Circulatory oscillations in post cardiac surgery patients
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The aim of this study was to monitor, analyse and compare circulatory oscillations in blood pressure (BP) and heart rate (HR) in ten patients undergoing elective CABG surgery. The subjects were monitored with continuous invasive blood pressure and 3-lead ECG before and after surgery. The recordings were limited to 1 hour in four different situations; pre-operative, post-operative while intubated, post-operative when extubated and the morning after surgery. Frequency analyses were performed with continuous wavelet transform (CWT), which revealed 4 - 6 specific frequency bands. The whole frequency range from 0,004 Hz to 2 Hz was then divided into six sub bands. The change in power in each band for each of the four situations were examined by mixed effects modelling. The CWT of heart rate showed a dramatic fall in power in all sub bands after surgery that remained throughout the study period. Blood pressure oscillations showed a gradually increase in the respiratory frequency band (band II). All other frequency bands showed a dramatic reduction while intubated, then an increase to a level approximately half of the preoperative level from extubation to the next day. This means that respiratory oscillations in blood pressure does not cause oscillations in HR. The enhanced oscillations in blood pressure might indicate a post-operatively intact mechanical system. The diminished oscillations in HR might therefore come from denervation during the surgical procedure.