Introduction

Air transport of critical patients by Helicopter Emergency Medical Services (HEMS) is more efficient than by other means of transport in terms of the resolution of urgent and emergency situations, as it allows access to complex geographical locations and also provides a safe transfer in adverse weather conditions in a minimum amount of time. Compared to other medical transports, it even reduces morbimortality during transport, especially in time-dependent pathologies.1

The main objective of helicopter transport is the rapid transport of a patient to a hospital that has the necessary resources to optimise care; in this sense, it is vital to ensure fluid communication between the receiving entity, the HEMS, and the Emergency and Urgent Care Coordination Centre to guarantee the availability of human and technological resources for the care of the patient.2

HEMS, fly with rules, the Cat A take-off profile is the type of take-off that medical helicopters have to comply with the JARPOPS3 regulations, governing the sanitary flight regulations, being a vertical exit with a small backward deviation from the verticality at take-off. On other continents EC-145, EC-135, Leonardo 109 and Leonardo 139 are the usual models, since it has been seen that the manoeuvrability.3

HEMS provides first responders, ie on-site emergency care, in the majority of cases worldwide (81.1%). The most common emergencies involve traumas and cardiovascular conditions. Only 18.9% are inter-hospital transfers.4 Among the common users of this service, it could include cardiovascular, respiratory and trauma patients.4,5

The need for inter-hospital transfer of intensive care patients has been gradually increasing in recent years. When local therapeutic resources for complex treatments are limited, or when specialised patient care is indicated, inter-hospital transfer is recommended to facilitate the most appropriate treatment for the patient. For optimal outcomes, transfers should be performed as early as possible in the disease/injury process, while adhering to a high level of standards of care.5

When providing health care during the transport, whether pre-flight, in-flight, or post-flight, it is necessary to bear in mind two factors. On the one hand, the haemodynamic destabilisation derived from the patient’s own pathology and, on the other hand, that derived from the air transport itself, which includes climatological factors, temperature variations, changes in atmospheric pressure, noise levels, vibration, and acceleration.6,7

These factors are described as physically stressful impulses, thus making the care of air transport pathophysiology unpredictable To ensure stable conditions to prevent haemodynamic destabilisation of patients, a variety of nursing care procedures is required, which in turn implies a high degree of specialisation.8–10

Patients transported by helicopter may suffer haemodynamic changes as a consequence of the HEMS transport itself, as it exposes the patient to physical situations that may predispose them to physiological alterations, accounting statistically for 13% of the morbidity.9 The physiological functions that may be altered are: heart rate, blood pressure, oxygen saturation percentage, and their emotional state. This gives rise to a direct relationship between the haemodynamic changes experienced by the patient and the pathophysiology of their transport.11–14

Several publications show the benefits of medical helicopter transport compared to other types of transport, is spite of the fact that haemodynamic changes may occur during transport that will require to act accordingly. However, not many studies have been found describing haemodynamic changes in helicopter transport.

In this context, the objective of this review was to identify, synthesise, and assess the existing literature on haemodynamic changes in adult patients transported by HEMS.

Methods Study Design

A literature review was conducted on haemodynamic changes during medical helicopter transport using the systematic review format by following the criteria of the updated PRISMA 2020 (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines.15 The implemented protocol has been registered in the International Prospective Register of Systematic Reviews (PROSPERO) with code CRD42023470716.

Search Strategy

The formulation of the research question was based on the standardised PECOT structure (Table 1).

Table 1 PECOT Format and Key Words

Research Question: What haemodynamic changes occur in adult patients transported by air in helicopters?

The following Medical Subject Headings (MeSH) descriptors were used to create the search string: Helicopter Emergency; Hemodynamics; and Transportation of Patients. In order to maximise the search, free terms were added to the search using the Boolean operators AND and OR (Table 2).

Table 3 shows the search process conducted on April 2023 in various databases (Pubmed, Scopus and Web of Science Complete) and the different search strings, filtering from January 2013 to April 2023.

