title: Size distribution of air bubbles entering the brain during cardiac surgery.
abstract: BACKGROUND
  Thousands of air bubbles enter the cerebral circulation during cardiac surgery, but whether high numbers of bubbles explain post-operative cognitive decline is currently controversial. This study estimates the size distribution of air bubbles and volume of air entering the cerebral arteries intra-operatively based on analysis of transcranial Doppler ultrasound data.
  
  
  METHODS
  Transcranial Doppler ultrasound recordings from ten patients undergoing heart surgery were analysed for the presence of embolic signals. The backscattered intensity of each embolic signal was modelled based on ultrasound scattering theory to provide an estimate of bubble diameter. The impact of showers of bubbles on cerebral blood-flow was then investigated using patient-specific Monte-Carlo simulations to model the accumulation and clearance of bubbles within a model vasculature.
  
  
  RESULTS
  Analysis of Doppler ultrasound recordings revealed a minimum of 371 and maximum of 6476 bubbles entering the middle cerebral artery territories during surgery. This was estimated to correspond to a total volume of air ranging between 0.003 and 0.12 mL. Based on analysis of a total of 18667 embolic signals, the median diameter of bubbles entering the cerebral arteries was 33 μm (IQR: 18 to 69 μm). Although bubble diameters ranged from ~5 μm to 3.5 mm, the majority (85%) were less than 100 μm. Numerous small bubbles detected during cardiopulmonary bypass were estimated by Monte-Carlo simulation to be benign. However, during weaning from bypass, showers containing large macro-bubbles were observed, which were estimated to transiently affect up to 2.2% of arterioles.
  
  
  CONCLUSIONS
  Detailed analysis of Doppler ultrasound data can be used to provide an estimate of bubble diameter, total volume of air, and the likely impact of embolic showers on cerebral blood flow. Although bubbles are alarmingly numerous during surgery, our simulations suggest that the majority of bubbles are too small to be harmful.
authors.affiliation: University of Leicester Department of Cardiovascular Sciences, Leicester, United Kingdom; University Hospitals of Leicester NHS Trust, Leicester, United Kingdom; Leicester Cardiovascular Biomedical Research Unit, Leicester, United Kingdom.
authors.forename: Emma M L
authors.initials: EM
authors.lastname: Chung
authors.affiliation: University of Leicester Department of Cardiovascular Sciences, Leicester, United Kingdom; University Hospitals of Leicester NHS Trust, Leicester, United Kingdom.
authors.forename: Caroline
authors.initials: C
authors.lastname: Banahan
authors.affiliation: University of Leicester Department of Cardiovascular Sciences, Leicester, United Kingdom; Leicester Cardiovascular Biomedical Research Unit, Leicester, United Kingdom.
authors.forename: Nikil
authors.initials: N
authors.lastname: Patel
authors.affiliation: University of Leicester Department of Cardiovascular Sciences, Leicester, United Kingdom.
authors.forename: Justyna
authors.initials: J
authors.lastname: Janus
authors.affiliation: University of Leicester Department of Cardiovascular Sciences, Leicester, United Kingdom; Leicester Cardiovascular Biomedical Research Unit, Leicester, United Kingdom.
authors.forename: David
authors.initials: D
authors.lastname: Marshall
authors.affiliation: University of Leicester Department of Cardiovascular Sciences, Leicester, United Kingdom.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Horsfield
authors.affiliation: Leicester Cardiovascular Biomedical Research Unit, Leicester, United Kingdom.
authors.forename: Clément
authors.initials: C
authors.lastname: Rousseau
authors.affiliation: Open University Department of Physical Sciences, Milton Keynes, United Kingdom.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Keelan
authors.affiliation: University of Leicester Department of Cardiovascular Sciences, Leicester, United Kingdom.
authors.forename: David H
authors.initials: DH
authors.lastname: Evans
authors.affiliation: Open University Department of Physical Sciences, Milton Keynes, United Kingdom.
authors.forename: James P
authors.initials: JP
authors.lastname: Hague
mesh_terms: D000368:Aged; D001921:Brain / Q000098:blood supply*; D006348:Cardiac Surgical Procedures / Q000009:adverse effects*; D002536:Cerebral Arteries / Q000000981:diagnostic imaging*; D002560:Cerebrovascular Circulation; D004618:Embolism, Air / Q000175:diagnosis / Q000000981:diagnostic imaging* / Q000209:etiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009010:Monte Carlo Method; D010316:Particle Size; D017585:Ultrasonography, Doppler, Transcranial
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