Helicopter inter-hospital transfer for patients undergoing extracorporeal membrane oxygenation: a retrospective 12-year analysis of a service system

The study protocol was approved by the Ethics Committee of Eastern Switzerland (EKOS 21/064, St. Gallen, Switzerland), and due to the retrospective study design and anonymised character of the data, the need for informed consent was waived by the Ethics Committee. All of the patient data were extracted from the Swiss Air-Rescue information system, and anonymised into an electronic research database. The study was performed according to the Declaration of Helsinki and the Swiss Act on Human Research. The reporting follows the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines [15].

The Swiss tertiary hospitals in Bern, Geneva, Lausanne, St. Gallen and Zurich offer an ECMO retrieval service, with the availability of cardiovascular surgeons or intensivists and perfusionists. The helicopter crew consists of a pilot, a flight paramedic and a board-certified specialist in anaesthesia or intensive care medicine who has additional pre-hospital emergency medicine training. There is no change in staffing composition and seniority regarding the time of day. The helicopter fleet for ECMO transfers comprises Airbus H145, all are equipped with avionics permitting night operations with and without night vision goggles under visual flight rules, but also under instrument flight rules. The standard ECMO device used was the Cardiohelp System (Maquet, Rastatt, Germany), and the helicopter was equipped with a custom-made fixation plate (Getinge, Rastatt, Germany) to avoid dislocation of the device during the flight. According to the ELSO guidelines [14], we defined primary missions when ECMO was implanted in the referral hospital by the retrieval ECMO team. In such cases, the HEMS crew was accompanied by a cardiovascular surgeon and a perfusionist. Secondary missions were defined as transport of patients with ECMO installed, and the HEMS crew was accompanied only by an additional perfusionist.

All patients undergoing ECMO who were transferred by helicopter from 1 February 2009 (start of service) until 30 April 2021 were included. Missions that only transported the ECMO team without a patient, missions without medical records available and non-helicopter transfers were excluded.

The patient data were extracted from the mission transport protocols, which included demographic data (age, sex), medical history and treatment (diagnosis, reason for ECMO, type of ECMO, medication, and additional devices), mission details (flight distance, flight-time, total mission time). Flights between 06:00 h and 20:00 h were classified as daytime missions, flights between 20:01 h and 05:59 h were classified as night-time missions. Mission times were defined as:

The main medical diagnoses were coded according to the 2019 World Health Organisation International Classification of Diseases (ICD-10) [16]. The reasons for ECMO were categorised into four main categories by the authors based on the main medical diagnoses of: (a) pulmonary, (b) cardiac, (c) combined, or (d) other. Two of the authors (AF, RS) screened the transport protocols independently for documented adverse events during the inter-hospital transfer, and investigated potential non-documented adverse events (e.g., sudden change in heart rate, blood pressure, intravenous fluids, peripheral oxygen saturation, or ECMO flow). The adverse events were further classified into medical (e.g., medical condition or treatment associated) and non-medical (e.g., organization or weather-related). Patient survival during the transport was documented for all patients in the transport protocol. Follow-up data were available according to the patient medical records for 28-day survival or Intensive Care Unit discharge, and survival to hospital discharge was also recorded. The primary endpoint of the study was the survival of the patient during the transfer, and the secondary endpoints were adverse events during the transfer and 28-day survival.

Statistical analysis was performed using R environment version 4.0.2 [17]. Due to the observational character of the study, no formal sample size calculation was performed. Continuous variables were summarised by median and the first and third quartiles (Q1, Q3, respectively), if skewed. Data was inspected for normality using histograms and formally tested using the Shapiro–Wilk tests. Normally distributed data was summarized by mean and standard deviation. Categorical variables were summarised by counts and percentage for each level of the variable, and compared using Pearson’s chi-squared tests. Continuous variables were compared using student’s t-tests if they were normally distributed, or using Kruskal–Wallis rank tests if they were skewed. P-values are two-sided with an α-level of 5%.

To analyse the factors associated with complications during transportation and the in-hospital mortality, multivariable logistic regression models were built that included the variables daytime (binary; from 06:00 h to 20:00 h), sex, age (years), total mission time (minutes), type of ECMO (veno-venous vs. veno-arterial or veno-venoarterial). Veno-venoarterial was added to the veno-arterial group due to the low number of patients (n = 2), and diagnosis group (pulmonary vs. cardiac vs. combined vs. other). Multicollinearity was checked by correlation coefficients. Multicollinearity was accepted up to a correlation of coefficients of 0.7. Multicollinear variables were eliminated based on clinical decisions. Interactions between covariates were tested and removed if they did not improve model fit.

