The definition of hypertension
in pregnancy is a systolic pressure of 140 mmHg or
a diastolic pressure of 90 mmHg on two consecutive
occasions 4 or more hours apart. Why chose these thresholds?
There is some evidence that suggests that a diastolic
pressure of >95 mmHg is associated with an increased
risk of stillbirth across all gestational ranges. In
addition to this further studies have shown that a
systolic pressure of between 140 and 150 mmHg taken
together with a diastolic pressure between 90 and 95
are associated with an increase in the perinatal mortality
rate. Over and above these pressures the perinatal
mortality starts to rise slowly and the curve becomes
steeper with the most significant increases in perinatal
mortality around pressures of 170/125 mmHg. Therefore
using a threshold diastolic pressure of 90 mmHg is
not without some evidence. However, it is still uncertain
as to whether this threshold should trigger treatment.
It is still not know whether the rise in blood pressure
associated with pre-eclampsia is a protective mechanism
to increase placental perfusion through already what
is a partly ischaemic placenta. In this way the mother
may be attempting physiologically to protect the fetus
from hypoxia. Alternatively the hypertension may be
a pathophysiological response to the release of "factor
X" perhaps from damaged endothelium. The rise
in blood pressure merely creates a vicious spiral with
more endothelial damage releasing further vaso active
substances. Which of these 2 explanations for the rise
in blood pressure in the disease pre-eclampsia is correct
is yet to be determined. Clearly if the rise in pressure
is a protective effect, lowering the blood pressure
may very well be detrimental to the fetus. It should
be remembered that autoregulation of the maternal cerebral
circulation only breaks down at blood pressures of
170/110 mmHg. The evidence that antihypertensive drugs
protect the mother from morbidity is most significant
at these higher levels.
In a recent meta analysis of antihypertensive therapy
in pregnancy it has been suggested that lowering the
blood pressure is associated with a reduction in fetal
birth weight. The researchers postulated that this
was due to poor placental perfusion. However, they
could not rule out specific drug effects. The question "which
levels of blood pressure require treatment in pregnancy?" still
needs to be answered.
Once a clinician has decided to start treatment then
it is clear that there are 2 main antihypertensive
drugs from which to choose. The first is Methyldopa
which has a long track record and is known to be safe.
However, in a significant number of women, it causes
severe side effects necessitating replacing it with
another drug. This has led to the use of Labetalol
as an alternative first line agent. This drug also
has a good safety profile in pregnancy. The second
line agent of choice is Nifedipine and although this
drug is not licensed for use in pregnancy it has been
used for many years by obstetricians with good effect.
In the acute situation Hydralazine has
traditionally been the drug of choice to control high blood pressure. However
recent data has suggested that oral Nifedipine may be just as effective in
controlling severe hypertension although there is a theoretical problem when
this drug is combined with magnesium sulphate resulting in precipitate falls
of blood pressure. If this does occur there may be a consequent reduction in
the placental flow causing fetal distress.
|
