Changes in flow velocity, resistance indices, and cerebral perfusion pressure in the maternal middle cerebral artery distribution during normal pregnancy

Background. There are few longitudinal data currently available detailing the normal changes in maternal cerebral hemodynamics during human pregnancy. This lack of information limits the study of pregnancy-associated cerebrovascular adjustments and, in particular, preeclampsia, where the brain appears to be especially susceptible to ischemic and encephalopathic injury. Our objective was to define the hemodynamic changes, specifically velocity, resistance indices, and cerebral perfusion pressure, in the middle cerebral artery (MCA) distribution of the brain during normal pregnancy. Methods and materials. Transcranial Doppler ultrasound was used to determine the systolic, diastolic and mean blood velocities in the middle cerebral arteries in non-laboring women studied longitudinally during normal gestation. The resistance index (RI), pulsatility index (RI), and cerebral perfusion pressure (CPP) were calculated using the velocity and blood pressure data. Data were analyzed using a longitudinal statistical model incorporating random patient effects and a homoscedastic (compound symmetric) variance-covariance structure over time (gestational age). The predicted mean value (Least Squares Mean), and the 5th and 95th percentiles, were defined for normal pregnancy. Results. MCA systolic velocity decreased (24%) as did the mean velocity (17%). The diastolic velocity did not change significantly. The MCA RI decreased by 19% and the PI decreased by 25%. The MCA CPP increased by 52% between 12 and 40 weeks of gestation. Conclusions. The normative ranges for MCA velocity, RI, and CPP have been defined in normal human pregnancy using longitudinally collected data. By having a defined normal range, identification of abnormalities in cerebral hemodynamics during pregnancy is now possible, and this may help researchers and clinicians to elucidate etiologies and treatments for pregnancy-related pathophysiologic states such as preeclampsia.