Incidence and Challenges of Helicopter Emergency Medical Service (HEMS) Rescue Missions with Helicopter Hoist Operations: Analysis of 12,000 Daytime and Nighttime Missions in Switzerland

Objective We aimed to investigate the medical characteristics of helicopter hoist operations (HHO) in HEMS missions. Methods We designed a retrospective study evaluating all HHO and other human external cargo (HEC) missions performed by Swiss Air-Rescue (Rega) from 1 January 2010 to 31 December 2019. Results During the study period, 9,963 (88.7%) HEMS missions with HHO and HEC were conducted during the day, and 1,265 (11.3%) at night. Of the victims with time-critical injuries (NACA ≥ 4), 21.1% (n=400) reached the hospital within 60 min during the day, and 9.1% (n=18) at night. Nighttime missions, a trauma diagnosis, intubation on-site, and NACA Score ≥ 4were independently and highly signicantly associated with longer mission times (p<0.001). The greatest proportion of patients, who needed hoist or HEC operations in the course of the HEMS mission during daytime, sustained moderate injuries (NACA 3, n=3,731, 37.5%) while practicing recreational activities (n=5,492, 55.1%). In daytime HHO missions, the most common medical interventions performed were an insertion of a peripheral intravenous access (n=3,857, 38.7%) and administration of analgesia (n=3,121, 31.3%). the only adequately trained and crew members should HHO


Introduction
Involvement of a helicopter emergency medical service (HEMS) can signi cantly shorten rescue times, especially in mountaineous areas, and improve the outcome of severely injured patients (1,2). Due to the challenging terrain in the mountains, landing the helicopter is not always possible, and alternative patient evacuation methods must be used. Helicopter hoist operation (HHO) to extricate patients in di cult terrain is a common operational rescue technique in mountain emergency medical services (MEMS) if landing at the scene of the incident is not possible. This helicopter procedures enable both immediate professional medical care onsite and an immediate safe evacuation of the patient to ensure further outcome-relevant timely treatment at an appropriate hospital. Only approx. 11% of all HHO missions take place during the night. Thus, there is less experience in night HHO missions per se. Nonetheless, severe incidents or accidents in Rega's HHO missions day and night have not been reported in the observation period.
The topic of HHO in HEMS missions is very rarely addressed in the scienti c medical literature (3)(4)(5). We do know, however, that nighttime imposes, due to reduced visible cues, an additional, but manageable, risk for HEMS operations. To improve our understanding of HHO in HEMS, especially in night operations, we investigated the medical characteristics of HEMS missions with HHO.

Data and Ethics
We conducted a retrospective study of all consecutive HHO missions performed by Swiss Air-Rescue (Rega) from January 1, 2010, to December 31, 2019. Data were extracted from Rega's HEMS staff mandatory electronic medical record system (SAP database) and an additional chart review was conducted.

Setting and Population
In Switzerland, ve organisations provide 24/7 physician-staffed HEMS operations and carry out primary (pre-hospital retrieval; about 2/3) and secondary (inter-hospital transfer; about 1/3) missions. Rega, the largest of these organisations with more than 88,000 HEMS missions in the observation period, provides operations for and in all cantons, and for some regions in neighbouring countries. Rega operates 12 helicopter bases, located throughout Switzerland as well as a partner base in Geneva, and can reach any location in the operational area within 15 minutes ight time day and night, provided the respective weather conditions are met. Since August 2019, it also has a training base in Grenchen, in Canton Solothurn. The helicopter eet comprises seven Airbus H145 at the midland bases in Zurich, Basel, Bern, Lausanne and St. Gallen, and 11 AgustaWestland AW109SP "Da Vinci" helicopters (performanceenhanced version for Rega of the AW109S Grand) at the alpine bases in Untervaz, Locarno, Erstfeld, Samedan, Wilderswil, Mollis, and Zweisimmen. With Rega's helicopters more than 11,000 HEMS missions are conducted per year and all are equipped with a certi ed rescue hoist and avionics that permit night operations with and without night vision goggles (NVG) under visiual ight rules (VFR) but also under instrument ight rules (IFR), including state-of-the-art satellite-based high-precision procedures to and from major hospitals and HEMS bases to provide relevant medical care to patients also in bad visibility.
In Switzerland, the HEMS crew includes a pilot, a HEMS physician and a paramedic, who serves as technical crew member and hoist operator. Inter alia, the requirements for HEMS physicians are a boardcerti cation in anesthesiology and a certi cation in pre-hospital emergency medicine. Several HEMS physicians hold additional certi cations in intensive and critical care medicine and/or mountain emergency medicine. In missions, when challenging terrain is expected, a rescue specialist with basic life support education is added to the crew on board. The HEMS physician is either winched down to the site rst or after the rescue specialist's initial safety assessment of the environment and situational circumstances.

