Decreasing cerebral oxygen consumption during upright tilt in vasovagal syncope

We measured changes in transcranial Doppler ultrasound (TCD) and near infrared spectroscopy (NIRS) during 70° upright tilt in patients with recurrent vasovagal syncope (VVS, N = 20), postural tachycardia syndrome (POTS, N = 20), and healthy controls (N = 12) aged 15-27 years old. VVS was included if they fainted during testing within 5-15 min of upright tilt. We combined TCD and NIRS to obtain estimates of percent change in the cerebral metabolic rate of oxygen consumption (CMRO₂), cerebral blood flow velocity (CBFv), and oxygen extraction fraction (OEF). Over the course of 10 min of upright tilt, CBFv decreased from a baseline of 70 ± 5 to 63 ± 5 cm/sec in controls and 74 ± 3 to 64 ± 3 cm/sec in POTS while decreasing from 74 ± 4 to 44 ± 3 cm/sec in VVS CMRO₂ was unchanged in POTS and controls during tilt while OEF increased by 19 ± 3% and 15 ± 3%, respectively. CMRO₂ decreased by 31 ± 3% in VVS during tilt while OEF only increased by 7 ± 3%. Oxyhemoglobin decreased by 1.1 ± 1.3 μmol/kg brain tissue in controls, by 1.1 ± 1.3 μmol/kg in POTS, and 11.1 ± 1.3 μmol/kg in VVS CBFv and CMRO₂ fell steadily in VVS during upright tilt. The deficit in CMRO₂ in VVS results from inadequate OEF in the face of greatly reduced CBF.