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- Brain Injury and Protection During Heart Surgery | SpringerLink
- Life support during heart surgery
A microembolism can arise from air bubbles, fat particles, platelet aggregates, tube fragments, silicone antifoam materials and glove powder. An embolism may also originate from diseased carotid arteries. Transcranial Doppler ultrasound technology has revealed microembolic signals in all patients undergoing cardiac surgery with CPB The number of microembolic signals is significantly higher in patients undergoing valve replacement than in those undergoing CABG surgery The clinical consequences of cerebral microemboli may be dependent not only on the number but also on the composition e.
Modern perfusion equipment venous reservoir, membrane oxygenator, arterial line filter appears to effectively remove air bubbles from the CPB circuit. Special attention must be paid to removing air from the ascending aorta and the heart open heart surgery before the air is ejected into the arterial circulation when the heart restores effective ejection at the end of CPB 45 , Recently, thousands of focal dilatations in terminal brain arterioles and capillaries were found in patients who died shortly after CPB These small capillary and arteriolar dilatations SCAD were thought to be gas bubbles or fat emboli.
Shed blood scavenged from the mediastinum and reinfused via the CPB circuit seems to be the main source of cerebral fat embolism 50 , Unfortunately, current venous and arterial line filters inadequately protect patients from lipid microemboli An increased duration of CPB was associated with a larger embolic load, and the embolic load declined with increasing survival time after CPB The number of large and medium size microemboli declined with time after CPB, which is consistent with the hypothesis that fat emboli break into smaller globules as they pass through the capillary network.
Since recovery of the SCADs, they have been suspected to be associated with neurological deficits or the decline in cognitive function after CPB Cerebral microemboli may cause neuronal dysfunction without killing the cells, as has been observed in cases of intermittent monocular blindness Studies in animal models have demonstrated that transpulmonary passage of marrow fat into the systemic circulation does occur Continuing neurological impairment, by contrast, was associated with brain atrophy and permanent lesions visible on MRI Cardiac output is approximately 3—3.
In conjunction with moderate or deep hypothermia, even more reduced pump flow rates have been used. However, patients with diabetes and cardiovascular autonomic neuropathy with orthostatic hypotension showed a significant drop of mean and diastolic flow velocities upon active standing suggesting impaired cerebral autoregulation Even with reduced pump flow rate, such low levels of cerebral blood flow have not been demonstrated However, regional or global reduction in cerebral blood flow and ischaemic injury can occur.
Factors influencing the incidence, severity and clinical outcome of the inflammatory response associated with CPB are not well understood Interaction between the patient's circulating blood and the artificial material surfaces of the CPB circuit induces humoral and cellular activation, i. This potential mechanism of brain injury remains speculative but would be consistent with the damage imposed by the systemic inflammatory response syndrome on other organs 4.
Furthermore, with microscopic gas emboli, inflammatory mechanisms probably play a greater role than ischaemia itself 66 , Independent risk factors have been identified for different types of brain damage after cardiac surgery. General predictors include advanced age and a history or the presence of significant hypertension After adjustment for other risk variables, female gender was shown to be a weak independent predictor of stroke On the other hand, female sex seems to be protective for late survival 68 , Individual patient counselling regarding the risk of stroke represents an important opportunity to aid patients as they weigh possible risks and benefits before the operation.
An atherosclerotic ascending aorta is the most significant predictive factor of adverse cerebral outcome after cardiac surgery, reflecting the role of aortic athero thrombo embolism as a cause of stroke A high correlation was present between age, severe atherosclerosis of the ascending aorta and atheroemboli An embolism from the ascending aorta may result from any manipulation of the vessel wall including surgery de cannulation de clamping, attaching proximal anastomoses and even a falsely directed cannula jet Evaluation of the ascending aorta with a CT scan underestimates lesions when compared with epiaortic echocardiography Transoesophageal echocardiography TEE often fails to identify lesions detected in epiaortic ultrasound, especially in the distal half of the ascending aorta.
Most dangerous lesions, i. Intraoperative epiaortic echocardiography revealed, however, a protruding atheroma of the anterior aortic wall white arrow. The proximal end of the radial graft was attached to the side of the left thoracic artery. The patient recovered uneventfully. Prospective, randomised, controlled trials evaluating the effect of modifying surgical techniques according to intraoperative epiaortic echocardiography do not exist and perhaps will never become due to ethical reasons.
Compared with general incidence rates, the incidence of postoperative stroke was reduced by modifying the procedure according to preoperative carotid duplex scanning and intraoperative epiaortic ultrasound 74 , Furthermore, screening by epiaortic echocardiography combined with exclusive use of Y grafts no aortic graft attachment resulted in significantly reduced late neurocognitive decline compared with conventional manual palpation of the aorta combined with proximal aortic anastomoses Based on epiaortic echocardiography manipulation, cannulation and clamping of diseased aortic segments can be avoided.
