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How to Prevent Neurologic Complications in Cardiac Surgery


How to Prevent Neurologic Complications in Cardiac Surgery

After cardiac surgery, neurologic complications are second only to heart failure as a cause of morbidity and mortality; and the likelihood of requiring long-term care is significantly increased by the presence of neurologic sequelae.

The three categories that most neurologic problems following cardiac surgery can be divided to are:

 

  • Stroke
  • Neuropsychiatric
  • Abnormalities or encephalopathy peripheral neuropaties.

 

Of over 400,000 cardiac surgeries between 1996 and 1997, there’s a 3.3% of overall incidence of a new neurologic event (unexplained coma lasting more than 24 hours, stroke, or transient ischemic attack) as reported by the Society of Thoracic Surgery National Cardiac Database. A second study has prospectively evaluated 2108 patients undergoing coronary artery bypass graft surgery (CABG) at 24 hospitals in the US between the years of 1991 and 1993. Overall, 6.1% of those patients suffered a cerebral complication, roughly equally divided between encephalopathy and stroke. One of the dominant risk factors was increased age.

 

HOW TO REDUCE NEUROLOGIC COMPLICATIONS:

 

Preoperative evaluation:

Preoperative evaluation allows formal identification and treatment of preexisting risk factors, reducing perioperative injuries. For instance, the risk of stroke during cardiac procedures is increased with significant carotid artery stenosis; in patients with significant bilateral carotid disease this risk could be as high as 20%. Performing a simultaneous carotid endarterectomy could reduce the risk of stroke in these patients during cardiac surgery.

 

In some patients with carotid stenosis, the risk of stroke could be reduced by performing carotid stenting procedures, but the efficacy of this procedure is still not clear. In patients with severe peripheral arterial disease, a previous stroke or transient ischemic attack, or those with an audible bruit, preoperative carotid duplex imaging should be considered. Additional knowledge concerning the risk for the individual patient is important for intraoperative management decisions, even if carotid revascularization isn’t planned. In the presence of significant carotid stenosis, the flow of cerebral blood will be improved when the doctor takes special care to avoid periods of sustained hypotension and to maintain a relatively high mean arterial pressure during and after bypass.

 

Aspirin:

The benefits of aspirin therapy in preventing thromboembolic complications have to be balanced by risk of associated bleeding complications in the case of patients undergoing CABG. Aspirin should be restarted within 6 to 24 hours after surgery in case it is stopped before surgery, which doesn’t appear to be necessary in many patients.

 

Atrial fibrillation:

The early postoperative period following CABG is complicated by atrial fibrillation in 15 to 40% of patients; after valve surgery, the risk becomes even higher. Beta blockers, sotalol, and amiodarone reduce the incidence of atrial fibrillation by 45 to 65% as reported by a meta-analysis of 58 trials involving over 8500 patients. Beta blockers had the greatest magnitude of benefit. Despite the noticeable reduction in AF, prophylactic drug therapy was linked with a nonsignificant reduction in stroke, which may have been due to a low rate of events, 1.2% vs. 1.4%.

 

Minimally invasive surgery:

The invasiveness of the operation is decreased, recovery time is shortened, and complications of cardiopulmonary bypass are eliminated when the doctor uses techniques to perform CABG without the use of cardiopulmonary bypass. In patients undergoing minimally invasive off-pump GABG (OP CABG) and conventional CABG, both randomized trials and observational studies have compared outcomes, including neurological outcomes. A few randomized trials have compared the incidence of neurologic complications in OP CABG and conventional CABG. The issues of stroke and neurocognitive dysfunction are addressed by the following observations from meta-analyses:

 

  • A 50% relative risk reduction of stroke with OP CABG that reflected an absolute risk reduction of one stroke per 100 procedures was reported by a meta-analysis of 41 randomized trials of almost 4000 patients.
  • There was a non-significant trend toward a lower rate with OP CABG at 30 days that wasn’t maintained at one year when a second meta-analysis of 37 randomized trials evaluated rates of neurocognitive dysfunction in 3369 patients.

 

It has been suggested by observational studies that high risk of subgroups may show more significant reductions in cerebral complication rates with OP CABG. Patients with severe atheromatous aortic disease, triple vessel disease, octogenarians, and peripheral arterial disease are included in this.

