalia, perineum or major joints, electrical and chemical burns. The exclusion criteria were patients that were transferred in from other BICUs for continuation of burn wound care and patients with missing data.

In our centre, major burns were defined as all partial and full-thickness burns with total burn surface area (TBSA) > 20% involving burns on the face, hands feet, genitalia, perineum or major joints, inhalational injury, electrical and chemical burns. TBSA estimation is performed using the Lund and Browder chart. Inhalational injury is defined as all burn patients with a history of being in an enclosed burn space and clinical features which may include a singed eye brow, soot in nostrils, laryngeal oedema and facial burns that may suggest possible inhalational injury. Indication for mechanical ventilation for patients with burn inhalational injury is based on clinical and blood parameters coupled with the clinical experience of the attending anaesthetist. Positive clinical findings of rales, ronchi, wheezing, tachypnea and suspicious visible supraglottic oedema are main factors for mechanical ventilation. Subsequent evidence of hypoxia by the arterial blood gas results further lowers the threshold for mechanical ventilation. All severely burn patients were managed and resuscitated initially according to the advanced trauma life support (ATLS) and American Burn Association (ABA) guidelines. All patients with over 20% partial to full thickness burns received the Parkland Formula for fluid resuscitation. Patients with inhalational injury received treatment in accordance to the hospital’s inhalational injury treatment protocol. The protocol includes humidified oxygen to maintain oxygen saturation above 90%, chest physiotherapy, two hourly turn and positioning, aerosolised N-acetylcysteine with bronchodilator, alternate aerosol 5000 units of Heparin together with normal saline, regular airway suction and sputum cultures. Nebulised Heparin was given to inhibit pulmonary fibrin clot formation and improve oxygenation for patients with inhalational injury.

Variables chosen to predict mortality during the hospital stay included gender, age, mechanism of injury, total body surface area burn, inhalational injury, mechanical ventilation, tracheotomy, length of time from injury to BICU admission and centre of initial emergency treatment was administered prior transfer/admission to BICU. Centre of initial emergency treatment were categorized into the hospital’s emergency department (ED BICU) and outside of the parent emergency department categorized as Periphery Primary and Secondary Health Care Centre (PPSHCC).

The risk of death from burn injury for each patient was estimated using five burn scores: the Baux score, Abbreviated Burn Severity Index (ABSI), Ryan score, Belgium Outcome Burn Injury (BOBI) and revised Baux score. These scores were calculated based on the recorded variables in the burns database. The Baux score is the summation of patient’s age and TBSA, equivalent to the percentage of mortality [5]. The ABSI is a scoring system based on the parameters of patient’s sex, category of age, presence of inhalational injury and TBSA which is numerically scored between 0 to 17. Higher scores denote a lower probability of survival [6]. The Ryan score uses 3 risk factors of age greater than 60 years, TBSA of 40% or greater and inhalational injury with a maximum score of 1 for each risk factor present. Higher score represents a higher risk of mortality for death [7]. The BOBI score uses categorical values of age, TBSA and presence of inhalational injury [8]. The maximum score is 10 which give a 99% risk of mortality. Lastly, the revised Baux score includes the presence of inhalational injury giving the percentage of mortality [5]. The details of these five scores are seen in Additional file 1.

All analysis of data was performed using SPSS for Windows version 16.0 (SPSS Inc., Chicago, USA). All continuous variables were expressed as mean and standard deviation (SD) and categorical variables were expressed as frequencies and percentages. For each variable/risk factor of interest, univariate analysis performed between survivors and non-survivors predicting death with significance accepted p < 0.05. The univariate analysis was performed using Chi square test, Student’s t test or Mann-Whitney test where appropriate. All variables with significant p value by univariate analysis were included in the multivariate binary logistic regression using enter method to determine association with mortality.

The five burn mortality prediction scores (Baux score, Abbreviated Burn Severity Index (ABSI) score, Ryan score, Belgium Outcome Burn Injury (BOBI) score and revised Baux score) were evaluated using area under receiver operating curve (AUROC) to determine the accuracy at distinguishing between survivors (false positive) and non-survivors (true positives). An area over 0.9 indicates high accuracy, 0.7–0.9 moderate accuracy 0.5–0.7 low accuracy and 0.5 indicates chance discrimination [9].

Older age is often associated with physiological reduction in immune function and thinning of the skin [10]. This is often the reason for longer recovery periods for older patients with burn. In general, the elderly population is at higher risk of mortality due to increased pre-existing medical co-morbidities, failure of immune system to combat post-burn infections and thinning of skin which leads to deeper burn injuries [11]. Although the acceptable definitions of elderly age group were usually taken as 65 years and older, there are reports that rebuke that age alone is not a significant predictor. The use of a frailty score may better predict mortality in elderly burn population [12].

Greater total body surface area burn is a well-known predictor of burn mortality. Jeschke et al reports that adults with 40% total body surface area burn are at a high risk for mortality and morbidity even in a highly specialised burn centre [12]. The common cause of death is due to reduced immune function due to loss of skin coverage that leads to wound sepsis and hospital complications of pneumonia [11].

Mechanical ventilation is indicated in the majority of patients with inhalational injuries. Although some may succumb to severe lung injury, the majority of patients recover. Complications of ARDS and pneumonia may occur after mechanical ventilation [13]. Although it is not clear whether these complications occur after the primary lung insult or secondary to mechanical ventilation, there is a strong statistical significance seen between mechanical ventilation and mortality seen in this current study.

Due to the limited BICU centres in Malaysia, majority of burn patients may be inevitably treated at a non-burn centre prior to referral to a BICU. Therefore, this study attempts to explore other associated risk factors of the centre administering emergency initial burn treatment with mortality. The time of burn injury to burn unit admission was longer in patients that were treated first at PPSHCC. Time from burn to admission was longer due to the initial phase of emergency resuscitation, logistic and communications with the burn centre and arranging appropriate transportation (land or air transfers) specific to geographical location. Although it was not statistically significant but the time to admission had positive odds for mortality in the multivariate analysis. Hrenjec et al stated that the admitting facility is one of the critical factors predicting mortality in burn patients [14]. This study did not reveal specific centres but identified high volume burn centres had better outcomes in terms of survival.

Amongst the five burn scores which were validated in this current study, the revised baux score had the highest area under receiver operating curve value of 0.94 to predict burn mortality in our local Malaysian burn population. The result is obvious as it incorporates well known burn mortality predictors of age, total body surface area burns and inhalational injury. The remaining burn scores had good predictive ability which is due to the incorporation of similar burn mortality predictors. From the results, it is advocated to use the revised Baux score in a Malaysian burn unit mainly because of its simplicity of the addition of age, TBSA burn and plus 17 (if there is presence of inhalational injury) which the sum, is equivalent to the percentage of mortality in the individual burn patient. With this validation, it may be crucial for prognostic and mortality risk counselling to patient relative during the initial resuscitative phase [15, 16].

Despite validating these burn mortality scores, there is a possible higher significant number of deaths in patients which were involved with blunt trauma. This is especially seen in road traffic accident patients which may have polytrauma which may lead to death instead of the burn injury. This is a limitation of this current study which may possibly addressed in future retrospective studies. In addition, although there were dedicated burn trauma nurses for data collection, we did not have a luxury to have a data manager. Most of the data collected were through the concerted of nurses under the direct supervision of the burns and trauma surgeon.