By Jonathan Gomez (left), Aarti Sarwal (left), John Bennett (center), Chandrika Garner (center), Adrian Lata (right), Charles Tegeler IV (right)
Neurological complications in the perioperative period of cardiothoracic (CT) interventions occur in about 50-60 percent of patients. These complications may range from focal neurological deficits to confusion and memory deficits, with stroke rate approaching 2-8 percent. Patients who experience operative complications exhibit an increased mortality rate of 10-21 percent. Neurointensivists may be called to see patients postoperatively for management of these complications. Yet, postoperative reduction of cerebrovascular events poses a challenge as these patients have multiple cardiovascular risk factors predisposing them to insults. Therefore, the emphasis of management for this group needs to be prevention of neurological injury by preoperative selection and intraoperative monitoring with selection of patients for screening based on the largest predictors for intraoperative stroke: so far known risk factors for post-operative stroke include advanced age, > 80 years old, non-elective surgery, renal disease and vascular disease including previous stroke.
There are two proposed mechanisms of operative neurological complications: embolic events and hypoperfusion. One mechanism of injury is due to either embolic phenomena, macroemboli (> 100 µm) or microemboli. Macroemboli arise from the disruption of atherosclerotic plaque during physical manipulation of vessels, mainly of the aorta: specifically, any type of aortic clamping, cannulation, aortic valve exposure, creation of proximal coronary graft anastomoses and placement of antegrade cardioplegia catheters during surgery. Animal models suggest that solid emboli are largely responsible for neurological deficits in coronary artery bypass graft (CABG). Microemboli formation, comprised of gaseous or solid particulate, is typically the consequence of cannulation of vessels or generated by bypass components when cardio pulmonary bypass is utilized intraoperatively.[4-5]. The ability of transcranial doppler (TCD) to accurately discern emboli composition is currently under investigation, requiring further validity but holds promise due to lack of any alternate diagnostic modality that helps distinguish the emboli by composition.[5-6] The second mechanism of injury is due to a persistent hypoperfusion state inherent of the bypass process leading to cerebral ischemia clinical effect of which depend on the ischemic threshold of the patient. This hypoperfusion state is more associated with generalized deficits versus focal neurological deficit. Neuronal injury is likely exacerbated by inflammatory processes resulting from inherent operative process or ischemia and reperfusion caused by it. In addition, the operative technique performed, on-pump vs. off pump, in CABG surgery affects the number of embolic signal detected intraoperatively, with on-pump producing more intraoperative emboli. [9-10] However, there has been no significant difference in level of neurocognitive function at six weeks and six months after surgery between the two techniques.
Preoperative evaluation can reduce the incidence of neurologic complications by allowing formal identification and treatment of preexisting risk factors as well as optimal planning for individualized surgical approach and anesthesia management, including neurointensivist consultation and intraoperative monitoring. Various neuromonitoring devices like bispectral index (BIS), near-infrared spectroscopy (NIRS) and TCD have all been used and validated as modes of monitoring during cardiothoracic surgery. NIRS is limited by hemodynamic resolution of areas scanned and BIS monitoring is limited by its interaction with anesthetics. TCD, on the other hand, directly assesses cerebral perfusion patterns and their responses and allows the ability of dynamic monitoring for embolic signals, including composition differentiation. TCD in both carotid endarterectomy and CABG surgery has been investigated for neuromonitoring for both hemodynamic changes and emboli detection.
TCD functions through the use of ultrasound and doppler shifts. Ultrasound waves are emitted through the temporal window and reflected from any tissues they encounter. The doppler shift is calculated from the reflected sound waves off red blood cells. Based on the magnitude of the shift, velocity and direction can be determined. Using these principles, real-time dynamic monitoring of cerebral hemodynamics can be recorded. With the advent of continuous TCD devices that utilize specialized head gear, continuous hands-free monitoring has become feasible, and TCD can be monitored for hours at a time once the device is set up.
We present two descriptive cases for neuromonitoring with TCD in CABG that highlights clinical algorithms that can be used to detect high risk patients and target them for neuromonitoring with the goal of preventing intra and postoperative cerebrovascular and global ischemic events. We identified a 72-year-old male with hypertension, diabetes, hyperlipemia, three-vessel coronary artery disease and recent right PCA stroke for CABG hence concern for further CABG related cerebrovascular events. Preoperative TCD showed no intracranial stenosis with adequate temporal windows. Preoperative CT angiography showed diffuse intracranial atherosclerosis with no significant stenosis except for a previously known right P2 occlusion. This pre-operative evaluation allowed us to screen this patient for adequate temporal windows for insonation, detect the presence of focal lesions and establish baseline perfusion parameters. On the day of surgery, preoperative neurological exam was performed. TCD monitoring was initiated after induction of general anesthesia. Baseline perfusion parameters were established and embolic monitoring for high-intensity transient signal (HITS) was initiated prior to initial incision by the surgeon. TCD data was then collected throughout the course of the surgery. Patient underwent on pump cardiopulmonary bypass surgery. HITS and MCA mean flow velocity monitoring were done. Both the anesthesia and the cardiothoracic surgical team were alerted to any detected emboli, with increased focus during cannulation, clamping and removal of clamp. The anesthesia team was informed of any significant change in perfusion from baseline, to allow optimization of systemic hemodynamics. BP was targeted to maintain adequate cerebral perfusion while avoiding excess hypertension that could contribute to intra-operative bleeding. A postoperative neurological exam was performed and did not show any neurological deficits.
Our second patient was a 61-year-old female with diabetes, hypertension and prior left subcortical stroke with no residual deficits. Her preoperative TCD was abnormal for focal intracranial stenosis in the left MCA and left vertebral artery. CTA confirmed 50 percent stenosis of left vertebral artery, chronic occlusion of the right vertebral and diffuse intracranial atherosclerotic changes, greatest in the left M1 and M2. Patient underwent on-pump CABG with monitoring. Intraoperative TCDs were monitored for focal changes in left MCA in response to systemic hypotension and BP was targeted to avoid excessive decrements. Postoperatively patient’s neurological exam was unremarkable with NIHSS of 0.
Currently there are no established guidelines that define at risk population for cerebrovascular risk factors based on neuroimaging or underlying focal intracranial stenosis which are intuitive risks for cerebrovascular events. There is no specific guidance on key parameters for neuromonitoring during CABG to prevent cerebrovascular events either. The American Society of Neuroimaging makes a positive recommendation, based on strong consensus of class III evidence, for its use. TCD is a helpful tool for intra-operative monitoring that should be explored on a larger scale through pragmatic trials in creating algorithms that help identify at risk patients pre-op, define key parameters for monitoring intra-op with the goal of preventing post-op CV complications. Neuro-intensivists can help with development of institutional clinical algorithms in conjunction with cardiothoracic surgeons and anesthesiologists to create a multidisciplinary team-based approach for pre-op selection, intraoperative neuromonitoring and post-operative care of these patients. By utilizing this strategy, reduction in the incidence of peri-operative cerebrovascular events may be possible and will allow neurointensivists to play a preventive rather than reactionary role in patient care. Randomized trials would be beneficial to determine the risk reduction ability of the strategies.
Figure 1. Figures highlight the large magnitude of HITs, > 100 that can guide better surgical practices for cannulation and manipulation of the aorta. [9,15] Microemboli of gaseous composition comprise a majority of the HITS visualized (about 85 percent) while the remaining HITS are of solid composition of differing sizes.[16-17]
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