Abstract
Purpose
This study aimed to investigate the efficacy of the keyword method in improving outcomes for patients with unstable circulation.
Methods
Patients with unstable circulation were selected from the Japan Doctor Helicopter (DH) Registry System (JDRS) database. Unstable circulation was defined as systolic blood pressure of less than 90 mmHg when emergency medical technicians (EMTs) contacted the patients. The following details of the dispatch activity were collected from the JDRS database: whether the request for the DH dispatch was made before (Keyword) or after (Control) the EMTs contacted the patient, time from first call to DH contact, age, sex, initial vitals, DH intervention, disease classification, and outcome.
Results
There were 650 subjects in the Keyword group and 1,085 patients in the Control group. All the participants were evacuated from the scene. The duration from the first call to contact, respiratory rate, and the ratio of tracheal intubation to transfusion were significantly smaller in the Keyword group than in the Control group. The average systolic blood pressure and survival ratio in the Keyword group were significantly higher than those in the Control group. After matching the analysis to align with the patients’ backgrounds, the observed tendencies persisted.
Conclusion
This is the first report to describe the keyword method as a prognostic factor in patients with unstable circulation who were evacuated by the DH using the JDRS. Future prospective studies are required to determine the accuracy of these results.
Keywords: shock, doctor helicopter, patient outcome, keyword method, unstable circulation
Introduction
Japan’s Doctor Helicopter (DH) system deploys physicians and nurses to emergency situations during daylight hours (HEM-NET, https://hemnet.jp/en). As of April 2025, 57 DH units were operational across all 47 prefectures. In Japan, DH dispatches are categorized into two main types: rendezvous dispatches for on-site patient retrieval, and interhospital transfers to higher-level medical facilities. Rendezvous dispatches are further divided into two subtypes: those initiated based on patient assessments by emergency medical technicians (EMTs) and those triggered by predefined keywords. The keyword-based system facilitates early DH activation and rapid physician intervention. Our previous study demonstrated that keyword-triggered dispatches were associated with improved survival outcomes in patients transported by DHs1).
Unstable circulation can result from a wide range of conditions, including cardiogenic, distributive (e.g., septic, neurogenic, anaphylactic, or drug-induced), hypovolemic, and obstructive shock. Previous studies have emphasized the importance of early medical intervention to improve outcomes in such cases2,3,4,5). In traumatic hemorrhagic shock, particularly in prehospital settings, such as tactical or austere environments, early bleeding control and hemostatic resuscitation have been shown to reduce mortality2). In cases of cardiogenic shock, severity classification facilitated prognostication and early management, often involving vasopressors, inotropes, and, in some cases, mechanical circulatory support, which served as a bridge to recovery3). For septic shock, early hemodynamic optimization was associated with improved outcomes, whereas in obstructive shock, timely recognition and immediate treatment of conditions such as tension pneumothorax were critical4, 5). However, few studies have specifically examined patients with shock who were transported by physician-staffed helicopters. In Japan, the DH system aims to improve patient outcomes not only through rapid transport but also through early medical intervention at the scene. This study aimed to evaluate the effectiveness of a keyword-based dispatch method in improving the outcomes for patients with unstable circulation.
Methods
We conducted a retrospective analysis of patients with unstable circulation who were transported by DHs under a keyword-triggered dispatch system, using data from the Japan Doctor Helicopter Registration System (JDRS). The protocol for this retrospective study, which involved data analysis using the JDRS, was approved by our institutional review board. All procedures were conducted according to the principles of Good Clinical Practice and the Declaration of Helsinki. The approval number for this study was 733.
