Mechanical ventilation in HEMS is a frequently performed procedure although there may be different rates across different health care systems. Almost 20% of the HEMS missions in our study involved the transport of mechanically ventilated patients. Approximately half of all mechanically ventilated HEMS patients were interfacility transports of critical care patients (8).
In general, there are only few studies on the process quality of mechanical ventilation in HEMS environments (4, 5, 9, 10). Key results of our small study sample demonstrate that participating HEMS-teams were entirely physician-staffed and provided a high expertise in critical care management and long experience in HEMS. Included patients had no transport-related complications of airway management and ventilator use.
In the literature, critical events (i.e., inadvertent extubation, loss of IV-lines, cardiopulmonary deterioration) occur in approximately 5% of HEMS transports of patients who are critically ill, rising up to 18% when focusing on systems with paramedics as the sole health care provider conducting interfacility critical care transfers. (11). Nevertheless, fatal events during HEMS transport are reported in less than 0.2% of cases (1, 9, 12).
Regarding ventilator settings, all patients of the present study were ventilated with low tidal volumes and almost every second patient underwent pressure controlled ventilation (PCV), and even though the evidence of low tidal volume ventilation or PCV in patients without ARDS is low the HEMS teams use ICU standards of ventilation during transport (13, 14). This is in contrast to published literature where tidal volumes have been reported to be above 6 ml/kg body weight in 86% of patients and most patients receive volume control ventilation during transport (6). This may be explained by the low pre-hospital availability of transport-ventilators with PCV-mode in older studies. Nowadays modern transport-ventilators with PCV-mode are widespread even in the pre-hospital setting. High FiO2 during HEMS transport are common and may be the result of safety measures to prevent critical desaturation and of rare ability to perform blood gas analyzes during transport (9).
End-tidal capnography monitoring has an essential value of safety during HEMS transport. First, tube dislodgement may be detected reliably, second, hypocapnia or hypercapnia can be adjusted by changing mechanical ventilation parameters, and third, it provides prognostic value in highly critical patients (15–18). Hypocapnia secondary to hyperventilation is a frequent iatrogenic complication during pre-hospital ventilation (19). This has been most commonly documented among patients with traumatic brain injury and occurred in up to 79% of patients (19, 20). Pre-hospital hyperventilation and the resulting hypocapnia are associated with poor outcomes, including increased mortality rates due to cerebral vasoconstriction causing cerebral ischemia (21). The use of pre-hospital capnography monitoring has been identified as an outcome relevant quality parameter in the German trauma registry (TraumaRegister DGU) (22). The high percentage (>95%) use of capnography in our study demonstrate the perception of the mentioned problems by the HEMS-teams.
Despite the risk of ventilator–associated critical events during HEMS-transport, there may also be opposite effects in terms of improvement of ventilation and respiratory function. In our small study, PEEP-levels were significantly higher after transportation compared with pre-transportation PEEP, but the clinical relevance of these higher PEEP-levels could not be assessed. For instance, If the change in PEEP-level caused a PEEP over the lower inflection point of the lung, this change could be clinically relevant (23).
Across multiple studies, critical care transport teams with training in complex ventilator management are associated with improved PaO2 after transfer (9, 10, 12). The transporting team changed ventilator settings during transport in most patients (decreasing tidal volume, increasing PEEP, and increasing FiO2). Furthermore, the use of neuromuscular blocking drugs is a common measure to improve respiratory function under mechanical ventilation in HEMS (9, 10). Our data confirm these findings (45% of patients) although there was a high proportion of on-scene rescue missions and rapid sequence intubation in our cohort compared with other studies.
Notably, mechanical ventilation parameters provided by HEMS teams are known to considerably influence initial hospital ventilation parameters after patient handover (6). Besides focusing on ventilated patients, our study-sample demonstrated a high level of expertise of participating HEMS-teams across Europe. In all cases the HEMS-teams were physician-staffed with many years of HEMS-experience along with a high proportion of board certification and ICU-training. No other studies have reported this kind of data.