EAAA provides prehospital critical care in support of the statutory emergency medical service in the East of England (East of England Ambulance Service NHS Trust). EAAA operates from two bases (Cambridge and Norwich), deploying a prehospital critical care team comprised of a physician and critical care paramedic, in either an Airbus H145 helicopter or rapid response vehicle, depending on patient location, weather constraints, and time of day [8]. EAAA predominantly responds to pre-hospital emergencies, but undertakes a small number of inter-hospital transfers.

As previously described, [8] EAAA physicians are predominantly from an emergency medicine or anaesthesia background with at least six years of post-graduate clinical experience, and competent in arterial catheterisation. Critical Care Paramedics (CCP) receive training in peripheral arterial catheterisation and point-of-care-ultrasound (POCUS), including vascular access, and then practice under physician supervision until achieving independent sign-off.

Arterial catheterisation has been performed by EAAA clinicians since 2014. A 20G arterial cannula (BD Arterial Cannula with Flow Switch, Sandy, UT, USA) and an arterial pressure transducer (Edwards Truwave 3 cc, Edwards Lifesciences, Germany) are currently used for peripheral arterial cannulation. Additionally, femoral arterial cannulation using a 5Fr sheath introducer (MERIT Prelude, MERIT Medical, South Jordan, UT, USA) has been added to the list of standard procedures since June 2021. Ultrasound guidance is mandated for femoral catheterisation and is available for radial artery catheterisation at the discretion of the operating clinician (Butterfly iQ, Butterfly Net Inc, MA, US).

Ultrasound-guided femoral arterial cannulation is formally trained as part of the local Specialist Percutaneous Emergency Aortic Resuscitation (SPEAR) programme utilising a Seldinger technique [9, 10].

This retrospective observational study included all patients attended by EAAA who underwent arterial catheterisation and initiation of IABP monitoring between 1st February 2015 (initiation of the electronic medical record) and 20th April 2023 (last data available at data extraction). Patients without a single systolic IABP > 20mmHg were excluded.

To ensure spurious and artefactual readings were not included in the analysis, cases meeting the following criteria were manually reviewed: <10 min of data recorded; all IABP measurements < 50mmHg; or all systolic IABP measurements were < 90mmHg. During manual review, the clinical notes and the IABP data were compared to determine whether the results were artefactual (Table S1). Cases of inter-hospital transfers were excluded from analysis as IABP monitoring is frequently initiated in-hospital prior to transfer.

The following data were retrieved: sex; age in years; aetiology (medical cardiac arrest, other medical emergency, trauma); anatomical site of arterial cannulation; operator role (physician/CCP); mission result (conveyed by air, conveyed by road, or died at scene). The time of insertion and, where applicable, time of pre-hospital emergency anaesthesia (PHEA), and return of spontaneous circulation (ROSC) following cardiac arrest were also retrieved.

To characterise interventions associated with IABP measurement, data were collected on the following: arterial blood gas analysis (CG8 + cartridge, iSTAT 1, Abbot Point of Care Inc, Abbot Park, IL, USA), intubation, PHEA, administration of blood products or intravenous crystalloid fluids, and vasoactive drugs (metaraminol, ephedrine, adrenaline).

The EAAA clinical governance database (RLDatix, Richmond, UK) was cross-referenced to identify any serious untoward incidents associated with arterial catheterisation.

IABP measurements were routinely downloaded from the prehospital monitor (ZOLL X Series Monitor/Defibrillator, ZOLL Medical Corporation of Asahi Kasei Corp., Tokyo) to the electronic medical record (HEMSbase, Medic One Systems Ltd, UK). Anonymised data were extracted from HEMSbase and stored in a secure data environment in Excel (Microsoft® Excel® for Microsoft 365, v2309), with data management and statistical analyses performed by a trained statistician using the R statistical programming language (R Core Team [2018]; R: A language and environment for statistical computing [R Foundation for Statistical Computing, Vienna, Austria]). Characteristics of the sample were described as number (percentage) for categorical variables and mean (± standard deviation (SD)) or median (interquartile range (IQR)) for continuous variables as appropriate.

This study met the UK Health Research Authority definition of service evaluation, and was registered with the EAAA Department of Research, Audit, Innovation, & Development (REF: EAAA 2023/01). The STROBE (Strengthening the Reporting of Observational studies in Epidemiology) reporting guideline was followed [11].