Global positioning system coordinates were used for geographic plotting and analysis of the direct distances between the hospitals (as the approximation for flight distance). The topographic relief was visualised using swisstopo data from 2016 [18].

There were 37 (19.4%) missions with adverse events, of which 7 (18.9%) had more than one event, as given in Table 3. Of these adverse events, two-thirds were considered medical (26, 70.3%), and one-third non-medical (11, 29.7%). Nine (29.1%) of the adverse events were considered to be ECMO related. Two-thirds (6, 66.6%) of these were observed during insertion (e.g., difficult cannulation due to hypovolaemia, severe bleeding with the need for vascular suture), and only one-third (3, 33.3%) during transport (“suck-down” of the ECMO, solved by either reduction of ECMO flow or administration of additional fluids). The medical events were more frequent during the take-over of the patient (e.g., change of syringe pumps). Non-medical events arose from weather-related delays (e.g., landing or take off not possible) and organisational issues (e.g., missing equipment, missing support to unload the patient from the helicopter). We did not observe any adverse events in the sub-cohort of patients under 18 years of age. Adverse events occurred more frequently during night-time missions (16, 28.1%, vs. 21, 15.7% during daytime; p = 0.047), as given in Table 3. This was not further confirmed in the logistic regression model, as shown in Fig. 3 and given in Table 4.

Table 3 gives the survival and follow-up data. All 191 patients survived the transport. Follow-up data for survival to 28 days after the transport or to Intensive Care Unit discharge were available for 157 patients (82.2% of total). The majority of these patients (86, 54.8%) were still alive, including 14 (14/32, 43.8%) of the patients with ECPR. Follow-up data for survival to hospital discharge were available for 136 patients (71.2% of total). Sixty-three of these patients (46.3%) were still alive, including 11 (11/28, 39.3%) of the patients with ECPR. The logistic regression model indicated a higher odds ratio for 28-day survival for patients with a veno-arterial ECMO (2.87; 95% confidence interval, 1.35–6.25; p = 0.006), as given in Table 5 and shown in Fig. 4.

Audits and analysis of adverse events are crucial to improve the operation of clinics and case management and flight logistics, with the aim to guarantee patient safety during these highly specialised services [7]. Our findings of rare adverse events during inter-hospital transfers for these ECMO patients are in-line with previously published incidence reports of adverse events, and they underline the overall safety of the system [10, 11]. All observed ECMO related adverse events are among the known complications for ECMO [1, 2], regardless of the need for a medical transfer. However, HEMS crews need to be aware that these are potential risks also during transfer, to be best prepared and ensure an improved safety culture. We also confirmed that where one adverse event happens, there can also be a second one [10]. Adverse events tended to happen more often during the night-time inter-hospital transfers. This might be explained as reduced cognitive performances [19] and environmental circumstances during the night-time (e.g., darkness), with increased difficulty for efficient trouble management. These circumstances might be trained for in high-fidelity training centres, as already reported for helicopter missions with human external cargo [20]. The logistic regression model indicated night-time missions are not to be associated with more adverse events. Hence, from a clinical point of view, it might be that during night-time, the relatively more critical patients might be transferred, and that the patient condition might lead to an accumulation of adverse events [21]. Organisation of such transport is highly complex, especially for primary missions where fast intervention times are of importance. Some organisational adverse events were identified as communication errors (e.g., patient weight not requested, additional devices not announced). Other challenges of the system here were the compatibility of different ECMO types and additional devices. Indeed, the transport of a patient with ECMO and simultaneous treatment with an intra-aortal balloon pump can only be provided by one specific helicopter base.

The feasibility, quality and ability to deliver a 24-h/7-days per week helicopter flight service was confirmed by the similar durations of the mission responses, handover times and total mission times, as well as by the similar flight distances between daytime and night-time missions. Taking the specific geographic circumstances of Switzerland into account, with its high Alpine mountains, deep valleys and large numbers of lakes, ground ambulance transfer needs substantially more time even for short distances [22], and therefore the use of a helicopter is reasonable, as recommended by the ELSO [14].