De nitions and Statistics
We analysed mission characteristics including mission duration, time of day, season (6), the National Advisory Committee for Aeronautics score (NACA) (7), and the medical interventions performed on scene (e.g., intubation). Nighttime was de ned according to the European Union Aviation Safety Agency (EASA) (8) as the period between the end of evening civil twilight and the beginning of morning civil twilight.
Continuous variables were summarised by mean ± standard deviation if normally distributed, or by median and interquartile range if skewed. Normality was tested using the Shapiro-Wilk test. Categorical variables were summarised with counts and percentages for each level of the variable. Changes in the number of missions per year were assessed by linear regression, and the total number of missions per base type was compared using Pearson's Chi-squared test. The Wilcoxon-Mann-Whitney test was used to assess differences in the duration of daytime and nighttime HHO missions. To further investigate factors that are potentially associated with a prolonged duration of HHO missions (mission time was de ned as the time between an emergency call and arrival at the hospital), a multivariable linear regression model was built including the binary variables intubation, daytime/nighttime, and trauma versus medical diagnosis as well as the NACA score as a factor variable. To obtain a more homogeneous sample, unharmed patients (NACA 0) were excluded from this analysis; minor to moderately injured patients (NACA 1-3) were merged; and deceased patients (NACA 7) were excluded, as rescue missions differ substantially for these patients and there was not a clear end-of-mission time point de ned for a substantial proportion of these patients. Two-sided p-values of <0.05 were considered statistically signi cant. All statistical analyses were performed using R Studio 3.6.0 on macOS 10.15.4.

Number of HHO Missions in Switzerland
During the study period, 88,213 HEMS missions were recorded; 11,228 of which were registered as HHO missions. The majority of HHO missions (9,963; 88.7%) were conducted during the day. There were 1,265 (11.3%) nighttime missions ( Table 1), most of which took place before midnight (n=1,050, 83%). All patients were winched up accompanied by either a rescue specialist or the HEMS physician. All patients and rescuers safely boarded the helicopter, without any procedure-related injuries or other adverse events to patients or crew members, as recorded for observation period.

Regional Distribution
There was a signi cant increase in the total number of HHO missions over the study period for all types of HEMS bases (i.e., lowland, intermediate, alpine; Figure 2). The number of HHO night missions did not signi cantly increase over the study period (Figure 3), but there was a positive tendency (i.e., positive regression coe cients for alpine and intermediate bases). The total number of HHO missions and the number of nighttime HHO missions was signi cantly higher in alpine bases compared to intermediate or lowland bases (p<0.001 for both comparisons).

Mission Duration
The overall median time from emergency call to landing at the hospital in a HHO mission was signi cantly shorter during the day compared to the night as well (67 min; IQR 54 to 83 min versus 83 min; IQR 73 to 129 min, p < 0.001) ( Table 1). Of the victims with time-critical and possibly life-threatening injuries (NACA ≥4), 21.1% (n=400) reached the hospital within 60 min in the daytime, and 9.1% of patients (n=18) reached the hospital within 60 min during the night. A trauma diagnosis, night missions, intubation on-site, and NACA Score ≥4 were independently and highly signi cantly associated with longer mission times (p <0.001 for all variables in univariate and multivariate analysis) (Table 3, Figure  1). In the univariate analysis, intubation prolonged the overall mission time by roughly 27 min. Multivariate analysis revealed that intubation itself is only accountable for an additional 13 min when adjusted for trauma diagnosis, night mission, and NACA score. The other variables (night mission, trauma diagnosis, and NACA score) were less affected by the multivariate adjustments.

Characteristics of HHO Missions and Medical Condition of Patients
About half of the HHO missions taking place at night in this study were performed for uninjured patients (NACA 0, n=610, 48.2%), whereas during the daytime only 22.7% (n=2,259) were uninjured. Most HHO missions during daytime were due to winter or summer sport-related injuries of moderate severity (NACA 3, n=3,731, 37.5%). The greatest proportion of patients requiring hoist operations during the daytime were practicing recreational activities in the mountains during the summer (hiking, mountaineering, climbing, etc.) (n=5,492, 55.1%), whereas 975 patients (9.8%) were doing winter sports such as skiing, snowboarding or free riding. Road accidents accounted for 1,388 (14.0%) evacuations by HHO (Table 1).
A substantial number of patients in HEMS missions with HHO during day or night were dead on arrival on scene or died on scene (NACA 7, n=859, 8.6%, and n=106, 8.4%, respectively). Return of spontaneous circulation (ROSC) could be achieved in 5% (n=56) of patients with cardiac arrest. Trauma victims were in signi cantly worse conditions according to the NACA score compared to medical patients (p <0.001, Chi2test).

