In 50 % of these patients ( n = 10 ) , COx became highly negative after vasoactive drug administration ( from -0.04 [ -0.25 to 0.16 ] to -0.63 [ -0.83 to -0.26 ] after administration of phenylephrine , and from -0.05 [ -0.19 to 0.17 ] to -0.55 [ -0.94 to -0.35 ] after administration of nitroprusside ) .