Table 2 Terminology Used in the Search Strategy

Table 3 Search Strategy in the Different Databases

Selection Criteria

The following inclusion and exclusion criteria were used for the selection of the articles:

Inclusion Criteria Articles published in English, Spanish, French, and Portuguese. Articles carried out between 2013 and 2023. Typology: descriptive studies, correlational studies, cohort or case-control studies. Articles assessing the following indicators: transport of adult patients by helicopter; in-flight haemodynamic changes such as blood pressure, oxygen saturation, and heart rate; in-flight, pre-flight, and post-flight assessment.Exclusion Criteria Articles of low methodological quality after assessment applying the quality assessment tool. Articles on research involving pregnant women and children. Not related to the objective. Articles on military helicopters.Data Collection and Extraction

The search was conducted independently by two reviewers using the agreed descriptors and the combination of Boolean operators indicated in the search strategy. Subsequently, the articles were read and selected according to the inclusion criteria and applying the exclusion criteria. In case of disagreements during the study selection and data extraction processes, a third reviewer acted as mediator to reach to a decision.

Methodological Quality Assessment

The assessment of methodological quality using a critical appraisal tool was carried out independently by both reviewers who applied the Joanna Briggs Institute (JBI) tool for studies at the University of Adelaide.16 The use of this tool allows the assessment of the methodology used in the research by identifying the absence of bias in its design, procedure, or analysis. In the present review, the version for cross-sectional quantitative studies consisting of 8 items was used and the cut-off point was set by consensus of both researchers at 6/8 to be considered eligible for inclusion in the present review.

Regarding methodological quality assessment, the Newcastle-Ottawa scale was utilized. This scale evaluates three categories: Selection (with a maximum of 6 points), Comparability (with a maximum of 2 points), and Outcome (with a maximum of 5 points). Studies with 6 points or more suggest a good quality. Fair quality receives between 5 and 6 points. Poor quality is indicated from zero to two points. The included studies were scored from 8 to 9 points (Table 4).

Table 4 Methodological Quality Assessment and Quality of Evidence

Results

A total of 4117 articles were identified from the aforementioned databases using the search strings specified in Table 3. After removing duplicate articles 1112, a total of 3005 articles were identified. Then, 2771 articles were excluded after reading the title and abstract.

Subsequently, 96 articles were eliminated for different reasons after reading the full text. These included the type of study (n=65), low methodological quality (n=23), not being related to the objective of the review (n=22), reporting no or little data analysis (n=5), no details of the tool used (n=5), and not having the full text available (n=10). Figure 1 details the process followed for the identification, screening, and selection of the studies included in this review.

Figure 1 Identification of studies via databases (PRISMA Flowchart).

Note: Adapted from Page MJ, McKenzie JE, Bossuyt PM, et al. The PRISMA 2020 statement: An updated guideline for reporting systematic reviews. BMJ. 2021;372:n71.15

Finally, the articles included for review were 8, that reported haemodynamic changes in adult patients transported by helicopter.

Main Results

To synthesise the articles included in the review, a table has been drawn up which presents the main characteristics of the studies, including information on the author(s), location, main objective, type of research, sample, tools used, main results, and methodological quality (Table 5).5,10,17–22

Table 5 Main Results

The origin of the different studies was varied: the studies had been conducted in the United States,18,22 Japan,5,19 United Kingdom,21 the Netherlands,17 Denmark,10 and England,20 one in each country; there were no multinational studies.

To summarise, all the articles reported on haemodynamic changes in vital signs during flight, such as blood pressure, heart rate, and oxygen saturation during HEMS missions; in one of them, the prevention of possible haemodynamic changes and instability through EFAST (Extended Focused Assessment with Sonography in Trauma) was also studied.18

Haemodynamic changes during HEMS missions were the main objective of the study and were assessed.5,17–19

In relation to the sample collection period, the studies were conducted in 2022,18 2015,22 between 2020-2021,17,21 between 2017-2020,19 between 2014-2019,20 between 2015-2019,5 and between 2010-2013.10

Heart Rate

Heart rate has been one of the most studied variables by most researchers, obtaining very similar results as they coincide in that changes do exist. Slagt et al17 analysed the variables that can be taken out of hospital to determine the increase in cardiac output at take-off and the decrease in systemic vascular resistance. Thus, this correlation was established based on the fact that there is a decrease in the heart rate during the flight. It should be noted that some patients required increased sedation.21 A similarity exists with Strony et al,18 where heart rate decreased during the flight, but increased before and after the flight. EFAST was used to assist in the presence of instability, in addition to cardiac monitoring.17,18

In the study by Nozawa et al,4 the same changes were identified, where the heart rate increased during pre-flight and post-flight and decreased during the flight. This is consistent with the study by Spoelder21 in which the results are similar.

However, there are studies in which stability was maintained, such as that by Kawai et al,19 in which heart rate did not change significantly.

