Five PaCO2 variables were defined : minimum ( Min ) , maximum ( Max ) , SD , average ( time-weighted ) , and a four level categorical variable ( hypercapnic ( highest quartile of Max PaCO2 ) , hypocapnic ( lowest quartile of Min PaCO2 ) , fluctuators ( hypercapnia and hypocapnia ) , and normocapnic ( middle two quartiles of Max and Min PaCO2 ) ) .