Inspiratory CO₂ increases orthostatic tolerance during repeated tilt

Introduction: The partial pressure of end tidal CO₂ (PetCO₂) is known to decrease with head-up tilt. Decreases in arterial CO₂ reduce cerebral blood flow (CBF) and may increase the incidence of presyncope. We measured cerebral and central cardiovascular responses to repeated tilt where: 1) PetCO₂ was allowed to change with tilt (eucapnic): and 2) PetCO₂ was clamped at supine levels (isocapnic). Methods: In eight healthy subjects breath-by-breath measurements were made of ventilation (V̇E) and PetCO₂ along with beat-by-beat measurements of blood pressure (BP), heart rate (HR) and middle cerebral artery mean flow velocities (MFV). Following 30-min in the supine position, a series of six 10-min 90° head-up tilts were performed, with 30-s of supine between each. Presyncopal subjects were returned immediately to the supine position. Results: Statistical comparisons were made between the supine, and the first and last minute of the first tilt. BP, HR responses were not different between the eu- and isocapnic conditions; however, by the end of the first tilt V̇E was significantly higher than supine. MFV and BP at brain level decreased and HR increased from supine to tilt. MFV was higher in the isocapnic compared with the eucapnic condition but decreased from the beginning to the end of the first tilt in both conditions (i.e., tilt #1: eucap. 49.4 to 46.7; isocap. 65.0 to 59.6 cm · s^-1; p < 0.05) while the BP remained constant. Five subjects were presyncopal in the study. With isocapnic tilt, presyncopal time was not reduced but was extended in four of the five subjects (2.2, 5.5, 6.3 and 31 min) yet at presyncope the values for MFV, BP and HR were the same in both conditions. Conclusions: Inspiratory CO₂ contributed to increased MFV at the beginning of tilt and increased orthostatic tolerance.