Blood Pressure

In the reviewed studies, changes in blood pressure have been observed mainly during flight. In the study by Nozawa et al,5 diastolic blood pressure was maintained during take-off and landing and increased during the flight. Similarly, in the study by Slagt et al,17 diastolic blood pressure increased during the flight, but systolic blood pressure also decreased during the flight in this study. In the research carried out by Spoelder,21 blood pressure was not disaggregated, but there is a reported increase in blood pressure during the flight, which does not exist in pre-flight and post-flight. However, in a retrospective observational study by Kawai et al,19 diastolic blood pressure increased post-flight and systolic blood pressure decreased.

Oxygen Saturation

Oxygen saturation showed very similar results across research, with the exception of patients on mechanical ventilation. Accordingly, in the study by Slagt et al,17 there was no significant variation in StO2 (oxygen saturation) levels. One factor to consider is that the flight altitude was at 800 and 1200 feet, where the application of physical laws results in little variation in StO2, adding that the patients were on mechanical ventilation and therefore receiving extra oxygen supply.17 In contrast, the research by Strony et al18 found that StO2 changed significantly, with a decrease during the flight phase and an increase during the pre-flight and post-flight phases. This is in discrepancy with the retrospective cohort study by Leicht et al,20 where the patients arrived with hyperoxia. Yet, this was addressed by the transfer staff and under anaesthetic sedation, as 61% of the patients presented with severe hyperoxaemia, 20.4% with mild hyperoxaemia, and only 19.7% with normoxia. These measurements were accurate as they were made by means of arterial blood gas measurement, while peripheral oximetry readings showed StO2 ≥ 97%.

Two studies reported similar outcomes, the one by Spoelder,21 in which StO2 decreased during flight and increased during pre-flight and post-flight, and the study by Broman et al,10 in which StO2 increased during pre-flight and post-flight and decreased during flight.

In contrast to the rest of the studies, the one by Kawai et al19 was the only one in which a small increase in respiratory rate during transport was assessed.

Discussion

In this review, we use the acronym PECOT (Population, Exposure, Comparator, Outcomes, Time) to address haemodynamic changes in adult patients transported by the Helicopter Emergency Medical Service. The changes were assessed during pre-flight (before the helicopter took off), during the flight (when the helicopter was in continuous flight), and post-flight (when the aircraft landed). Potential medical risks from exposure to high altitude during flight and travelling at high speeds on board an aircraft are among the factors contributing to physiological changes.6,23

Many epidemiological studies have described out-of-hospital emergency health care, mainly regarding ground transport, but very few have referred exclusively to emergency medical air transport. HEMS offers many benefits, including the ability to quickly access rural or remote locations, provide multiple advanced on-board configurations, and intervene when ground units are unavailable. Other considerations for the use of HEMS include changes in physiology associated with the flight.24,25

The analysed studies showed, as the main haemodynamic modification, changes in heart rate, blood pressure, and oxygen saturation.

Oxygen saturation is predisposed to change, and oxygen compensation mechanisms must be in place to avoid developing other pathologies during the helicopter transport. Atmospheric pressure decreases with increasing altitude, with a consequent decrease in oxygen partial pressure, causing hypoxia. Hypoxia can be defined as a deficiency of oxygen in body tissues sufficient to cause impairment of the physiological function.9,26 The most threatening aspect of hypoxia is its insidious onset. It is taken for granted that Gay Lussac’s physical law, Charles’ Law, and Boyle’s Law (which can be observed when a person inflates a balloon because, if there is more pressure exerted inside the balloon, its volume increases),19 will be observed in patients who are transported by helicopter as they will obviously be flying at a height above the sea level.3,27,28

The research conducted by Thomas et al29 in which 30 patients were transported on different modes of invasive mechanical ventilation, 10 of them on Oxilog® ventilators, showed variations in oxygen saturation due to a failure in the ventilator that failed to favour minute volume, causing hyperventilation in these patients. However, in the study by Strony et al,18 of 28 patients who were intubated and connected to an Oxilog®-type ventilator during patient transport, 43% of them suffered alterations in oxygen saturation during the flight, recovering their baseline state once they arrived at their destination.

According to the study conducted by Andrews et al,30 patients who were transported suffered haemodynamic changes before, during, and after transport in a proportion of 80% of the total, of which 51% of patients suffered these changes during transport and 66% of patients suffered them prior to transport. This leads to the conclusion that in order for the haemodynamic changes in the patients not to be further aggravated during transport, this should be started with as much stabilisation as possible and under proper haemodynamic control.

