The effect of 50% compared to 100% inspired oxygen fraction on brain oxygenation and post cardiac arrest mitochondrial function in experimental cardiac arrest

Background and aim We hypothesised that the use of 50% compared to 100% oxygen maintains cerebral oxygenation and ameliorates the disturbance of cardiac mitochondrial respiration during cardiopulmonary resuscitation (CPR). Methods Ventricular fibrillation (VF) was induced electrically in anaesthetised healthy adult pigs and left untreated for seven minutes followed by randomisation to manual ventilation with 50% or 100% oxygen and mechanical chest compressions (LUCAS^®). Defibrillation was performed at thirteen minutes and repeated if necessary every two minutes with 1 mg intravenous adrenaline. Cerebral oxygenation was measured with near-infrared spectroscopy (rSO₂, INVOS™5100C Cerebral Oximeter) and with a probe (NEUROVENT-PTO, RAUMEDIC) in the frontal brain cortex (PbO₂). Heart biopsies were obtained 20 min after the return of spontaneous circulation (ROSC) with an analysis of mitochondrial respiration (OROBOROS Instruments Corp., Innsbruck, Austria), and compared to four control animals without VF and CPR. Brain rSO₂ and PbO₂ were log transformed and analysed with a mixed linear model and mitochondrial respiration with an analysis of variance. Results Of the twenty pigs, one had a breach of protocol and was excluded, leaving nine pigs in the 50% group and ten in the 100% group. Return of spontaneous circulation (ROSC) was achieved in six pigs in the 50% group and eight in the 100% group. The rSO₂ (p = 0.007) was lower with FiO₂ 50%, but the PbO₂ was not (p = 0.93). After ROSC there were significant interactions between time and FiO₂ regarding both rSO₂ (p = 0.001) and PbO₂ (p = 0.004). Compared to the controls, mitochondrial respiration was decreased, with adenosine diphosphate (ADP) levels of 57 (17) pmol s⁻¹ mg⁻¹ compared to 92 (23) pmol s⁻¹ mg⁻¹ (p = 0.008), but there was no difference between different oxygen fractions (p = 0.79). Conclusions The use of 50% oxygen during CPR results in lower cerebral oximetry values compared to 100% oxygen but there is no difference in brain tissue oxygen. Cardiac arrest disturbs cardiac mitochondrial respiration, but it is not alleviated with the use of 50% compared to 100% oxygen (Ethical and hospital approvals ESAVI/1077/04.10.07/2016 and HUS/215/2016, §7 30.3.2016, Funding Helsinki University and others).