Effect of age on the interaction between heart rate and mortality in prehospital trauma care

2.1 Study design and setting

The Korea Central Fire Services operates the prehospital emergency medical system (EMS) exclusively for trauma care and regional fire headquarters manages the system in each 17 provinces. Emergency medical service providers performed basic to intermediate levels of services. Basic procedures including oxygen supplement from nasal cannula to back-valve masks, spinal immobilization, splints, and wound care can be conducted based on standing orders. Korean emergency medical technicians (EMTs) are divided into level 1 (equivalent to EMS-Intermediate) and level 2 (equivalent to EMS-Basic). Level 1 EMTs can provide intravenous fluid administration for hypotensive patient and advanced life support for traumatic cardiac arrest including endotracheal intubation under direct medical control [24].

Korean EMS operates ground ambulance response systems in most cities and helicopter EMS can be activated in some rural areas. All trauma patients are transported to nearby emergency departments (EDs), which are categorized as level 1, 2 and 3. Level 1 or 2 EDs can provide definite care including surgery and embolization followed by critical care in intensive care units. Level 1 EDs operates emergency board physicians 24/7. Level 3 EDs can provide appropriate care by general physicians. There are about 30 Level 1 EDs, 120 Level 2 EDs, and 300 Level 3 EDs. Level 1 trauma centers are designated separately for each province, but most overlap with Level 1 ED. Trauma centers require independent facilities and trauma team. The designation varies according to annual performance and quality evaluations conducted by the Ministry of Health and Welfare.

In 2012, Korea Prehospital Trauma Triage and Scheme (KPTTS), which has 4 triage categories modified from the U.S. CDC’s triage guidelines, was implemented for prehospital protocol for destination hospitals [25, 26]. For severe trauma cases recognized by EMS personnel, the protocol encourages transport to level 1 or 2 EDs or level 1 trauma centers. Regional level 1 trauma centers covered by emergency physicians or trauma surgeons should provide a specialized trauma team, independent operating rooms, and trauma intensive care units from general ED space. The Korean government has designated regional level 1 trauma center in each province. A total of 13 institutions were designated from five institutions from 2012 to 2015.

2.2 Data source

This multicenter cross-sectional observational study was conducted using the Korean Emergency Medical Services-Severe Trauma Registry (EMS-STR) database. It was constructed in 2012 through a collaboration between the Korean Central Fire Service and the Korean CDC. Three data resources including EMS run sheets, the EMS trauma registry, and the hospital trauma record review registry were merged to construct the EMS-STR. All EMS-transported traumatic injuries were screened and severe trauma cases were selected according to KTPPS criteria. EMS run sheets and the EMS trauma registry were recorded by EMS providers after transport. The EMS run sheet includes demographic findings, EMS operational variables, time elapse variables, destination hospital levels, and transport-related information for all trauma patients. EMS providers fill out the EMS trauma registry, which includes in-depth information such as clinical findings and prehospital procedures, for patients with positive KTPPS criteria [26]. A hospital trauma record review was conducted by professional medical record reviewers hired by the Korea CDC. They collected clinical information about patient status, procedures, and outcomes from hospital medical records by visiting the hospitals where EMS providers transport patients. A medical record review was conducted in 6 provinces in 2012 and 10 in 2013, and all of the provinces were surveyed in 2015.

The data quality managed by the committee consisted of epidemiologists, biostatistics experts, emergency medicine physicians, and trauma surgeons. It holds monthly meetings for education and feedback to medical record reviewers.

2.3 Study population

Adult severe trauma patients who were treated and transported by EMS from 2012 to 2015 were screened. Since the surveillance was not conducted in any area in 2014, data for 3 years were enrolled. Prehospital cardiac arrest cases, transfer cases, and patients without heart rate data or hospital outcomes were excluded.

2.4 Data variables

The main exposure was tachycardia, and the interaction variable was age. We defined tachycardia as the heart rate measured by pulse oximetry or electrocardiogram monitor by EMS providers higher than 90 beats per minute, which was frequently used for the Shock Index cut-off value [27]. The Korean EMS trauma standard operation protocol recommend measuring vital signs at least 3 times during EMS transport, immediately after encountering the patient, during ambulance transport, and at the time of ED arrival. We used the first recorded heart rate if multiple measurement was conducted.

We selected confounders including age, sex, Charlson Comorbidity Index, residential area, mechanism of injury, Injury Severity Score (ISS), mental status, shock status in the field, and alcohol consumption status. Age was categorized into 3 groups: 15–64 years, 65–84 years, and over 85 years. We categorized the Charlson Comorbidity Index into 3 groups: two or more according to the number of comorbidities as the high-risk group [28, 29], one or none. The mechanisms of injury were traffic accidents, falls, other blunt force trauma, other penetrations, and other mechanisms. Injury severity was categorized into 2 groups: severe (ISS 16 or higher) and not severe (ISS 1–15) [30]. We divided prehospital shock status and mental status into 2 groups: based on systolic blood pressure measured with the heart rate simultaneously by EMS providers, shock status was defined as less than 90 mmHg, and mental status which was not alert on AVPU scale (alert, response to verbal stimulation, response to pain, and unresponsiveness) was defined as altered mental status. Hospital management and clinical outcome data included surgery under general anesthesia, transfusion in the first 24 hours, emergent embolization for traumatic bleeding, discharge from ED, admission to the intensive care unit, disability, and mortality. The Glasgow Outcome Scale (GOS) was used to measure disability including death, vegetative status, severe disability, moderate disability, and good recovery [31]. It was retrospectively measured from medical records before the time of injury and at the time of discharge.