The new generation of anastomotic devices that permit rapid proximal aortic graft attachment without the need for aortic clamping may further help reduce aortic manipulation and thus the risk of embolism. In some centres, the severely diseased ascending aorta has been replaced with a prosthesis in DHCA In experienced hands, prophylactic carotid endarterectomy is superior to conservative therapy for prevention of stroke in patients with asymptomatic or symptomatic severe stenosis 83 , In patients scheduled for both carotid endarterectomy and heart surgery, the procedures may be done in either a combined or staged fashion.
The staged approach is probably more popular, particularly in elective operations. The superiority of neither approach has been established by prospective, controlled trials.
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Because of this gap in knowledge, the surgical approach is left to local team strategies based on careful assessment of team outcomes 3. In patients aged 65 or more, previous TIA or stroke, history of smoking, left main disease, peripheral vascular disease and female sex have been shown to be associated with severe carotid artery stenosis For safety and simplicity, it is customary in many centres to screen all patients aged 65 or more, while those with left main disease are screened at any age 3. In patients with chronic atrial fibrillation and unsatisfactory control of anticoagulation, intraoperative surgical manipulation or spontaneous resumption of sinus rhythm in the postoperative period may cause an embolism due to a left atrial clot.
In such cases, the left atrium can be evaluated by preoperative TEE. In urgent surgical cases, however, the procedure cannot be postponed and careful handling of the heart and ligation of the left atrial appendix are recommended. Although the mechanism of this association is unknown, prevention of atrial fibrillation may result in improved neurocognitive function.
In these patients, the role of anticoagulation has not yet been established. In general, aggressive anticoagulation and cardioversion may reduce cerebral complications associated with atrial fibrillation. In unpredictable situations, TEE may be helpful in detecting or excluding left atrial clots. In patients with recent or acute myocardial infarction, urgent or emergency CABG operations are performed today. Systemic embolism occurred in six of the 38 patients with thrombus formation, and in all of them, a left ventricle clot was detected before hospital discharge In such patients, preoperative TEE should be used to demonstrate or exclude thrombus formation.
Depression has commonly been perceived to be associated with an increased incidence of cognitive decline, particularly that of memory, after cardiac surgery. However, the majority of patients who are depressed after surgery have recently been shown to already be depressed before surgery Moreover, no correlation was detected between depressed mood and cognitive decline after CABG surgery, suggesting that depression alone cannot account for postoperative cognitive dysfunction.
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However, the evidence of Pl A2 polymorphism resulting in increased risk of thrombosis is still controversial 96 , Neurological outcome after DHCA is affected by several factors such as the method and rate of cooling and rewarming, blood glucose levels, haemodilution, pharmacological agents and age of the patient 14 , Selective antegrade cerebral perfusion SCP was first introduced in to protect the brain from ischaemic injury in aortic arch aneurysm surgery SCP necessitates additional time to identify and cannulate the carotid arteries, which in turn may damage the fragile endothelium of these vessels or dislodge atheromatous debris into the cerebral circulation.
A simpler technique of cerebral perfusion, retrograde cerebral perfusion RCP , was initially described in as a treatment for massive air embolism during CPB Continuous RCP was subsequently adopted in the early s for cerebral protection during aortic arch surgery despite the lack of evidence for its efficacy or safety.
Different techniques to perform RCP exist but most effective of these seems to be a method in which the entire venous system is pressurized The role of RCP in providing cerebral nutrition may be insignificant. The role of epiaortic ultrasound and carotid Duplex scanning, as well as measures in cases of atrial fibrillation, were already discussed.
To shorten the length of CPB and the time spent in the operating theatre, routine CABG surgery is increasingly performed also at normothermia. Hypothermia has been shown to reduce the risk of neurological complications in animal models. Postoperative neurological tests have shown a lower rate of neurological complications in the group with MBP above 80 mmHg.
It should also be highlighted the role of carbon dioxide PaCO 2 and glycemia. As mentioned above, lower PaCO 2 levels would play some role in microembolization prevention when heartbeats are restarted 8. Hence, the strict control of glycemia by anesthesiologists in the intraoperative period is a basic recommendation today 8,9.
Among these substances, special attention has been given to propofol for its effect on EEG spikes suppression. A question has been raised whether such effect could play a role in decreasing brain metabolism during CPB, thus acting as neuronal protector. According to Souter et al. Cell ischemia pathway inhibitors, such as calcium antagonists are being evaluated as potential protectors. Antidepressant agents could aid for a new strategy aiming at leukocytes inhibition, endothelial protection and excitatory aminoacid receptors inhibition, among others, promoting some degree of organic protection during CPB 9.
However, clinical confirmation of such results still requires further investigations. Even with protective methods and the proper care during pre, intra and postoperative periods, there are still patients who, submitted to similar risks, present different clinical evolutions, making us believe in a case-by-case genetic influence.
