This study aims to better understand how short periods of exposure to high oxygen levels affect blood flow in the brain of patients who are intubated and mechanically ventilated in the Intensive Care Unit (ICU). Many ICU patients receive more oxygen than strictly necessary, and high blood oxygen levels (hyperoxemia) are very common. However, the immediate effects of short hyperoxic exposures on cerebral circulation and autoregulation remain poorly understood.
In this study, patients who already require mechanical ventilation for medical reasons will undergo a brief and controlled increase in the oxygen delivered through the ventilator (FiO₂). During this time, we will continuously monitor blood flow in one of the main brain arteries using a non-invasive ultrasound technique called transcranial Doppler (TCD). The goal is to evaluate how cerebral blood flow, pulsatility, and autoregulatory capacity change during and after a short hyperoxic stimulus.
No additional invasive procedures are required beyond standard ICU monitoring, except for the temporary adjustment of the ventilator's oxygen settings and arterial blood gas sampling, which are part of usual care in critically ill patients. Participation does not provide direct clinical benefit but may help improve future oxygen management in ICU patients. The study involves minimal risk, as short hyperoxic exposures are already common in routine care and will be interrupted immediately in case of any adverse event.
Adult patients aged ≥18 years
Admitted to the intensive care unit (ICU)
Intubated and mechanically ventilated for ≤72 hours
Receiving volume-controlled mechanical ventilation
Age <18 years
Pregnancy
Extracorporeal membrane oxygenation (ECMO)
Continuous renal replacement therapy (CRRT)
Single group
Normobaric Hyperoxic Stimulus (NBHO)
Other interventionThe intervention consists of a short, controlled increase in the inspired oxygen fraction (FiO₂) delivered by the mechanical ventilator. Depending on baseline FiO₂, patients will receive: Depending on baseline FiO₂, patients will receive: Two-step NBHO (baseline FiO₂ \< 0.5): FiO₂ raised to 0.5 and then to 1.0 One-step NBHO (baseline FiO₂ ≥ 0.5): FiO₂ raised to 1.0 Each step includes 5 minutes to reach steady state followed by a 10-minute recording period. Cerebral blood flow velocity and autoregulation are continuously assessed using transcranial Doppler ultrasound.
Change in Mean Flow Index (Mx) During Normobaric Hyperoxia
Dynamic cerebral autoregulation will be assessed using the mean flow index (Mx), calculated as the moving Pearson correlation coefficient between mean arterial pressure (MAP) and mean middle cerebral artery (MCA) blood flow velocity measured by transcranial Doppler ultrasonography. The outcome will be reported as the absolute change in Mx from baseline, calculated as the average Mx value during normobaric hyperoxia minus the baseline average.
Change in Middle Cerebral Artery Cerebral Blood Flow During Normobaric Hyperoxia
Cerebral hemodynamics will be assessed by transcranial Doppler ultrasonography of the middle cerebral artery (MCA). The cerebral blood flow velocity (cm/s) will be evaluated and analyzed as a change from baseline during normobaric hyperoxia. It will be expressed as the absolute change from baseline, calculated as the average value during the last 5 minutes of normobaric hyperoxia minus the baseline average.
Change in Autoregulation Index (ARI) During Normobaric Hyperoxia
Cerebral autoregulation will also be evaluated using the Autoregulation Index (ARI) derived from transfer function-based step response analysis, expressed on a scale from 0 to 9. The outcome will be expressed as the absolute change from baseline during normobaric hyperoxia.
Change in Transfer Function Analysis Parameters During Normobaric Hyperoxia
Transfer function analysis (TFA) between mean arterial pressure and middle cerebral artery blood flow velocity will be performed. Gain (cm/s/mmHg) will be analyzed in the low-frequency band (0.07-0.20 Hz). It will be expressed as absolute change from baseline during normobaric hyperoxia.
Change in Transfer Function Analysis Parameters During Normobaric Hyperoxia
Description: Transfer function analysis (TFA) between mean arterial pressure and middle cerebral artery blood flow velocity will be performed. Phase (radians) will be analyzed in the low-frequency band (0.07-0.20 Hz). It will be expressed as absolute change from baseline during normobaric hyperoxia.
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