This study of the GRR registry demonstrated for the first time a significant difference between the outcomes of OHCA patients treated with mCC or ACCD, stratified by the airway used in the out-of-hospital setting. The most important finding of our study was that patients treated with SAD alone during mCC and ACCD in OHCA showed the lowest survival rates to hospital discharge and the lowest survival rate with good neurological outcome in comparison to all other airway and compression strategies.
Advanced airway management, such as ETI or SAD, is one of the most prominent interventions in OHCA treatment. In the past, some studies investigated the pitfalls and limitations of ETI, including unrecognized misplacement and dislodgement, multiple failed ETI attempts, and interruption of chest compression continuity [15,16,17,18]. The ERC guidelines discussed the airway strategy according to the skill level and stated that there are no data supporting the routine use of any specific approach to airway management during cardiac arrest [3]. Recently published studies tended to use an observational design with the well-known methodological flaws, and it was stated that large-scale randomized trials are required to solve ongoing uncertainty in this area of clinical practice [19]. Our findings were in line with the recently published results from Sulzgruber et al. [20]. The authors used a propensity score matched analysis and found significant outcome differences between different airway strategies (including laryngeal tube and ETI) during OHCA treatment. Another recently published cluster randomized study compared i‑gel vs. laryngeal mask airway supreme vs. current practice (principally tracheal intubation) and found no significant differences in outcome between the three groups [21]; however, the study was a feasibility study and declared by the authors to be underpowered to detect survival differences. In line with our study, a meta-analysis of 10 studies including 34,533 ETI and 41,116 SAD treated OHCA patients found that patients who receive ETI by EMS are more likely to obtain ROSC, survive to hospital admission, and survive neurologically intact when compared with SAD [22]. Additionally, another analysis of the cardiac arrest registry to enhance survival (CARES) registry compared the outcomes of 5591 patients treated with ETI and 3110 patients with SAD found that survival was higher among OHCA receiving ETI than receiving SAD [15]. Altogether, these results provide further data to support ETI as the gold standard during OHCA; however, the previously mentioned studies did not take into account the type of chest compressions (manual vs. automated) provided during cardiac arrest.
The recently published guidelines on CPR from the ERC highlighted the importance of high-quality chest compressions [3, 4]. The use of ACCD during CPR of OHCA patients may to be associated with some advantages: minimization of interruptions, constant compression depth and high compression ratio. Despite the fact that the three major randomized controlled trials on the use of ACCD (CIRC [6], LINC [7] and PARAMEDIC [8]) did not show a benefit of ACCD over mCC in OHCA, no profound risks or evidence of inferiority of ACCD in comparison to mCC were found. These previous findings were not in line with the findings of the presented GRR study showing that the outcomes of OHCA patients who received mCC were significantly better in comparison to patients who received ACCD.
Several confounders have the potential to influence these results. One of the confounders may be the airway and ventilation strategy. Thereby, the current guidelines do not address specific ventilation problems with ACCD. The recommendations made in 2005, 2010 and 2015 by the ERC concerning the compression-to-ventilation ratio before and after advanced airway management are identical [3, 23, 24].
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A compression to ventilation ratio of 30:2 before intubation/supraglottic airway device and uninterrupted chest compression after intubation or use of supraglottic airway device as airway strategy.
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Once a SAD has been inserted, uninterrupted chest compressions should be attempted; if excessive gas leakage causes inadequate ventilation, chest compressions should be interrupted to enable ventilation (using a compression to ventilation ratio of 30:2).
Notably, and not surprisingly given the general lack of evidence, no special recommendations were made regarding the use of SAD during ACCD. There is a lack of data to support the safety and effectiveness of the recommendation for uninterrupted chest compression using ACCD and ventilation in combination with SAD. There is insufficient or missing evidence for the effectiveness of any airway and ventilation strategy and the use of ACCD.
In the presented GRR study we did not investigate the compression to ventilation ratio used in the OHCA patients stratified according to the airway device used; however, to the best of our knowledge, there are no clinical studies that focus on effective oxygenation and elimination of carbon dioxide in patients suffering from OHCA who are treated with ACCD. Furthermore, there is a notable lack of data on upper airway pressure limits (e. g., avoidance of barotrauma) during mCC. During ACCD airway pressure may exceed 20 cmH2O, which can make ventilation using SAD ineffective. This may be the reason for the observed lowest survival rate and neurological outcome in OHCA patients treated with SAD only in comparison to all other airway strategies in the present investigation. Although not investigated in the present study, ventilation problems might occur in the setting of SAD use during ACCD. In the authors’ clinical experience, ACCD makes continuous high-quality ventilation difficult and sometimes impossible [10]. None of the available ACCD cited in the ERC guidelines were constructed with particular regard to effective and safe ventilation [3].
In general, the use of a SAD itself can be complicated by numerous problems that lead to inadequate ventilation, hypoxemia and hypercapnia (e. g., displacement, leakage, incorrect placement and tongue/pharyngeal swelling; [25, 26]). As chest compression alone without oxygenation and ventilation are recommended only for the brief time period of basic life support performed by lay persons (compression-only CPR), a safe strategy for airway management and ventilation is an integral part of any resuscitative measures [3, 27]. After the arrival of healthcare professionals (e. g., paramedics and EMS physicians) and during advanced life support, ensuring oxygenation and elimination of carbon dioxide is crucial, even if the optimal strategy for managing the airway has not yet been determined [19]. In this context, our findings showed that a SAD first strategy (e. g. mCC, SAD/ETI) may be beneficial resulting in the mCC group to the best results for delta RACA ROSC. High-quality chest compressions only, with and without ACCD, will remain unsuccessful without oxygenation and decarboxylation of the blood. Desaturated blood does not contribute to myocardial and cerebral reoxygenation, and hypercapnia may be detrimental (e. g. acidosis and cardio-depressive effects; [28]).
During ventilation with SAD, perceived as a secured airway according to the ERC guidelines [3], while using ACCD there is considerable potential for ineffective ventilation with continuous and uninterrupted ACCD chest compression. It can be hypothesized that SAD was not so effective for securing the airway during CPR as expected and that SAD should be perceived as an unsecured airway. Thus, it is conceivable that the results of the three major ACCD trials (CIRC [6], LINC [7] and PARAMEDIC [8]) may reveal a significant difference between the study arms if patients ventilated with a bag-valve mask or SAD were excluded from the analysis. We presume that, particularly with SAD and continuous ACCD, ventilation is ineffective and a significant confounder in the three mentioned major ACCD studies. This may be particularly important for patients transported to a hospital with prolonged and ongoing CPR during transport as recommended by the current ERC guidelines [3]. Interestingly, in the presented registry study more patients in the ACCD group were admitted under ongoing chest compression. This may be consequence of the ERC recommendations to use an ACCD during patient transport due to safety reasons while transportation under mCC is associated with high risk and less often high-quality chest compressions. Keeping these findings in mind, we suggest an extended raw data analysis of the three major ACCD trials with respect to the impact of ACCD on effective ventilation. We hope that this will lead to better recommendations on safe and effective airway management and ventilation strategies during the use of ACCD, regarding, in particular, the use of SAD in these patients.