Decreasing cerebral oxygen consumption during upright tilt in vasovagal syncope

Abstract

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.

Keywords: Autonomic nervous system; near‐infrared spectroscopy; orthostatic intolerance; transcranial doppler ultrasound.

Figure 1

Heart rate (HR), end tidal carbon dioxide (ETCO₂), cardiac output (CO) and total peripheral resistance (TPR) during upright tilt for control subjects (solid line), vasovagal syncope patients (VVS, dashed line) and postural tachycardia syndrome patients (POTS, dash and dot line). HR is most increased above control in POTS and to a lesser degree in VVS. TPR is increased in POTS compared to control, and falls throughout tilt in VVS.

Figure 2

Percent change mean in arterial pressure (MAP), in cerebral blood flow velocity (CBFv), in cerebral metabolic rate of oxygen consumption (CMRO₂) and in oxygen extraction fraction (OEF) during upright tilt for control subjects (solid line), vasovagal syncope patients (VVS, dashed line) and postural tachycardia syndrome patients (POTS, dash and dot line). MAP declines in VVS but is stable in control and POTS. CBFv and CMRO2 decline monotonically and in parallel throughout tilt in VVS. OEF increases by approximately half as much as control in VVS. CBFv decreases equally in control and POTS, while CMRO₂ is well maintained.