In contrast to other transfer systems, the transfer service here that is operated from different bases is embedded in the well-functioning pre-hospital emergency medical system. This system minimises helicopter maintenance expenses and costs for special trained crews, who are solely deployed for ECMO inter-hospital transfers. The helicopters are staffed with advanced HEMS physicians. These are trained to ensure a continuous evaluation and appropriate treatment of the patient during the mission. Considerations regarding the operational environment (e.g., high altitudes) and the potential influence on medical equipment (e.g., need for recalibration) are part of the training.

However, we can also report here some limiting factors for these inter-hospital transfers through the helicopter system. One patient with a bodyweight of 180 kg could not be loaded into the helicopter due to exceeding the permissible total weight of the load. In this case, the usual inquiry in advance of the patient body weight failed, and he was finally transported by ground ambulance services. Therefore, this case was excluded from the analysis. One primary mission had some time delay because of bad weather conditions during the arrival landing. Therefore, the helicopter could not land on top of the hospital, and instead landed close by, with the team then carried by ambulance on the ground to the final destination. Nevertheless, the final air-medical transport of the patient was without further problems. The weather is a possible system-related factor for ECMO transfer by helicopter, even though this was of low incidence in our analysis and might be generalised to all HEMS missions. These limitations justify the provision of a ground ambulance transfer system as a backup whenever a helicopter transfer is not possible due to such weather-related or organisational issues.

Patient survival under ECMO treatment in the literature has been reported in a relatively heterogeneous manner, which might be explained by various factors, such as age, case mix and ECMO type. The 28-days survival for the present cohort was a little higher than what was reported in a systematic review [2]. In this review, most patients had a veno-arterial ECMO, while the ECMO type in the present cohort was more homogeneously distributed. Roch et al. reported a rate of 44% hospital survivors in patients transferred on ECMO with acute respiratory distress syndrome [23]. Of the total 142 patients considered for ECMO by the mobile team, only 85 were finally treated with ECMO, and 91% had a veno-venous ECMO. As might have been expected, the study identified age, sepsis-related organ failure assessment score and a diagnosis of influenza as factors in the evaluation of risk of death. Biscotti et al. reported on 100 transported patients with a relatively high survival to 30 days (71%), where 79% of the patients were treated with veno-venous ECMO [9]. Bryner et al. also reported a relatively high survival rate of 70% for patients with respiratory indications and 50% for patients with cardiac indications who were transferred on ECMO [12].

Interpretation of the survival rate in the present study needs to be careful. First, there is the possibility of a selection bias of the patients; e.g., the sicker patients are transferred. According to the medical records of some of the patients, withdrawal of ECMO treatment was decided shortly after transfer to the tertiary centre. These decisions require experience, extended neuro-functional diagnostics, and finally interdisciplinary consensus, which might not be available in all referral hospitals.

The present cohort included 35 patients with ECPR. Traditionally, survival rates for patients with an in-hospital cardiac arrest treated by conventional cardiopulmonary resuscitation is reported to be around 35% [24], and 10% with an out-of-hospital cardiac arrest [6]. A systematic review of patients with out-of-hospital cardiac treated with ECPR reported survival rates up to 18% [6]. The reported survival rates for patients with ECPR in the present cohort are slightly higher than the survival rate for adults of 41% given in the ELSO registry [4]. However, a systematic review showed that patients who were cannulated with ECMO during refractory cardiac arrest had better neurological survival with protocols using a strict time cut-off from the start of resuscitation to the start of ECMO [25]. In this report, for at least one patient in normothermia, ECMO was only started 2 h after the collapse (1 h pre-hospital, and an additional hour of in-hospital conventional cardiopulmonary resuscitation).

An obvious limitation of the present study is the retrospective character of this analysis, with a possible under-reporting bias of adverse events. Due to the low overall number of adverse events, there is the possibility of random error in the interpretation of the data. Poor data entry occurred due to the heterogeneous medical data documentation of ECMO case details in the transport form, which was performed on the standard helicopter transport form that does not have specific ECMO-related items, like flow, pressure changes, and oxygenator settings and placing of cannulas. Our presented analysis of survival rates might have a rather descriptive character. Findings should be carefully interpreted and might not explain causality. However, as a result of this study, the Swiss Air-Rescue have developed a specific ECMO medical data recording form. Follow-up data on survival were reported with the existing medical reports in the database, which were missing for many cases. For many patients with ECPR, it remained unclear if they had had an in-hospital or out-of-hospital cardiac arrest.