The JDRS, managed by the Japanese Society for Aeromedical Services, collected dispatch data from April 2015 to March 2020, including patient outcomes at one-month post-transport6). The JDRS was originally designed to focus on field emergency cases involving stroke, acute coronary syndrome, and trauma with the aim of comparing the outcomes between DH and ground ambulance transport. However, it was later decided that all field emergency cases involving DH dispatches should be registered. Data entry into the JDRS is performed by staff at each DH base hospital in Japan, with each base responsible for its own registration. For patients transported to hospitals other than base hospitals, outcomes were followed up via telephone and written surveys. The outcomes that could not be confirmed were recorded as unknown. The dataset was cleaned and validated by the Japanese Society for Aeromedical Services. These records have been available to base hospitals since December 2022.
In eastern Shizuoka, keywords such as unconsciousness, suffocation, and convulsions prompted the simultaneous dispatch of an ambulance and a DH. However, previous studies have not identified the specific conditions that benefit the most from this keyword-triggered approach1). Notably, the selection of keywords was independently determined by each fire command center responsible for DH requests; currently, a standardized list of keywords is not available across Japan.
First, patients with unstable circulation were selected from the JDRS database. Unstable circulation was defined as systolic blood pressure of less than 90 mmHg when the EMTs contacted the patients. The exclusion criteria included patients with cardiac arrest when the EMTs contacted them and missing cerebral performance category (CPC) data at one month. Then, the following dispatch details were collected from the JDRS database: whether the request for DH dispatch was made before (Keyword) or after (Control) the EMTs contacted the patient; the duration from the first call to when the DH staff contacted the patient; age, sex, and vital signs (Japan Coma Scale [JCS]7), systolic blood pressure, heart rate, and respiratory rate) when contacted by the EMTs; contents of the medical intervention (securing venous route, tracheal intubation, infusion of drugs) by the DH staff; classification of disease causing unstable circulation (exogenous disease other than trauma, trauma, endogenous disease, or unclassified); the final outcome (CPC at one month and the survival outcome: CPC1, good cerebral performance; CPC2, disabled but independent; CPC3, conscious but disabled and dependent; CPC4, vegetative state; CPC5, dead). Endogenous diseases were further classified into the following categories: acute coronary syndrome, aortic disease, other cardiac conditions, sepsis, vasovagal reflex, stroke, gastrointestinal bleeding, non-gastrointestinal hemorrhage, dehydration, anaphylaxis, and other miscellaneous conditions. First, the subjects were divided into two groups: Keyword and Control. Then, variables were compared between the two groups. The primary results assessed were patient survival and functional outcomes as measured using the CPC score. Secondary analyses included a comparison of vital signs between the prehospital phase, when the EMTs first assessed the patients, and upon arrival at the hospital. In addition, Propensity Score Matching (PSM) was performed to adjust for differences in patient backgrounds. A logistic regression model was used to calculate propensity scores, with the presence or absence of the keyword-based dispatch method as the response variable and patients’ vital signs as explanatory variables. The standardized mean difference was used to assess the balance between the matched groups.
Data were analyzed using Wilcoxon’s test for the duration from the first call to contact, age, systolic blood pressure, respiratory rate, and heart rate; the median test compared the JCS scores and CPC data; the χ2 test was used for sex, medical intervention, ratio of patients, and mortality rate; cross-tabulation analysis classified diseases and endogenous diseases. Changes in vital signs between the prehospital and hospital periods were determined using a paired t-test. Statistical significance was set at P<0.05. Data are shown as the mean ± standard deviation or the median with interquartile range. Standardized mean differences were assessed after PSM to confirm the balance between groups.
Results
During the study period, 41,592 patients were registered with the JDRS. Unstable circulation was present in 3,255 patients. After excluding 980 patients who experienced cardiac arrest and 540 patients with missing CPC data, 1,735 patients were included in the analysis. All patients were evacuated directly from the scene and none involved interhospital transport. Among the patients analyzed, 650 were assigned to the Keyword group, and 1,085 to the Control group.
