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Changes in morphology of the pulse oximeter waveform during the onset of spinal anaesthesia for caesarean section

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Changes in the Morphology of the Pulse Oximeter Waveform

During the Onset of Spinal Anaesthesia for Caesarian Section L Hammon, I Wrench, S Handa*, R Mahajan*

Anaesthetic Department, Sheffield Teaching Hospitals NHS Foundation Trust, *Anaesthetic Department, Nottingham University Hospitals NHS Foundation Trust


Pulse oximeter wave represents digital blood flow.1 In young people, digital pulse volume has two clearly defined peaks.2 1st represents pressure wave from the heart to finger. 2nd (dicrotic) is a backward-travelling ‘reflected’ pressure wave (speed of which depends on the peripheral vascular resistance).

We report an ethically approved observational study -investigating changes in pulse oximeter waveform morphology during spinal anaesthesia (SA) for LSCS -and if these changes may be used for early detection of hypotension.


Ten patients presenting for elective LSCS Radical Seven® pulse oximeter and finometer (continuous NIBP) attached prior to SA

Management of case at discretion of attending anaesthetist


For seven patients the systolic blood pressure (SBP) fell >25% within 10 minutes of SA. Dicrotic notch migrated from the primary waveform before hypotension in six of these. This change was observed in only one of the patients not experiencing hypotension.

DISCUSSION The interval between the dicrotic wave and the

primary waveform increases with vasodilatation. 1-4 Such vasodilatation in the hand is likely during SA for LSCS from sympatheic blockade.5

This study suggests it may be possible to analyse the position of the dicrotic wave and detect the onset of hypotension during the establishment of SA. We used a crude method here to investigate this phenomenon.

Future studies should involve downloading the signal for analysis, rather than capturing screen images.

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