Objective: To determine any differences in cerebral perfusion pressure in patients with chronic hypertension compared with those with chronic hypertension and superimposed pre-eclampsia.
Design: A prospective observational study.
Setting: University hospital clinic and labour and delivery suite.
Participants: Fifteen women with chronic hypertension and 15 with superimposed pre-eclampsia.
Methods: Transcranial Doppler ultrasound was used to measure blood velocity in the middle cerebral arteries of the patients. Systemic blood pressure in the brachial artery was measured simultaneously. Middle cerebral artery. resistance index, pulsatility index, and cerebral perfusion pressure were calculated and plotted on the same axes as data from normal pregnant women. Cerebral perfusion pressure values outside of the 5th and 95th centiles were regarded as abnormal. Cerebral perfusion pressure data from the chronic hypertension and superimposed pre-eclampsia groups were also expressed in terms of the number of normative standard deviations from the mean value for normal pregnancy (Multiples of the Standard Deviation: MOS). All studies were conducted before labour, under similar conditions, and before volume expansion or treatment. Statistical analysis was by Student's t test and Fisher's exact test as appropriate with significance set at a two-tailed P<0.05.
Results: Patient demographics and blood pressure were not significantly different between the two groups. The resistance index and pulsatility index were not significantly different (neither absolute nor multiples of the standard deviation values). The absolute cerebral perfusion pressure was significantly higher in the patients with superimposed pre-eclampsia. The group of women with superimposed pre-eclampsia had a significantly higher mean value of cerebral perfusion pressure measured as multiples of the standard deviation from the mean value for normal pregnancy, despite there being no blood pressure difference.
Conclusions: Superimposed pre-eclampsia is associated with significantly higher cerebral perfusion pressure measurements compared with women with uncomplicated chronic hypertension. This is not directly related to a higher blood pressure. The difference in cerebral perfusion pressure may be used to speculate upon the pathophysiology of the increased risk for eclampsia seen in patients with superimposed pre-eclampsia.