Discussion
This study of 11,228 HHO rescue missions performed day and night and is the largest known study to date. Our data show that HHO missions in Switzerland occur frequently, even at night. Although most of the patients evacuated by HHO had no or minor injuries, almost one fth was in severe condition, with NACA scores between 4 and 6, and in many cases advanced medical interventions had to be performed at the scene before HHO evacuation. Night missions, a trauma diagnosis, intubation on-site, as well as NACA Score ≥4 were independently and highly signi cantly associated with longer mission duration. Nevertheless, the aforementioned factors increase in general the mission time, regardless of a hoist mission or not. With regard to Rega's additional safety procedures for night ight operations, there is a natural increase of the mission time owed to ight and patient safety. These operational safety procedures are usually mitigating measures to address the operational risks resulting from with the lack of daylight and the subsequent natural de ciencies of the human eye in dark environments. Flight and patient safety must never be compromised. Thus, a safety compromise for a potential bene t to save 2 or maybe 4 minutes is not an eligible or recommendable option.

Need for HEMS Crews with Advanced Skills
A relevant observation in our study is that the proportion of severely injured patients (NACA 4-6) is similar in daytime and nighttime HHO missions. This emphasises the need for HEMS teams with advanced critical care capabilities 24/7, and adds weight to the discussion of personal skills in HEMS services (9). In our study, the condition of trauma vs. medical victims was more critical judged by the NACA score (p <0.001). This nding contradicts a previous Swiss study which showed that in HEMS, patients with medical emergencies had higher NACA scores than trauma patients (10,11). A possible explanation is an overall predominance of trauma in our study population, due to the fact that the greatest proportion of patients in need of HHO rescue are practicing recreational activities in the mountains.
We found that most of the basic medical interventions we provide -such as vascular access (n=4,125; 36.8%), analgesia (n=3,323; 29.6%) and immobilisation (n=2,179; 19.4%) -were performed on the scene and before the HHO procedure. In 425 patients (3.8%), advanced critical care interventions (cardiopulmonary resuscitation, ventilation, rapid sequence induction, endotracheal intubation, pleural decompression) had to be performed urgently due to immediate life-threatening conditions such as cardiac arrest, acute respiratory failure, cardiocirculatory collapse, or pneumothorax. These ndings are in accordance with previous reports and again emphasize that the medical team being involved in the HHO rescue missions must be able to perform the entire spectrum of life-saving emergency procedures in often extremely di cult environmental conditions, and with limited skilled human resources (12,13). In Europe, primarily anesthesiology and intensive care medicine physicians have the experience needed to perform these invasive procedures safely (14). Health systems in other countries may have different legal settings that render other specialties or professions more relevant for HEMS sta ng, but the goal should always be to provide the highest level of care possible.
There are some studies analysing prehospital times in alpine HEMS (5,15,16). Several factors are discussed, which potentially contribute to longer time intervals for alpine rescue compared to urban or suburban rescue missions. These include the necessity for complex HHO rescue procedures to gain access to the patient, bad weather conditions, and longer ight times from remote areas to level 1 trauma centres. Recent data support conducting medical interventions prehospital rather than upon hospital arrival (15,17). This might be even more important in rescue missions with in general longer pre-hospital times such as alpine or night HEMS missions.
Data from the international alpine trauma registry show that prolonged pre-hospital times are not necessarily harmful per se, an argument supported by data showing that in-hospital mortality in trauma patients transported from alpine regions is comparable to that of patients from urban areas, despite longer pre-hospital times and higher injury severity scores (15,18). Furthermore, Kulla et al. reported that the overall time from the accident until the end of emergency department (ED) treatment is equal for severely injured patients undergoing procedures such as endotracheal intubation and pleural decompression, regardless of the location (pre-hospital or in-hospital) at which these interventions are performed (16).
We found a signi cantly higher proportion of HHO missions in the alpine HEMS compared to the intermediate and lowland bases. Additionally, there was a tendency towards an increase in HHO mission volume over the 10 year study period in the alpine HEMS bases. Both ndings could be connected to an increase in recreational activities in the mountains, and more extreme and more remote leisure behaviour over time.

Strengths
This is the rst study analysing >10,000 HHO missions including data of night missions (3,11,19).

Limitations
Our study has limitations inherent to a retrospective chart review, as data quality depends on documentation quality. Second, we were unable to validate the pre-hospital diagnosis made by the HEMS team, or to determine in-hospital outcome because of the lack of related hospital follow-up in our database. Finally, composition of HEMS crews and legal aspects elsewhere may have an impact as well.

Conclusions
Nearly 20% of patients, who needed to be evacuated by a hoist, were severely injured, and complex and lifesaving medical interventions were necessary before the HHO procedure. Therefore, only adequately trained and experienced medical crew members should accompany HHO missions.