The clinical relevance of plasma potassium abnormalities on admission in trauma patients: a retrospective observational study

Study design

This study was a retrospective observational study conducted at the Fukuyama City
Hospital. The Fukuyama City Hospital is a community hospital with 506 beds, located
in the eastern area of Hiroshima prefecture in Japan, with an Emergency Medical Center
corresponding to the tertiary trauma center in this area. This study was approved
by the Institutional Review Board of the Fukuyama City Hospital.

Patients

All trauma patients with neither cardio-pulmonary arrest on arrival nor burn injury
were eligible if they had been transported to the Fukuyama City Hospital between January
1, 2010 and December 31, 2013 and if their Injury Severity Score (ISS) was greater
than 15 13].

The patients were excluded if they met the following criteria: (1) were transported
to the Fukuyama City Hospital after 24 h or later from the injury onset, (2) the dates
when they were injured were unknown, (3) the serum creatinine values on admission
were greater than 2 mg/dL, or (4) a blood gas analysis was not performed on admission.

Demographic and clinical information

Information was obtained from the hospital medical records and our database of trauma
patients conforming to the Japan Trauma Data Bank and included the following: age,
sex, type of trauma, situation of transport, vital signs on admission, laboratory
values, injury profile according to the Abbreviated Injury Score (AIS), ISS, and therapeutic
interventions, including surgery 14], 15].

Scoring for trauma severity

To assess the severity of trauma, the following scores were coded and calculated by
trained assistants and physicians. To assess the anatomical severity, AIS was coded
by the AIS90 update98 and ISS was calculated 15]. To assess the physiologic severity, the Revised Trauma Score (RTS) was calculated
according to the vital signs on admission 16]. Using the method of the Trauma Score and the Injury Severity Score (TRISS), we estimated
the probability of survival to describe the study population 17].

Measurement of potassium and other values

In the trauma medicine department at the Fukuyama City Hospital, blood sampling is
typically performed on admission for the analysis of blood gas, laboratory tests and
quick preparation for blood transfusions. The analysis of blood gas was performed
with ABL700 (Radiometer, Copenhagen, Denmark), and we obtained the following values
of the plasma from this analysis: potassium, sodium, pH and bicarbonate. The serum
creatinine level was measured in the Hospital Central Laboratory Division with LABOSPECT
008 (Hitachi High-Technologies Corporation, Tokyo, Japan).

Outcome

The primary outcome was death during the hospitalization from all causes. The secondary
outcome was the occurrence of a life-saving intervention (LSI). According to the current
literature available on the epidemiology of trauma deaths, we categorized the LSI
into 2 groups, the LSI for neurological emergency and the LSI for bleeding 18], 19].

The LSI for neurological emergency was defined as craniotomy. The LSI for bleeding
was defined as intra-aortic balloon occlusion, transcatheter arterial embolization,
massive transfusion, thoracotomy or laparotomy. The transcatheter arterial embolization
and surgeries in the LSI were defined as those performed within 24 h of arrival. The
intra-aortic balloon occlusion was defined as that performed between the arrival and
start of hospitalization. A massive transfusion was defined as a transfusion of 10
or more units of packed red blood cells within 24 h of arrival.

Statistical analysis

Continuous variables were assessed for graphically normality and are described as
the mean (standard deviation) or median [inter-quartile range]. Categorical variables
are described as n (%). To adjust for the severity of trauma, the RTS was divided
into two groups (4 and ?4) and the ISS was divided into three groups (16–24, 25–40,
and 41–75) 16].

The normal range of potassium was set between 3.5 and 5.0 mEq/L for describing the
distribution of the potassium disturbance. Additionally, six groups of different potassium
levels by 0.5 mEq/L was created to investigate the association of potassium concentration
with the clinical characteristics and outcomes (3.0, 3.0–3.5, 3.5–4.0, 4.0–4.5,
4.5–5.0, and ?5.0 mEq/L).

We used the following tests to compare the characteristics between the six potassium
groups: Fisher’s exact test for categorical variables, the Kruskal-Wallis test for
continuous variables without normality, and a one-way analysis of variance for continuous
variables with normality. We used a logistic regression analysis to examine the association
between the potassium levels and outcomes while adjusting for covariates, including
age, gender, the RTS, and the ISS. In this logistic regression analysis, the potassium
concentration of 3.5–4.0 mEq/L was set at the reference group. The results of the
logistic regression analyses were reported using the odds ratio (OR) and 95 % confidence
interval (CI).

All tests were two sided, and statistical significance was considered to exist at
p??0.05. All analyses were performed using the R software package (version 3.1.1).