The results of the analysis comparing the two groups are presented in Table 1. There were no significant differences between the groups in terms of age, sex, JCS score, heart rate, drip infusion rate, drug administration, or disease classification. However, the time from the initial call to patient contact, respiratory rate, rates of tracheal intubation and transfusion, and the median CPC score were significantly lower in the Keyword group than in the Control group. In contrast, the average systolic blood pressure and survival rate were significantly higher in the Keyword group than in the Control group. Regarding endogenous diseases, significant differences were observed in the distribution of specific disease categories (Table 2).
Table 1. Results of comparison between the two groups.
| Keyword (n=650) | Control (n=1,085) | P-value | ||
|---|---|---|---|---|
| First call – physician contact (minute) | 30.6 ± 13.8 | 38.4 ± 14.5 | <0.0001 | |
| Age | 65.1 ± 22.9 | 66.9 ± 19.4 | 0.81 | |
| Sex (female/male) | 240/410 | 368/717 | 0.2 | |
| Japan Coma Scale | 2 (0, 10) | 3 (1, 20) | 0.58 | |
| Systolic blood pressure (mmHg) | 78.1 ± 10.6 | 75.9 ± 11.5 | 0.0001 | |
| Respiratory rate (breaths per minute) | 23.6 ± 7.2 | 24.3 ± 7.4 | 0.01 | |
| Heart rate (beats per minute) | 85.7 ± 27.8 | 85.5 ± 31.3 | 0.4 | |
| Tracheal intubation (yes/no) (%) | 46/604 (7.0) | 155/930 (14.2) | <0.0001 | |
| Drip infusion (yes/no) (%) | 596/54 (91.6) | 1,011/74 (93.1) | 0.25 | |
| Transfusion (yes/no) (%) | 0 | 17 (1.5) | 0.001 | |
| Drug administration (yes/no) (%) | 298/352 (45.8) | 504/581 (46.4) | 0.8 | |
| Cardiopressor (yes/no) (%) | 69/581 (10.6) | 122/963 (11.2) | 0.68 | |
| Classification of disease | 0.13 | |||
| Exogenous disease other than trauma (%) | 105 (16.1) | 132 (12.1) | ||
| Trauma (%) | 183 (28.1) | 316 (29.1) | ||
| Endogenous disease (%) | 349 (53.7) | 612 (56.4) | ||
| Unclassified (%) | 13 (2.0) | 25 (2.3) | ||
| Cerebral performance category | 1 (1,1) | 1 (1,2) | 0.006 | |
| Survival/Death (survival rate) | 589/61 (90.6) | 931/154 (85.8) | 0.003 | |
Table 2. Results of comparison between the two groups in endogenous disease.
| Keyword | Control | P-value | |
|---|---|---|---|
| <0.0001 | |||
| Acute coronary syndrome | 43 (12.3) | 107 (17.4) | |
| Aortic disease | 13 (3.7) | 39 (6.3) | |
| Other cardiac conditions | 47 (13.4) | 66 (10.8) | |
| Sepsis | 64 (18.3) | 127 (20.8) | |
| Vasovagal reflex | 57 (16.3) | 41 (6.7) | |
| Stroke | 31 (8.9) | 44 (7.1) | |
| Gastrointestinal bleeding | 23 (6.6) | 73 (11.9) | |
| Non- gastrointestinal hemorrhage | 5 (1.4) | 12 (2.0) | |
| Dehydration | 17 (4.9) | 47 (7.7) | |
| Anaphylaxis | 7 (2.0) | 8 (1.3) | |
| Other miscellaneous conditions | 42 (12.0) | 48 (7.8) | |
| Total | 349 | 612 | |
The results pertaining to changes in vital signs from the prehospital phase to hospital arrival are presented in Tables 3, 4. Compared to prehospital values, systolic blood pressure upon hospital arrival was significantly higher, and the respiratory rate was significantly lower in both groups.
