Effect of pre-spinal non-invasive blood pressure management in caesarean section on incidence and magnitude of hypotension
Dr T Orr, ST5 Anaesthesia, Dr I Wrench, Consultant Anaesthetist, Sheffield Teaching Hospitals NHS Foundation Trust
The incidence of maternal hypotension during spinal anaesthesia for caesarean section is as high as 71%.1 A consensus definition of hypotension in this context is lacking but is commonly accepted as a fall in systolic blood pressure (sBP) below 80% of baseline or an absolute value below 100mmHg.1 Maternal hypotension has been associated with lower foetal pH at delivery2 and increases the incidence of maternal nausea, vomiting and dizziness.3 We looked at the variation in practice among anaesthetists in the measurement of maternal non-invasive blood pressure at the beginning of caesarean section under spinal anaesthesia at a tertiary obstetric centre.
We analysed data recorded for a blinded, ethically approved research project where sBP was measured by a researcher using a Finometer4 and conventionally by the anaesthetist according to their usual practice to determine the timing of non-invasive blood pressure measurements at the beginning of anaesthesia. Phenylephrine was given as repeated boluses at the discretion of the anaesthetist. Where non-invasive blood pressure was not measured by the anaesthetist before performing spinal anaesthesia, the pre-spinal value was extrapolated from the Finometer.5 This method gives an accurate estimate of the changes of systolic and diastolic pressure but is relatively inaccurate at measuring absolute values.5
We analysed data from 14 anaesthetics for elective caesarean section. A non-invasive blood pressure reading was taken before spinal anaesthesia in 6 (43%), and no baseline was taken in 8 (57%). There was an even spread of consultant and specialty trainee anaesthetists. Where a non-invasive blood pressure was taken prior to performing spinal anaesthesia, the mean lowest sBP throughout the operation was 99mmHg (range 80-108) and the mean fall in sBP before delivery was 19mmHg (13%, range +10- -40%). Three out of six cases in this group (50%) met the definition of hypotension. In cases where a non-invasive blood pressure measurement was not taken before performing the spinal, the mean lowest sBP was 87mmHg (70-98mmHg) with a mean fall before delivery of 40mmHg (29%, range 0-50mmHg). All of the patients in this group (100%) met the definition of hypotension.
In this sample, in more than half the cases a baseline blood pressure was not determined prior to performing spinal anaesthesia, despite all of the cases being elective. Although the use of a Finometer instead of blinded NIBP readings may reduce accuracy, it is well validated in tracking changes in blood pressure and the extrapolation will have minimised the effect of this. In any case, those who did not determine the baseline blood pressure had greater haemodynamic instability with a greater incidence and severity of hypotension. It is not possible to determine from this study the reasons for this but it is possible that those who ascertained a baseline blood pressure were more aware of what constitutes relative hypotension in an individual patient and may have been more attentive in reacting to this, thus minimising haemodynamic instability.