Abstract
BackgroundThousands 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.MethodsTranscranial 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.ResultsAnalysis 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.ConclusionsDetailed 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.
Highlights
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
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
Transcranial Doppler (TCD) ultrasound techniques are highly sensitive to small air emboli and typically reveal thousands of embolic signals originating from tiny air bubbles entering the brain during surgery
Summary
Air bubbles entering the cerebral circulation intra-operatively have potential to obstruct blood flow, and are thought to be a source of endothelial irritation and inflammation.[1,2] bubbles entering the circulation during heart surgery have long been implicated as a possible cause of post-operative cognitive decline, the potential for adverse clinical sequelae due to bubbles in the bloodstream remains controversial.[2,3,4,5] Potential alternative explanations for cognitive decline include the effects of age, pre-existing cognitive decline, and pre-existing cardiovascular disease, combined with peri-operative stressors, such as the use of anaesthesics, haemodynamic changes during cardiopulmonary bypass, impact of particulate emboli, impaired regulation of cerebral blood flow, and inflammatory responses. Since there is currently no method for determining the size distribution and volume of bubbles reaching the cerebral circulation, it is difficult to assess whether quantities of air typically introduced during surgery are high enough to result in cognitive decline. Since any potential adverse impact of bubbles on cerebral blood flow is sensitively related to bubble size, the ability to estimate the sizes of bubbles in the bloodstream may be useful for distinguishing clinically significant macrobubbles from tiny microbubbles that pass harmlessly through the vasculature and are outgassed by the lungs.[2] existing technologies are capable of detecting and sizing bubbles moving through Cardiopulmonary Bypass (CPB) lines,[8] no studies have so far been performed to investigate the size distribution and volume of air entering the brain intra-operatively. 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.
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