Background

Studies have not explored consciousness during deep hypothermic circulatory arrest (DHCA). However, as studies in cardiac arrest have identified a spectrum of consciousness, we sought to establish the feasibility of studying consciousness during DHCA.

Methods

This was a prospective study across 10 hospitals with 36 DHCA patients undergoing thoracic aortic aneurysm repair or pulmonary endarterectomy. A tablet computer delivered audiovisual stimuli (images and names of three fruits) using headphones during each procedure as a potential test of implicit learning and explicit recall. We also established electroencephalography (EEG) and near-infrared spectroscopy (NIRS) to measure electrocortical markers of consciousness and cerebral oxygenation. Post-procedure interviews were carried out to test patients’ ability to recall the audiovisual stimuli as well other explicit memories. Primary outcomes: 1) Feasibility of establishing tests of explicit recall and implicit learning, 2) Electroencephalography testing during DHCA. Secondary outcomes: 1) Signs of explicit recall of memories or implicit learning, and 2) identification of electrocortical biomarkers of consciousness during DHCA.

Results

Overall, 29/36 (81%) had the tablet set up. All 36 had NIRS and EEG set up, but 9 (25%) had useable EEG data, and 23 (66%) NIRS data. Delta EEG waves were observed during circulatory arrest in 3/9 (33%) patients, while 1/9 (11%) had theta waves just prior to circulatory arrest. All others showed isoelectric pattern. 35/36 (97%) agreed to follow up interviews. None had explicit recall of the names of the three fruits, but 3/36 (9%) correctly guessed them as a potential sign of implicit learning and 3 (9%) recalled other memories including events around the procedure and themes consistent with a recalled experience of death.

Conclusions

A spectrum of consciousness and awareness, including signs of implicit learning and electrocortical biomarkers of consciousness may be present during DHCA, despite absence of visible signs of consciousness. This can be further used to help explain the negative psychological outcomes that cardiac arrest survivors face.