Brain Injury and Protection During Heart Surgery | SpringerLink
Currently, with the development of genetic research labs, it is possible to identify neuronal function markers. These markers would be involved in brain protection or a higher trend for cell injury after an aggression, or would even help patient's screening for more accurate prognosis or diagnosis. One of the several investigations in this area has targeted S protein, which may be a promising neuronal injury marker.
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S protein is not an usual part of blood plasma substances, but it is present in plasma after stroke, subarachnoid hemorrhage, brain trauma and CPB Trying to understand how this protein is released after CPB, Johnson et al. Data have shown that early S release was related to age and longer CPB time, with no correlation with neurological tests.
Late S release, however, was associated to positive neurological tests and the incidence of brain injuries. Although being a possible indicator of brain injury severity, S protein is unable to inform its anatomic extension or clinical consequences. While investigations progress toward finding effective substances to improve diagnosis, there are speculations about other already well-known substances, among them glutamate, which could be a brain injury marker. Reynolds et al. Glutamate concentration changes were obtained during surgery. Neuro-cognitive tests were applied before and after surgery.
It has been observed that glutamate concentration during surgery would vary according to each patient and with time. Based on intraoperative serum glutamate concentrations and neurological tests, Reynolds et al.
In the search for brain injury predictors, others substances were identified which could influence prognosis. Toward this goal, attentions were turned to apoliprotein E ApoE. Some studies already indicate that it could be involved with cognitive decrease after heart surgery. A multi populational study 18 has shown that ApoE e 4 allelic variation increases the risk for Alzheimer's disease, while e 2 allele has a protective effect.
It has been postulated that allele e 4 could be involved in brain blood flow auto-regulation. A study by Ti et al. Brain blood flow was measured during surgery using Xe and brain metabolic rate was calculated by the difference between O 2 concentrations in arterial and mixed venous blood obtained in the jugular bulb. The conclusion was that allele e 4 had no global effect on brain blood flow, O 2 supply and extraction, suggesting that allele e 4 effects on brain injuries are due to different mechanisms.
With that in mind, Drabe et al. Drabe's group has studied 22 patients by monitoring IL-8 and TNF- a concentrations thru radioimmunoassay in hour intervals after surgery, at the same time that they determined the presence of ApoE e 4 allele. Heart surgery has experienced major technological advances in recent years, not only with the advent of new anesthetic-surgical techniques, but also with the development of different drugs, which knowingly decrease the incidence of short and long-term cardiovascular complications.
Nevertheless, it has been observed that, even with good results in long-term follow-ups, there are still some complications for which it is not possible to determine, with a reasonable level of certainty, the predisposing event leading to morbidity. Postoperative neurological dysfunction is an important source of problems, which may vary from transient distress such as disorientation or attention deficits, to irreversible central nervous system injuries, such as brain hemorrhage and ischemia.
It is known that there are some risk factors for central nervous system related problems in the postoperative period of heart surgeries, such as CPB, severe ascending aorta and carotid atheromatous disease, inadequate anticoagulation during surgery with CPB, older age, induction of intraoperative body temperature changes, hyperglycemia, intraoperative acid-base correction methods, micro and macroembolization in CPB, intracardiac procedures, advanced cerebrovascular disease and previous cardiac disease.
Even so, these risk factors are still under investigation to determine their actual impact on postoperative neurological prognosis. Genetic predisposition to many diseases has been extensively studied. Results of a simple neurological evaluation, known as mini-mental, are compared to ApoE phenotyping to detect whether patients with phenotype for ApoE e 4 are prone to postoperative neurological complications. Sampling test suggests that an expressive number of patients is needed to achieve any result, still in the early stage of the study. This mini-mental evaluation is composed of a questionnaire with simple questions to test recent and late memory, before, at 48 hours and at hospital discharge.
If more severe complications or mini-mental changes are detected, the neurologist is called for specialized tests.
Life support during heart surgery
Therefore, it is necessary to establish, as accurately as possible, which are the factors predisposing to pathogenic processes and their effects. The significant risk of adverse neurological events during heart surgery explains the renewed interest on protective strategies, injuries pathophysiology and, more recently, on the genetics likely to be involved in main central nervous system injuries Department of Anesthesiology, Tokyo Medical University. Brain injury after cardiovascular surgery is associated with significantly reduced prognosis. The ability to predict and prevent brain injury during the perioperative period is thus important.
There are two categories of brain damage after cardiac surgery: type 1, focal neurologic deficits, coma and stupor; and type 2, decline in intellectual function and memory impairment. These types have an incidence of 3. With both types, mortality rates are increased. Important risk factors for brain injury after cardiovascular surgery include age, atherosclerosis of the central artery, intraaortic balloon pumping, diabetes mellitus, lung disease and alcohol abuse.
Cardiopulmonary bypass CPB can induce brain injury after cardiovascular surgery, as CPB can evoke embolization, low perfusion rates and inflammatory response. However, many reports suggest that these considerations are insufficient in low-risk patients.