Internal jugular venous oxygen saturation does not reflect sagittal sinus oxygen saturation in piglets

Can blood samples from the internal jugular vein (IJ) be used reliably in place of sagittal sinus (SS) samples in the calculation of cerebral oxygen extraction? To test this question we compared the O2 saturation (Sat) of blood samples drawn from SS, IJ vein, and pulmonary artery (MV) during hypercarbia, eucarbia and hypocarbia in 7 paralyzed, ventilated piglets. Cerebral blood flow was assessed by measuring unilateral internal carotic artery blood flow (ICABF), determined by an electromagnetic flow probe placed around the common carotid artery after ligation of the external carotid artery. During hypocarbia, eucarbia and hypercarbia SatSS (37.3 +/- 9.3, 48.9 +/- 10.2, 70.8 +/- 11.8%, respectively) was significantly different from SatIJ (54.8 +/- 8.9, 54.5 +/- 9.0, 62.0 +/- 15.1%) and SatMV (55.9 +/- 5.5, 58.7 +/- 5.3, 53.5 +/- 11.2%). The mean slope of the SatSS vs. PaCO2 regression lines was +0.583 +/- 0.303%/mm Hg, significantly greater than the mean slope of the regression lines for SatIJ vs. PaCO2 (+0.087 +/- 0.310%/mm Hg) or SatMV vs. PaCO2 (-0.112 +/- 0.230%/mm Hg). The relationship of ICABF vs. PaCO2 (mean slope = 0.444 +/- 0.294 ml/min/mm Hg) was statistically significant, while the relationship of cardiac output (determined by an electromagnetic flow probe placed around the pulmonary artery) vs. PaCO2 (mean slope = 0.470 +/- 1.617 ml/min/mm Hg) was not. We conclude that blood samples from the IJ do not reliably reflect cerebral venous blood and cannot be substituted for SS samples in piglets. It is most probable that the substitution of IJ for SS blood is not valid in piglets because the IJ receives venous effluent from noncerebral tissue.