Blood pressure plays an important role in helicopter transport since, according to evidence, minimal changes in blood pressure are observed, mainly with a tendency to increase systolic blood pressure during the flight as a response to physiological stress and hypoxia, which requires careful monitoring to prevent complications.29,31,32

Regarding heart rate, we observed that it undergoes relevant changes with a downward trend during the flight and increasing before and after the flight without causing tachycardia, however in the study by Taylor carried out in which he had a sample of 50 patients of which, 42 presented cardiac arrhythmias with tendencies towards tachycardia during the transfer, and 22 of them were considered serious, deducing that the mere fact of transferring patients already produced interaction in the changes in heart rate.25

There are studies that, although they do not involve emergency helicopters, are relevant to our study. They showed that haemodynamic and physiological changes were observed in patients with different pathologies who were transported intra-hospital. 66% of the patients transferred from the ICU for tests experienced physiological changes before, during, and after the transfer, but these changes were not due to the physiopathology of the transport, but to the severity of the patient’s illness.32

The vehicle dynamics on the transported patient have an influence on their health status. Specifically, linear accelerations can increase blood pressure and could cause rapid changes in heart rate and blood pressure.33 Thus, the importance of monitoring and managing hemodynamic changes during HEMS transportation is noted in this study. HEMS teams are likely not only provide advanced clinical skills, but also to support clinical decision-making in terms of triage and transport conditions. In this sense, in order to maintaining hemodynamic control, the immediate identification of blood loss, pneumothorax, and cardiovascular diseases via ultrasound examinations is known to be effective for patients with trauma and cardiovascular diseases in prehospital settings.34,35 Indeed, out-of-hospital standards of monitoring should not be interrupted at any time, so that patients can be safely transported.36 Also, high-fidelity invasive arterial blood pressure monitoring and arterial blood gas analysis become relevant in the HEMS prehospital setting to assure a safe transportation.37

Among the limitations of the study, the difficulty in selecting articles related to medical helicopter transport and its relationship with haemodynamic changes, as well as the relationship between air transport in terms of each pathology and the meteorological and physical parameters physiological variables involved, such as altitude, vibrations, temperature, etc., were addressed. Also, the selected studies had a cross-sectional design which avoid generalizations or conclusions about prevalence.

Conclusion

Based on the available literature, it can be concluded that the haemodynamic changes that occur during HEMS missions modify most notably heart rate, blood pressure, and oxygen saturation. These parameters physiological variables remain similar during take-off and landing and decrease during the flight, thus applying the laws of physics.

Ill patients benefit from medical transport by helicopter, especially when they are in areas far from a reference hospital. It would be interesting to carry out more studies on this subject, in order to increase the knowledge of health professionals on the matter. In clinical practice, the different emergency services should also carry out studies on the activity of their HEMS, both in first response and in interhospital transfer, taking into account pathologies, and monitoring of vital signs.

Key Points Question

Not many studies have been found describing haemodynamic changes in helicopter transport. In this context, the objective of this review was to describe haemodynamic changes in adult patients transported by Helicopter Emergency Medical Services.

Findings

Haemodynamic changes that occur during HEMS missions modify most notably heart rate, blood pressure, and oxygen saturation. These parameters physiological variables remain similar during take-off and landing and decrease during the flight, except for blood pressure, which rises during the flight, thus applying the laws of physics.

Meaning

Healthcare HEMS professionals must be aware of these alterations of in order to assess the appropriateness of the transport and apply optimal care.

Abbreviations

EFAST, Extended Focused Assessment with Sonography in Trauma; HEMS, Helicopter Emergency Medical Services; JBI, Joanna Briggs Institute; PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses; PROSPERO, Prospective Register of Systematic Reviews; StO2, Oxygen Saturation.

Data Sharing Statement

All data are available within this article.

Author Contributions

Conceptualization, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Data curation, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Formal analysis, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Investigation, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Methodology, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Project administration, AAG, LRD.

Resources, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Software, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Supervision, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Validation, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Visualization, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Writing—original draft preparation, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

Writing—review and editing, AAG, JGS, FJFC, JMVL, JJGI, BMY, FJMV, LRD.

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Funding

The authors declare the non-existence of funding in relation to this article.

Disclosure

The authors declare that they have no conflicts of interest in relation to this article.

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