Can Changes in the Pleth Variability Index be Used to Predict Significant Hypotension During Spinal Anaesthesia for Caesarean Section?

BACKGROUND: When a baby is delivered by a surgical procedure (a Caesarean Section) the women will usually have a spinal anaesthetic. With a spinal anaesthetic local anaesthetic is injected into the spinal fluid to make the woman numb from her chest to her feet and this allows her to be comfortable but awake during the birth. One of the common side effects of a spinal anaesthetic is that the woman's blood pressure may fall making her feel unwell and (rarely) causing stress to the unborn baby. During a caesarean section the blood pressure is usually measured every few minutes and it may fall too quickly to be detected in a timely fashion by this intermittent means. The monitor the Masimo Rainbow SET® (MRS) uses a peg attached to a finger to detect (amongst other things) the flow of blood through it. One of the measures it takes is called the Pleth Variability Index (PVI) which (unlike standard blood pressure measurement) it measures continuously. The investigators have used the MRS on one of their patients who was having a caesarean section under a spinal anaesthetic and the PVI was seen to change before the patients blood pressure fell. Because this was only one case they would like to try the MRS on some more patients to see if the PVI can be used to predict falls in blood pressure. To keep a close eye on the blood pressure they would use another monitor called a finometer. A finometer measures blood pressure continuously by means of a small cuff that is placed around a finger. The finometer should allow them to follow changes in blood pressure more closely and see how these affect the PVI reading.

AIMS: The aim of this study is to see whether changes in PVI happen before a fall in blood pressure when a woman is having a spinal anaesthetic for a caesarean section.

METHODOLOGY: The women will have exactly the same anaesthetic as they would have had if they were not in the study. The investigators will simply be using two pieces of equipment (the MRS and the Finometer) to see what happens when and if the blood pressure falls.

EXPECTED OUTCOMES: The investigators hope that they will find that the PVI changes before the blood pressure falls significantly.

IMPLICATIONS: If this study were to be promising then the investigators could go on to perform another study where they use the MRS to help them decide when to give drugs and intravenous fluids to stop the blood pressure from falling.