 

It is believed that a lower rate of embolization contributes to the lower rate of neurologic complications with OP CABG. This was illustrated in a small randomized trial in patients who were assigned to on-pump or OP CABG in which there was reduced retinal microvascular damage, a lower frequency of transcranial Doppler high-intensity transient signals (HITS), and lower S100 protein concentrations which is a marker of cerebral injury, among patients assigned to OP CABG.

 

CARDIOPULMONARY BYPASS:

Temperature, acid-base management and blood pressure are the three aspects of CPB that seem to influence the risk of brain injury during cardiac operations.

 

Bypass temperature:

The cerebral metabolic rate is reduced by systemic hypothermia by a factor of three to four for every 10ºC decrease in core temperature, and could provide protection against transient periods of cerebral ischemia. Conflicting results were produced by randomized trials that compared the safety of normothermeic and hypothermic CPB. However, in the case of closed chamber cardiac operations that require CPB, moderate hypothermia in the range of 32ºC is commonly employed and 28ºC for open chamber procedures. When re-warming on CPB is initiated, the possible protective effects of hypothermia cease. To prevent an increased risk of neurologic injury from central nervous system hyperthermia and air embolus formation as the solubility of gases in blood decreases, the re-warming should be performed gradually.

 

Acid-base status:

Acid-base balance is altered by hypothermia, affecting cerebral blood flow. Alpha-stat and pH-stat are the two main techniques for regulating acid-base balance during hypothermia. These techniques vary depending on whether pH and CO2 are based directly on values corrected to the patient’s actual temperature (pH-stat) or on blood gas measurements made on blood warmed to 37ºC (alpha-stat). the addition of CO2 to the blood during hypothermic CPB is required in the case of pH-stat management, but not in alpha-stat. Because of this, when pH-stat is employed, cerebral blood flow and possibly the risk of embolization is greater during hypothermia, since CO2 is a cerebral vasodilator. In comparison, cerebral autoregulation is preserved with alpha-stat management and, in a randomized trial of 70 patients, the risk of postoperative cognitive dysfunction was significantly lower with alpha-stat management; 20% vs. 49% at six weeks.

 

Blood pressure:

During CPB, mean arterial pressures between 50 and 70 mmHg range are common and well tolerated by most patients. However, higher mean arterial pressures may be desirable for patients with hypertension or known stenosis of the cerebral vessels, and may decrease cardiac and neurologic complications. Cerebral ischemia and infarction have been linked to relative hypotension during surgery, which could be mediated in part by reduced clearance of microembolic debris from the brain.

 

There has been concerns that the risk of intraoperative and perioperative bleeding may increase due to higher blood pressure. This issue was addressed in a randomized trial in which 248 patients undergoing nonemergent cardiopulmonary bypass were assigned to mean arterial pressures of 80 to 100 mmHg or 50 to 60 mmHg. A significantly lower rate of neurologic and cardiac complications or death at six months (4.2% vs. 12.9%) was associated with high pressures during cardiopulmonary bypass, without an increase in bleeding or in transfusion requirements or a difference in functional and cognitive status. Acute increases in central venous pressure caused by poor positioning or kinking of the venous drainage cannula should be avoided because they reduce cerebral perfusion pressures.

 

Patient positioning:

Even though all nerve injuries aren’t preventable, but the risk of injury can be reduced by proper attention to patient positioning and padding of potential pressure points. Cardiac surgery patients are usually placed supine with their arms either abducted at <90º or tucked at the sides. Care should be taken to avoid resting the arm over wires or the edge of the bed and not to abduct the patient’s elbow more than 90º. A cushioned pillow should support the weight of the head, and an interscapular towel roll should be placed to extend the neck. To reduce the incidence of plexopathies, minimal sterna retraction should be employed. The likelihood of cerebral air embolization, especially during open chamber procedures prior to the separation from CPB could be decreased by putting the patient in the Trendelenburg position intraoperatively. Retained air within the cardiac chambers and aorta collects within the aortic root in the Trendelenburg position, from which air can be vented prior to full ventricular ejection.

 

Intraoperative monitoring:

The incidence of neurologic complications during cardiac surgery could be decreased with the use of several intraoperative monitoring modalities, including electroencephalography (EEG), somatosensory evoked potentials, transesophageal echocardiography (TEE), and trascranial Doppler ultrasound (TCD). Data are strongest that the information obtained with transesophageal echocardiography could be used to reduce the risk of stroke, but the use of other techniques is still controversial.