2.5 Outcomes measures

The primary outcome was hospital mortality includes ED death or death after hospitalization. Secondary outcome was worsened disability, which was used to assess the impact of current trauma on a patient’s disability in a previous study [32]. It was defined as a 2-point decline in the Glasgow Outcome Scale (GOS) score between discharge and the time before injury. The tertiary outcome was intensive care unit (ICU) admission from ED. All of the outcomes were identified by medical record reviews.

2.6 Statistical analysis

A descriptive analysis was conducted to compare the distribution of risk factors between main exposure and interaction variables. Chi-squared tests for categorical variables and Wilcoxon’s rank sum tests for continuous variables were used.

To evaluate the association between tachycardia and mortality, a multivariable logistic regression analysis was conducted before and after adjusting for multiple confounders. The confounders included age, gender, Charlson Comorbidity Index, residential area, mechanism of trauma, Injury Severity Score, prehospital shock status, prehospital altered mentality, and alcohol consumption status. The adjusted odds ratios (AOR) with 95% confidence intervals (95% CIs) were calculated to measure the effects of tachycardia and age. To compare the effect size of tachycardia according to age, we conducted an interaction analysis using interaction terms (tachycardia*age) added to the multivariable logistic regression model.

Additional sensitivity analyses were conducted for specific study participants. We divided entire cohort according to shock status and ISS score greater than or equal to 16, which was used to define severe trauma. Multivariable logistic regression was conducted for the association between tachycardia and outcomes adjusted for confounders in the main analysis. An interaction analysis with same model in the main analysis for subgroups was used to evaluate the interactive association between tachycardia and age for outcomes.

2.7 Ethical statements

This study complied with the Declaration of Helsinki, and its protocol was approved by the Seoul National University Hospital’s Institutional Review Board with a waiver for informed consent (IRB No. H-1206-024-412).

3.1 Demographic findings

A total of 81,910 trauma patients were treated and transported by EMS in the study period. After excluding 7561 pediatrics, 8721 prehospital cardiac arrests, 6732 who lacked heart rate values, and 23,354 who were transferred to other hospitals or lacked clinical outcomes, 35,542 patients were analyzed (Fig. 1). The tachycardia group demonstrated higher mortality than the non-tachycardia group (11.5% vs 6.6%) and also showed higher worsened disability (15.3% vs 9.9%) and ICU admission rates (26.1% vs 19.7%), respectively (all p values < 0.01). The tachycardia group was more likely to be male, under 65 years old, involve alcohol consumption, and have higher ISS and altered mental status. Unlike transfusion, surgery was more likely to occur in the non-tachycardia group (Table 1).

Fig. 1.Patient flow.

Age over 85 years group demonstrated higher mortality than the other groups (19.7% vs 15.6% vs 6%) and higher worsened disability (24.7% vs 20.4% vs 9.1%), respectively (all p values < 0.01). The ICU admission rate was similar between the age over 85 years and 65–84 years groups. There was no specific pattern of tachycardia proportion according to aging. The age over 85 years group was more likely to be female and injured by falling. Low blood pressure and higher Charlson Comorbidity Index values were found in the age over 85 years group (Table 2).

3.2 Main analysis and subgroup analysis

The multivariable logistic regression analysis showed significant associations between tachycardia and mortality; AOR (95% CIs) for tachycardia was 1.41 (1.28–1.54). For worsened disability and ICU admission, the AORs and 95% CIs were 1.25 (1.15–1.36) and 1.04 (0.97–1.12), respectively (Table 3). C statistics for discrimination of the final models were calculated: 0.903 for mortality, 0.895 for worsened disability, and 0.890 for ICU admission. Hosmer and Lemeshow Goodness-of-Fit Test were conducted to evaluate calibration and found that p-value were under 0.01 for all final models.

The same statistical model was used for tachycardia with interaction terms (age group), AORs, and 95% CIs for mortality were 1.2 (1.11–1.3) for the 15–64 years group, 1.4 (1.29–1.52) for the 65–84 years group, and 1.14 (0.92–1.42) for the age over 85 years group (p for interaction <0.01). For worsened disability, similar results were found: 1.16 (1.07–1.24) for the 15–64 years group, 1.38 (1.28–1.48) for the 65–84 years group, and 1.1 (0.9–1.35) for the age over 85 years group. No significant interaction effect was found in ICU admissions (Table 4).

3.3 Sensitivity analysis

We conducted a sensitivity analysis for mortality according to the patients’ initial shock status. The same multivariable logistic regression analysis with interaction terms was used for each subpopulation. For the prehospital shock cohort, AORs and 95% CIs for mortality were 1.29 (1.02–1.62) for the 15–64 years group, 1.26 (1–1.6) for the 65–84 years group, and 1.56 (0.82–2.95) for the age over 85 years group. For the prehospital non-shock cohort, AORs and 95% CIs for mortality were 1.21 (1.12–1.32) for the 15–64 years group, 1.4 (1.29–1.53) for the 65–84 years group, and 1.17 (0.93–1.46) for the age over 85 years group (p for interaction <0.01) (Table 5).