Oxytocin is an“old” vasoactive peptide with a complex hormonal activity. Specific receptors for oxytocin have been described in all kinds of tissue such as the myocardium, vessels, central nervous system, lactating glands, and the myometrium.
5 IU IS THE RECOMMENDED DOSE NOT 10 IU
Oxytocin has been considered to be a safe, harmless drug and it is used on a daily basis in obstetric practice to induce labour, stimulate uterine contractions, and limit postpartum bleeding. The CEMACH report of 1997–9 from UK, where the death of two mothers with cardiovascular instability was related to cardiac arrest after oxytocin, has changed practice from 10 IU to 5 IU at most anaesthesia departments. Recently, it was demonstrated that 5 IU also leads to hypotension and tachycardia when given as a rapid IV bolus, compared with more modest cardiovascular disturbances during slow infusion.
According to Cochrane reports, prophylactic oxytocin produces benefits in terms of postpartum haemorrhage, when compared with no uterotonic agent use, but there is still insufficient information about other outcomes and side-effects.
CARDIOVASCULAR EFFECTS OF OXYTOCIN AND ECG CHANGES
IV bolus of 10 IU of oxytocin can produce a transient hypotension, tachycardia, ECG ST-T depression, and the elevation of spatial ST-change vector magnitude (STC-VM) in women during CS under spinal anaesthesia and in healthy non-pregnant, non- anaesthetized controls.
The cardiovascular and ECG changes often observed during CS are related to the administration of oxytocin mostly and not necessarily due to pregnancy, spinal anaesthesia, surgical procedure, or delivery.
Oxytocin induces striking ECG changes and subjective symptoms that are typically associated with myocardial ischaemia. The symptoms of flush, chest pain, and dyspnoea are closely related to the injection of oxytocin and the cardiovascular effects.
Most previous studies of this topic have used Holter ECG with averaging periods of 30–60 s and intermittent non-invasive arterial pressure measurements. With continuous monitoring and short averaging periods of 15 s, the short-lasting cardiovascular and ECG-changes related to the injection of oxytocin can be observed more frequently.
Some researchers have used ECHO but were unable to demonstrate myocardial ventricular wall motion abnormalities as a sign of myocardial ischaemia during CS. The method is, however, difficult to perform during this procedure.
Another group of researchers demonstrated frequent episodes of ST-segment depression by continuous Holter monitoring both perioperatively and up to 12 h after delivery. They also found a release of cardiospecific troponin T as a biochemical marker of myocardial ischaemia in two of the patients with ST-T depression.
Use of both continuous 12-lead ECG and computerized VCG that has the advantage to provide both a magnitude and direction of a spatial vector reflecting changes in myocardial electrical activity in three dimensions (STC-VM) are better for studying this response of oxytocin. An increase in STC-VM of more than 50 mV is considered to be a more sensitive and specific sign of myocardial ischaemia than conventional ECG. It need to be stressed thought that the spatial vector is sensitive to postural changes of the heart.
The first small increase in STC-VM changes observed initially may be due to change in position of the operation table and reduction of abdominal content at delivery. The large increase in STC-VM may be related to myocardial ischaemia induced by oxytocin injection.
A combination of profound hypotension, tachycardia, and coronary vasoconstriction can cause a mismatch between myocardial oxygen demand and supply, leading to myocardial ischaemia even without co-existing coronary disease. Coronary artery disease in pregnant women is extremely uncommon. A summary by Kulka and colleagues in 2001 reported 136 cases of myocardial infarction during pregnancy. Half of them had normal coronary vessels, which suggests that a mechanism of coronary spasm or thrombosis could have been involved. Several case reports of myocardial infarction and even cardiac death in mothers show that cardiac complication is a risk that has to be considered in some parturients.
Use 5 IU and as slow infusion rather than rapid bolus.
Monitor patients closely if ECG changes persist into the peri-op period.
Myocardial infarction do occur with Oxytocin treatment.