Table 3. Comparison of vital signs between prehospital and at hospital in the Keyword group.
| Prehospital | At Hospital | P-value | |
|---|---|---|---|
| Systolic blood pressure (mmHg) | 78.1 ± 10.6 | 110.8 ± 31.3 | <0.0001 |
| Respiratory rate (breaths per minute) | 23.6 ± 7.2 | 20.2 ± 6.8 | <0.0001 |
| Heart rate (beats per minute) | 85.7 ± 27.8 | 84.3 ± 27.8 | 0.41 |
Table 4. Comparison of vital signs between prehospital and at hospital in the Control group.
| Prehospital | At Hospital | P-value | |
|---|---|---|---|
| Systolic blood pressure (mmHg) | 75.9 ± 11.5 | 109.0 ± 33.2 | <0.0001 |
| Respiratory rate (breaths per minute) | 24.3 ± 7.4 | 21.3 ± 7.5 | <0.0001 |
| Heart rate (beats per minute) | 85.5 ± 31.3 | 84.4 ± 30.2 | 0.33 |
The results of the analysis comparing the two groups after PSM are presented in Table 5. There were no significant differences between the groups in terms of age, sex, vital signs, drip infusion rate, drug administration, or disease classification. However, the time from initial call to patient contact, rates of tracheal intubation and transfusion, and median CPC scores remained significantly lower in the Keyword group than in the Control group. Additionally, the survival rate of the Keyword group was significantly higher than that of the Control group.
Table 5. Results of comparison between the two groups after Propensity Score Matching.
| Keyword (n=519) | Control (n=519) | P-value | Standardised mean difference | ||
|---|---|---|---|---|---|
| First call – physician contact (minute) | 30.3 ± 14.3 | 38.9 ± 15.1 | <0.0001 | 0.59 | |
| Age | 64.6 ± 23.0 | 66.6 ± 19.4 | 0.8 | 0.09 | |
| Sex (female/male) | 195/324 | 167/352 | 0.06 | 0.1 | |
| Japan Coma Scale | 2 (0, 10) | 2 (1, 20) | 0.99 | 0.06 | |
| Systolic blood pressure (mmHg) | 78.3 ± 10.4 | 78.2 ± 10.5 | 0.99 | 0.01 | |
| Respiratory rate (breaths per minute) | 23.6 ± 7.2 | 23.6 ± 6.6 | 0.6 | 0.002 | |
| Heart rate (beats per minute) | 86.3 ± 28.2 | 86.7 ± 31.2 | 0.82 | 0.01 | |
| Tracheal intubation (yes/no) (%) | 32/487 (6.1) | 70/449 (13.4) | <0.0001 | 0.25 | |
| Drip infusion (yes/no) (%) | 475/44 (91.5) | 482/37 (92.8) | 0.41 | 0.04 | |
| Transfusion (yes/no) (%) | 0 | 9 (1.7) | 0.0004 | 0.14 | |
| Drug administration (yes/no) (%) | 241/278(46.4) | 232/287 (44.7) | 0.57 | 0.02 | |
| Cardiopressor (yes/no) (%) | 47/472 (9.6) | 44/475 (8.4) | 0.74 | 0.04 | |
| Classification of disease | 0.35 | ||||
| Exogenous disease other than trauma (%) | 75 (14.4) | 57 (10.9) | |||
| Trauma (%) | 145 (27.9) | 156 (30.0) | |||
| Endogenous disease (%) | 287 (55.3) | 291 (56.0) | |||
| Unclassified (%) | 12 (2.3) | 15 (2.8) | |||
| Cerebral performance category | 1 (1,1) | 1 (1,2) | 0.01 | 0.19 | |
| Survival/Death (survival rate) | 477/42 (91.9) | 447/72 (86.1) | 0.002 | 0.16 | |
Discussion
This is the first report to describe using the keyword method as a prognostic factor in patients with unstable circulation who were evacuated by DHs using data from the JDRS. The present study included many patients with unstable circulation, followed up after one month, and collected data from across Japan.
