ECG changes during caesarean section are common. Incidence of ST depression on the ECG is up
to 81% in some studies. Although this may indicate inadequate oxygen supply to the heart
muscle (myocardial ischaemia) many other theories have been suggested including air entering
the circulation from the placental bed, high heart rate, hormone or nervous system influences
and spasm of the coronary blood supply. Perioperative ST depression often reflects an
imbalance between heart muscle oxygen supply and demand. At the time of delivery, high heart
rate is common and there is a further increase in the amount of blood the heart has to pump
every minute due to blood coming back to the circulation from the placental bed. This
increases oxygen demand and most ST changes are seen at the time of delivery or within 30
minutes. The clinical significance of these changes is much debated, and apart from a few
case reports do not appear to be associated with poor heart muscle function or ischaemia
(lack of oxygen supply). Management of the mother's blood pressure during caesarean section
has changed greatly in recent years. Intermittent boluses of ephedrine, given when blood
pressure is low, have been replaced with prevention of low blood pressure and phenylephrine
has become the drug of choice. Ephedrine increases heart rate and contractility of the heart
muscle and is likely to increase oxygen demand. Phenylephrine reduces heart rate while
maintaining blood pressure which may result in a more favorable oxygen supply demand ratio.
The investigators aim to compare the incidence of ECG changes if the mother's blood pressure
is maintained with phenylephrine as compared to ephedrine. To see if these ECG changes are
associated with myocardial ischaemia, the investigators will perform troponin T analysis
after delivery. Troponin T is a molecule released by ischaemic heart muscle.