Neurological impact of emboli during adult cardiac surgery

Abstract

Objectives: This study draws on advances in Doppler ultrasound bubble sizing to investigate whether high volumes of macro-bubbles entering the brain during cardiac surgery increase the risk of new cerebral microbleeds (CMBs), ischemic MR lesions, or post-operative cognitive decline (POCD).

Methods: Transcranial Doppler (TCD) ultrasound recordings were analysed to estimate numbers of emboli and macrobubbles (>100 μm) entering the brain during cardiac surgery. Logistic regression was used to explore the hypothesis that emboli characteristics affect the incidence of new brain injuries identified through pre- and post-operative MRI and neuropsychological testing.

Results: TCD, MRI, and neuropsychological test data were compared between 28 valve and 18 CABG patients. Although valve patients received over twice as many emboli per procedure [median: 1995 vs. 859, p = .004], and seven times as many macro-bubbles [median: 218 vs. 28, p = .001], high volumes of macrobubbles were not found to be significantly associated with new CMBs, new ischaemic lesions, or POCD. The odds of acquiring new CMBs increased by approximately 5% [95% CI: 1 to 10%] for every embolus detected in the first minute after the release of the aortic cross-clamp (AxC). Logistic regression models also confirmed previous findings that cardiopulmonary bypass time and valve surgery were significant predictors for new CMBs (both p = .03). Logistic regression analysis estimated an increase in the odds of acquiring new CMBs of 6% [95% CI: 1 to 12%] for every minute of bypass time over 91 mins.

Conclusions: This small study provides new information about the properties and numbers of bubbles entering the brain during surgery, but found no evidence to substantiate a direct link between large numbers of macrobubbles and adverse cognitive or MR outcome. Clinical Trial Registration URL - http://www.isrctn.com. Unique identifier: 66022965.

Keywords: Cardiopulmonary bypass; Cerebral microbleeds; Cognitive outcome; Embolization; MRI brain imaging.

Fig. 1

Bubble diameters estimated for a 55 year old male undergoing aortic valve replacement who had pre-existing cerebrovascular disease (total lesion volume: 1876 mm³) but no pre-existing CMBs. This patient exhibited cognitive decline in 2/8 tests with 10 new CMBs but no new ischemic lesions. Markers denote individual emboli (blue: left MCA events, orange: right MCA events). The vertical axis and marker size reflect estimated bubble diameter [7]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Positions of (A) new cerebral microbleeds, and (B) new MRI FLAIR lesions seen in the combined imaging data for all 46 patients. Only 4 new lesions were observed in the right hemisphere MCA territory compared to 14 in the left MCA territory; consistent with fewer emboli identified entering the right MCA in the 1 min following release of the AxC (right: 601 emboli, vs. left 1247 emboli). A similar observation was made for the CMBs, with 38 found in the right MCA territory and 58 in the left MCA territory. Estimated total numbers of bubbles and volume of air entering the MCA territories were similar for both hemispheres.