In this study, the mean contact time by the DH staff was delayed by approximately 8 min in the Control group compared to the Keyword group. This delay may have contributed to the increased need for tracheal intubation and blood transfusion, as the condition of patients with unstable circulation may have deteriorated during that time. Furthermore, the observed differences in vital signs between the two groups suggest that this delay may have influenced patient outcomes. A PSM analysis was performed to address this issue. Although this analysis revealed no significant differences in patient background characteristics between the two groups, except for the time from the initial call to contact with the DH staff, significant differences in patient care and outcomes remained. These findings suggest that delayed contact between DH staff and patients with unstable circulation may negatively affect clinical outcomes. As noted in a previous study, even in cases in which helicopter transport enabled rapid hospital access, unfavorable outcomes still occurred in patients with severe shock8). This study highlights the critical importance of timely medical interventions and rapid transport by helicopter for patients with unstable circulation.
The keywords used by fire departments responsible for DH dispatches in eastern Shizuoka Prefecture included impaired consciousness, asphyxiation, and seizures. Similar keywords have been employed in other prefectures, such as Yamagata, Osaka, Hyogo, Tottori, Hiroshima, and Kagoshima, to facilitate early DH activation9,10,11,12,13,14). Since patients with impaired consciousness may also experience circulatory failure, early circulatory management, followed by airway and breathing support, may improve patient outcomes15). Prompt correction of hypoglycemia can also lead to clinical improvement in patients with impaired consciousness caused by low blood glucose levels16). In cases of asphyxia, effective airway clearance and foreign body removal performed by skilled physicians can improve circulatory status and outcomes17). If not controlled with antispasmodic treatment, seizures can increase the risk of circulatory failure and death. We have previously reported that early dispatch and medical intervention based on keyword activation contributed to improved outcomes in such cases18).
An early request for DH dispatches could allow medical personnel to contact the patient sooner, which could facilitate timely medical intervention for patients with unstable circulation. However, patients with unstable circulation exhibit various symptoms and signs, and their chief complaint is not constant19). Therefore, a significant issue that must be addressed in the future is how to infer circulatory instability based on chief complaints.
This study has several limitations. First, the JDRS did not collect data specifically from patients with unstable circulation. Second, although unstable circulation in this study was defined simply as having a systolic blood pressure of 90 mmHg or lower, such a threshold does not necessarily indicate the presence of organ hypoperfusion. Therefore, it is possible that some patients included in the study were not in shock. Additionally, significant differences in the distribution of specific disease categories within endogenous diseases may have affected the results of this study. Moreover, owing to the non-disclosure policies of the participating hospitals, the researchers were unable to obtain detailed information regarding the treatments provided after hospital arrival, which limited further analysis of patient outcomes. Furthermore, this study did not compare the final outcomes between patients transported by a DH and those transported by ground ambulance. Finally, there were various reasons why paramedics requested DH dispatch after initial patient contact, many of which were not captured by the variables used for the case-mix adjustment in this study. Therefore, the presence of unmeasured confounding factors was a significant limitation.
Further prospective studies are warranted to address these limitations and provide more comprehensive insights into the effectiveness of early DH interventions for patients with unstable circulation.
Conclusion
This is the first report to describe the use of the keyword method as a prognostic factor in patients with unstable circulation who, based on data from the JDRS, were transported by the DH. Future prospective studies are needed to validate these findings and determine which specific keywords should be considered indicative of circulatory failure.
Conflict of interest
The authors have no conflicts of interest to declare.
Funding information
This work was supported in part by a Grant-in-Aid for Special Research in Subsidies for ordinary expenses of private schools from The Promotion and Mutual Aid Corporation for Private Schools of Japan.
Ethics approval
The research protocol was approved by the Juntendo University Review Board (approval number: 733).
Consent for publication
The authors consent to the publication of this article in the Journal of Rural Medicine.
Author contributions
All authors have made substantive contributions to the study.
Data availability statement
Permission from the Japanese Society for Aeromedical Services is required.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
Permission from the Japanese Society for Aeromedical Services is required.