BACKGROUND: The concept of personalized cardiopulmonary resuscitation (CPR) requires a parameter that reflects its hemodynamic efficiency. While intra-arrest ultrasound is increasingly implemented into the advanced life support, we realized a pre-hospital clinical study to evaluate whether the degree of compression of the right ventricle (RV) and left ventricle (LV) induced by chest compressions during CPR for out-of-hospital cardiac arrest (OHCA) and measured by transthoracic echocardiography correlates with the levels of end-tidal carbon dioxide (EtCO2) measured at the time of echocardiographic investigation. METHODS: Thirty consecutive patients resuscitated for OHCA were included in the study. Transthoracic echocardiography was performed from a subcostal view during ongoing chest compressions in all of them. This was repeated three times during CPR in each patient, and EtCO2 levels were registered. From each investigation, a video loop was recorded. Afterwards, maximal and minimal diameters of LV and RV were obtained from the recorded loops and the compression index of LV (LVCI) and RV (RVCI) was calculated as (maximal - minimal/maximal diameter) × 100. Maximal compression index (CImax) defined as the value of LVCI or RVCI, whichever was greater was also assessed. Correlations between EtCO2 and LVCI, RVCI, and CImax were expressed as Spearman's correlation coefficient (r). RESULTS: Evaluable echocardiographic records were found in 18 patients, and a total of 52 measurements of all parameters were obtained. Chest compressions induced significant compressions of all observed cardiac cavities (LVCI = 20.6 ± 13.8%, RVCI = 34.5 ± 21.6%, CImax = 37.4 ± 20.2%). We identified positive correlation of EtCO2 with LVCI (r = 0.672, p < 0.001) and RVCI (r = 0.778, p < 0.001). The strongest correlation was between EtCO2 and CImax (r = 0.859, p < 0.001). We identified that a CImax cut-off level of 17.35% predicted to reach an EtCO2 level > 20 mmHg with 100% sensitivity and specificity. CONCLUSIONS: Evaluable echocardiographic records were reached in most of the patients. EtCO2 positively correlated with all parameters under consideration, while the strongest correlation was found between CImax and EtCO2. Therefore, CImax is a candidate parameter for the guidance of hemodynamic-directed CPR. TRIAL REGISTRATION: ClinicalTrial.gov, NCT03852225 . Registered 21 February 2019 - Retrospectively registered.

Department of Anesthesiology Perioperative Medicine and Intensive Care J E Purkinje University Masaryk Hospital Usti nad Labem Socialni pece 3316 12A 400 11 Usti nad Labem Czech Republic Emergency Medical Service of the Central Bohemian Region Vancurova 1544 272 01 Kladno Czech Republic Department of Anesthesiology and Intensive Care Charles University Faculty of Medicine in Hradec Kralove University Hospital Hradec Kralove Sokolska 581 500 05 Hradec Kralove Czech Republic

Department of Anesthesiology Perioperative Medicine and Intensive Care J E Purkinje University Masaryk Hospital Usti nad Labem Socialni pece 3316 12A 400 11 Usti nad Labem Czech Republic Emergency Medical Service of the Central Bohemian Region Vancurova 1544 272 01 Kladno Czech Republic Department of Research and Development Charles University Prague Faculty of Medicine in Hradec Kralove University Hospital Hradec Kralove Sokolska 581 500 05 Hradec Kralove Czech Republic Department of Anesthesia Pain Management and Perioperative Medicine Dalhousie University Halifax Nova Scotia B3H 4R2 Canada

Department of Anesthesiology Perioperative Medicine and Intensive Care J E Purkinje University Masaryk Hospital Usti nad Labem Socialni pece 3316 12A 400 11 Usti nad Labem Czech Republic Usti and Labem Region Emergency Medical Services Socialni pece 799 7a 400 11 Usti and Labem Czech Republic

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