 

Transesophageal echocardiography (TEE):

By detecting the presence of a patent foramen ovale, mural thrombus within the left ventricle, left atrial thrombus in patients with atrial fibrillation, proximal aortic dissection, vegetations in patients with suspected endocarditis, retained intracardiac air after open chamber procedures, prosthetic valve thrombosis, and severe atherosclerosis of the thoracic aorta, intraoperative TEE could reduce the rate of intraoperative strokes. TEE can guide aortic cross-clamp placement and construction of the proximal coronary artery bypass anastomoses to minimize dislodgement of atheromata because it is superior to palpation for detecting the site and severity of atherosclerotic disease.

 

As mentioned above, an increased risk of stroke is associated with large or mobile aortic atheromas detected by intraoperative TEE or epiaortic echocardiography. This observation provides the rationale for possible benefit from aortic endarterectomy in such patients. However, the incidence of stroke was significantly higher in a report in which 43 such patients underwent endarterectomy, than in those without endarterecomy; 35% vs. 12%.

 

Electroencephalography (EEG):

Because the frequency and amplitude of spontaneous cortical electrical signals decrease when cerebral blood flow falls to critical levels, EEG is able to detect cerebral ischemia. However, the utility of EEG during cardiac procedures is still debated. A significantly lower risk of postoperative stroke or death in patients who received intraoperative EEG monitoring was found by one retrospective review of 3328 patients undergoing carotid endarterectomy in 46 institutions during 1981, but the cause and effect haven’t been demonstrated.

 

Others:

 

Transcranial Doppler:

Arterial microemboli entering the cerebral circulation that is most frequent during placement and removal of the aortic cross-clamp and the aortic cannula can be detected by transcranial Doppler monitoring of the middle cerebral artery during cardiac surgery. In some patients, the number of microemboli exceeds 60. Intraoperative somatosensory evoked potential monitoring has shown that intraoperative strokes occur during periods when emboli are most frequently seen of TCD, despite the fact that the absolute number of emboli found on TCD doesn’t directly correlate with the risk of stroke or cognitive impairment. Whether TCD monitoring can be used to reduce the incidence of neurologic injury is still controversial in spite of these findings.

 

Somatosensory evoked potentials:

The onset and laterality of stroke during cardiac operations are detected by somatosensory evoked potentials monitoring, but its efficacy in minimizing the severity of an intraoperative stroke hasn’t been demonstrated.

 

S100 beta:

After neurologic surgery, the central nervous system protein S100 beta is released into the peripheral blood. During cardiac surgery, early studies have suggested that measurement of the serum concentration of S100 beta could be predictive of neurologic injury. However, because of cross reactivity of the assay antibody with unidentified proteins from other sources, subsequent reports have determined that intraoperative measurements of S100 beta are often falsely elevated. Because of this, S100 beta isn’t considered a useful marker of intraoperative neurologic complications.

 

PHARMACOLOGIC NEUROPROTECTION:

The incidence or severity of neurologic complications associated with cardiac operations could be decreased by pharmacologic interventions. Barbiturates decrease the demands of cerebral oxygen, and could theoretically provide some protection in the setting of incomplete ischemia. Other agents that reduce reperfusion injury, the consequences of the inflammatory reaction to tissue injury, or cerebral edema, could also be beneficial.

 

One clinical trial randomly assigned 192 patients undergoing open chamber cardiac procedures at 34ºC to receive either standard care or sodium thiopental (to maintain a burst suppression pattern on EEG throughout the period of cardiopulmonary bypass). Compared with the 7.5% of patients in the control group who suffered neuropsychiatric disturbances 10 days postoperatively, none of the thiopental patients suffered from it. But because of the risk of intraoperative hypotension, treatment with thiopental isn’t used routinely.

 

The use of recombinant erythropoietin as a neuroprotectant during CABG surgery was examined in a small randomized pilot trial in 32 patients; the treatment appeared safe, and there was a trend to benefit in neurocognitive outcomes that didn’t meet statistical significance.

------------------------------------------------------

Prepared By: Dr. Mehyar Al-khashroum
Edited By: Miss Araz Kahvedjian


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