Middle cerebral O2 delivery during the modified Oxford maneuver increases with sodium nitroprusside and decreases during phenylephrine


The modified Oxford maneuver is the reference standard for assessing arterial baroreflex function. The maneuver comprises a systemic bolus injection of 100 $μ$g sodium nitroprusside (SNP) followed by 150 $μ$g phenylephrine (PE). On the one hand, this results in an increase in oxyhemoglobin and total hemoglobin followed by a decrease within the cerebral sample volume illuminated by near-infrared spectroscopy (NIRS). On the other hand, it produces a decrease in cerebral blood flow velocity (CBFv) within the middle cerebral artery (MCA) during SNP and an increase in CBFv during PE as measured by transcranial Doppler ultrasound. To resolve this apparent discrepancy, we hypothesized that SNP dilates, whereas PE constricts, the MCA. We combined transcranial Doppler ultrasound of the right MCA with NIRS illuminating the right frontal cortex in 12 supine healthy subjects 18-24 yr old. Assuming constant O2 consumption and venous saturation, as estimated by partial venous occlusion plethysmography, we used conservation of mass (continuity) equations to estimate the changes in arterial inflow ($Δ$Qa) and venous outflow ($Δ$Qv) of the NIRS-illuminated area. Oxyhemoglobin and total hemoglobin, respectively, increased by 13.6 ± 1.6 and 15.2 ± 1.4 $μ$mol/kg brain tissue with SNP despite hypotension and decreased by 6 ± 1 and 7 ± 1 $μ$mol/kg with PE despite hypertension. SNP increased $Δ$Qa by 0.36 ±.03 $μ$mol˙kg-1˙s-1 (21.6 $μ$mol˙kg-1˙min-1), whereas CBFv decreased from 71 ± 2 to 62 ± 2 cm/s. PE decreased $Δ$Qa by 0.27 ±.2 $μ$mol˙kg-1˙s-1 (16.2 $μ$mol˙kg-1˙min-1), whereas CBFv increased to 75 ± 3 cm/s. These results are consistent with dilation of the MCA by SNP and constriction by PE. © 2013 the American Physiological Society.

American Journal of Physiology - Heart and Circulatory Physiology