Systemic circulation and cerebral oxygenation during head-up tilt in spinal cord injured individuals

To compare tilt-induced alterations in cardiovascular homeostasis and cerebral oxygenation of spinal cord-injured (SCI) to able-bodied (AB) individuals. Subjects underwent 10 min supine rest followed by 10 min 70 degrees head-up tilt. The last 5 min of supine rest and head-up tilt were analyzed, provided a steady state existed. SCI individuals (n= 11), with lesions between C4 and T4, and AB individuals (n= 10), all males and balanced for age and weight. Calf circumference, mean arterial pressure (MAP), stroke volume, heart rate and cerebral oxygenated ([O2Hb]), deoxygenated ([HHb]) and total ([tHb]) haemoglobin concentration changes were measured. Head-up tilt evoked a greater fall in MAP (mean (SD): -9 (12) vs 2 (6) mmHg P=0.02) and stroke volume (-43 (12) vs -22 (10)%, P=0.005), and a greater increase in heart rate (27 (12) vs 18 (6) beats, P=0.04) in SCI than AB. Cardiac output decreased during head-up tilt in SCI but not in AB (-17 (15) vs 1 (15)%, P=0.01). The change in cerebral oxygenation ([HHb]: 3.9 (2.8) vs 2.8 (1.4) micromol x l(-1), P=0.1 and [O2Hb]: -6.1 (5.0) vs -2.1 (5.5) micromol x l(-1), P=0.1) was similar in SCI and AB. All variables mentioned showed a change significantly different from zero in both groups, apart from [O2Hb] in AB and [tHb] in both groups. SCI demonstrated a greater decrease of MAP and stroke volume with a similar decrease in cerebral oxygenation compared to AB. This suggests that although systemic circulation was less well regulated in SCI compared with AB, cerebral circulation in SCI was maintained as in AB.