Within a six-year period from 2016-2021, this retrospective cohort study aims to: 1) report the national incidence of drowning related OHCA's among cases attended by the Danish Emergency Medical Services (EMS), 2) assess survival defined as return of spontaneous circulation (ROSC) on scene, by hospital admission and 30-day survival.

Furthermore, aspects associated with better outcome are evaluated including actions taken by EMS-personnel and laypersons, geographical localization, type of activity, witnessed event, EMS response times, bystander CPR, initial rhythm, use of defibrillator, airway devices, pre-hospital medication, and patient demographics.

This can potentially result in recommendations towards certain educative, preventative, rescue, or treatment strategies to reduce OHCA from drowning.

Introduction

Preventing drowning accidents is of great interest, as a large proportion of drowning events can be avoided by proper educative and targeted preventive measures. Yet, effective educative, preventative, rescue and treatment interventions to reduce fatal drowning events require accurate, reliable and sufficient data to be scientifically analysed. However, the complexity of the disease, various drowning terminology, definitions, and an inappropriate classification of drowning events even in high-income countries possess a great challenge when trying to gather sufficient data.

Currently, drowning is a major cause of mortality worldwide, and a leading cause of cardiac arrest in children and adolescents. Even in highly developed countries, the incidence of drowning is highest among children less than 5 years of age and in young adults between 15 and 24 years of age. The duration of submersion plays a key role in determining the outcome from drowning, as submersion exceeding 10 minutes is associated with poor outcome. Furthermore, drowning leading to hypoxic cardiac arrest is a reversible cause that can be treated. Therefore, bystanders, trained rescuers (e.g., trained lifeguards) and EMS personnel play a critical role in the initial attempts at rescue and resuscitation to prioritize oxygenation and ventilation.

Yet, research on out-of-hospital cardiac arrest (OHCA) due to drowning is limited, and under reporting contributes significantly to misunderstanding of the process of drowning. This leads to a potential for improvement, which requires first and foremost clarification of the most obvious causes and risk factors associated with OHCA from drowning following recommended guidelines for uniform reporting of data from drowning.

Thus, within a six-year period, this study aims to: 1) report the national incidence of drowning related OHCA's among cases attended by the Danish Emergency Medical Services (EMS), 2) to assess survival defined as return of spontaneous circulation (ROSC) at any time followed by status at admission and 30-day survival.

Furthermore, aspects associated with better outcome are evaluated including actions taken by EMS-personnel and laypersons, geographical localization, type of activity, witnessed event, EMS response times, bystander CPR, initial rhythm, use of defibrillator, airway devices, pre-hospital medication, and patient demographics.

This can potentially result in recommendations towards certain educative, preventative, rescue, or treatment strategies to reduce OHCA from drowning.

Methods

Terminology: Drowning is defined by the WHO in 2002 as "the process of experiencing respiratory impairment from submersion or immersion in liquid". Submersion is the situation where the victim's entire body, including the airway, goes below the surface of the liquid. Immersion is to be covered in water, and for drowning to occur, usually at least the face and airway are immersed. If the victim is rescued at any time, the process of drowning is interrupted, which is termed a nonfatal drowning. If the person dies at any time because of drowning, this is termed a fatal drowning. Any submersion or immersion event without evidence of respiratory impairment is considered a water rescue and not a drowning. Terms such as "near drowning", "dry or wet drowning", "secondary drowning", "active and passive drowning", and "delayed onset of respiratory distress" should be avoided.

Setting: In 2016 the Danish Emergency Medical Services (EMS) introduced a nationwide electronic medical reporting system recording all following cases of OHCA in which a resuscitative attempt was initiated in the Danish Cardiac Arrest Registry. Together with a manual validation process, this electronic system is the cornerstone in the Danish out-of-hospital cardiac arrest (OHCA) registry, a solid base for identification and verification of OHCA. Because of the changeover a validation team manually processed all records extracted from the electronic medical records. This ensured high-quality data from the approximately 5,400 annually registered OHCA in Denmark, within the period from 2016 to 2018. From the direct registration and later manual verification, several additional data sources were coupled with each registered OHCA (e.g., survival, initiation of bystander CPR and potential drowning accident).

This enables the possibility for advanced text searching in the entire pre-hospital medical record for cases with OHCA. In a pilot-study performed at Zealand EMS in 2019, a feasible and efficient method for identifying OHCA from foreign body airway obstruction was developed, based on identification of trigger-words in the electronic medical record.

Identification of fatal drowning accidents: During the manual validation of OHCA in the changeover period, 211 cases were flagged as drowning accidents. However, the obstacle of manual verification across EMS regions persists and further data cleansing and revised search-strings are needed to provide high-quality data. Therefore, the investigators will identify trigger-words in the 211 electronic reports from the initial screening by manually reading through the medical records. A text-string based on these trigger-words will be structured and fed into a text-search algorithm able to search note-fields within the electronic medical records. When searching the electronic medical records from the OHCA register, the algorithm will return several records, consisting of OHCA presumably caused by drowning. However, the initial search alone will miss some of the manually registered OHCA caused by drowning and conversely, non-fatal drowning accidents not registered in the Danish Cardiac Arrest Registry will also be identified from the search, which need to be excluded in this research project. After a manual review of the specified reports, new trigger-words will be added to the search-string to include reports not identified during the initial manual registration ultimately leading to a revised text-string. This iterative process will be performed until no new reports appear, and the investigators will have identified the final number of OHCA presumably caused by drowning. The remaining four regions have all pledged data permission and agreed to initiate the study when funding has been acquired.

Perspectives: This study provides novel information regarding OHCA related to drowning; the descriptive statistics provides relevant data based on a reviewed, high-quality database. Furthermore, this study will enable targeted campaigns aimed at increasing survival from OHCA caused by drowning, targeting preventative actions in means of educating the public, creating awareness, increasing signage and potentially allocating lifeguards to high-risk areas, or establishing public lifeguard training and bystander initiatives.

Inclusion Criteria:

• Out-of-hospital cardiac arrest (OHCA)

Exclusion Criteria:

• Obvious clinical signs of irreversible death (decapitation, decomposition, postmortem lividity, postmortem rigidity)

All Danish citizens with out-of-hospital cardiac arrest in the period from 2016-2021.

The study protocol and statistical analysis plan are available in Danish from the corresponding author upon request.

The study protocol and SAP will be available after publication and up to 5 years after the publication date.

Upon reasonable request.