NEUROLOGICAL OUTCOME AFTER CORONARY ARTERY BYPASS SURGERY

INTRODUCTION

It is well recognized that cardiac surgery with cardiopulmonary bypass can potentially induce a wide spectrum of Neurocognitive disturbances and central nervous system (CNS) sequelae.^1-6 Coronary artery bypass grafting (CABG) is the most commonly done cardiac operation worldwide⁷ and neurological outcome determines to a large degree the mortality and the morbidity associated with this procedure. It is a devastating complication of the cardiac surgical procedures and is responsible for prolonged hospital stay and increased healthcare costs.^8-9 Our awareness of the prevalence of these CNS complications is dependant on the validity of the available diagnostic methods. Current assessment methods designed to detect both focal and diffuse cerebral ischemia, include standard neurological examination, imaging techniques, biochemical markers, neuropsychological assessment, and patient perceived outcomes.

The wealth of available data suggests that the incidence of overt CNS injury has declined since the 1980s and now varies between 2-7%.^10-11 A multitude of causes have been postulated as mechanisms for cerebral dysfunction in the cardiac surgical patients and many risk factors have been implicated as contributors to a higher occurrence of peri-operative neurological dysfunction. Investigators have implicated hypotension, hypoperfusion, air embolism, atheromatous disease, and intracranial and extracranial vascular diseases as the causative factors. Other investigators have identified groups of patients at high risk for the perioperative stroke, for example patients with carotid artery stenosis, patients with mural thrombi, and patients with previous transient ischemic attack (TIA). The purpose of this project was to determine the frequency of neurological deficit after CABG in our population and to evaluate the preoperative, intraoperative and postoperative factors in regard to their relationship with postoperative neurological outcome and to determine the contribution of each factor in order to be able to predict the groups of patients at a higher risk to plan strategies for prevention of this devastating complication of cardiac surgery.

MATERIAL AND METHODS

One thousand consecutive patients who underwent primary isolated CABG from July 2000 to December 2001 were enrolled for this study. All the charts and flow sheets of the patients were reviewed and the data were entered into the cardiac surgical database for subsequent analysis. All patients were operated at the Punjab Institute of Cardiology, Lahore. which is the largest facility of its kind situated in the most populated province of the country. To create a homogeneous population of the patients undergoing CABG, patients who had previous CABG, previous or concomitant valve replacement, corrective procedures for congenital anomalies or any other associated procedure were excluded from the study. A total of 691 men and 309 women were included in the study. Risk factors evaluated were diabetes mellitus, hypertension, cholesterol and triglyceride levels and smoking. Other preoperative variables included prior transient ischemic attacks (TIA), carotid artery disease on Doppler scan and presence of intramural thrombus on echocardiography. Generally, patients with asymptomatic carotid stenosis underwent CABG and followed up. Patients with symptoms or with bilaterally severe stenosis underwent concomitant carotid endarterectomy (11 patients). Operative considerations evaluated were the number of grafts (intermittent cross clamping), total bypass time, total aortic cross clamp time, and the presence of aortic disease as determinants from the operation notes. Preoperative echocardiogram and thallium scans were used for comparison where available. Postoperative arrhythmias were defined as any cardiac rhythm necessitating pharmacological intervention other than the frequent Ventricular Premature Contractions (VPCs) requiring lignocaine boluses. These were not separated into atrial or ventricular arrhythmias in terms of their relevance to perioperative neurological deficits. All patients who had a postoperative neurological deficit were placed into two groups, those who had a temporary neurological deficit (<30 days) and those who had a permanent deficit (>30 days). In all cases the diagnosis was documented by a neurologist and confirmed by a CT scan where possible. Patients who died within 30 days of having the neurological deficit were placed in the permanent deficit group.

RESULTS

Of the 1,000 patients who underwent CABG, 43 (4.3%) had a neurological deficit and 16 (1.6%) had a temporary deficit and 27 (2.7%) had a permanent deficit.

The presence of mural thrombus was associated with an increased risk of permanent neurological consequences and 17% patients with mural thrombi developed postoperative neurological derangements (p<0.000l).

Age over 55 years was associated with a higher risk and presence of asymptomatic preoperative carotid disease (15%) was not associated with risk of neurological complications (p<0.005). Similarly gender showed no significant association with the postoperative neurological deficit (Table 1& 2). Extensive aortic calcification noted by the surgeon at operation was a significant risk factor for a permanent neurological deficit. Of the 57 patients noted to have aortic atheromatosis / calcification, 11 (19%) had a CVA compared with 16 of the 943 patients without calcification who suffered postoperative neurological complications (p<0.0001). (Table3).

Table-1: Male Patient characteristics

Age(years)	Neurological deficit
Age(years)	yes	No
58-60	8	234
60-62	11	198
62-64	22	228
Total	31	660

Table2 Female patients’ characteristics

Age(years)	Neurological deficit
Age(years)	yes	no
58-60	1	104
60-62	4	87
62-64	7	106
Total	12	297

Operative bypass time and aortic cross clamp time had no bearing on the risk of neurological consequences in this group. Intermittent aortic clamping used for CABG was associated with a significantly higher neurological complications rate; however the number of patients with calcified aorta was not higher in this group of patients. Patients with postoperative arrhythmias also had a higher risk of having postoperative neurological sequelea. Of the 383 patients with a postoperative arrhythmia, 20 had an untoward neurological outcome as opposed to only 7 among 617 patients without arrhythmias.

Table-3: Patients with calcified and normal aorta.

	Normal aorta		Calcified aorta		Total
	No.	%	No.	%	No.	%
No neurological deficit	886	93.9	25	43.8	911	91.1
Transient neurological deficit	41	4.3	21	36.8	62	6.2
Permanent neurological deficit	16	1.6	11	19.2	27	2.7
Total	943	99.8	57	99.8	1000	100

DISCUSSION

The frequency of perioperative neurological sequelea in our patients undergoing primary isolated CABG is 4.3%. This is consistent with the reported frequency in the recent literature^6-8 as patients with complex procedures and combined operations were excluded. Although the incidence of overt neurological sequelae has traditionally been higher in patients undergoing isolated intracardiac procedures such as a valve replacement or repair, recent studies show that the incidence of neurologic outcome for the intracardiac procedures now approximates that for isolated CABG in the range of 1-4%. Our study, however, has several limitations of its own. First, limited expertise and resources are available. Second, no previous example of such study is available locally to compare with. Thirdly, the neurological findings were assessed by investigators available at the moment, not by a single neurologist performing all preoperativeand postoperative examinations; there may be significant variationsin clinical practice, and thus diagnosis. Fourthly, neuropsychologicaldeficits were not formally assessed because of several constraints,including a lack of technical experience in neuropsychological testing and the time required for such testing. Our assessmentof deterioration in intellectual function is thus open to criticism. And finally, we detected aortic atherosclerosisby surgical palpation. Recent studies have shown that ultrasonography is superior to palpation in detecting aortic atheroma, butour study was not designed in such an aspect. Adverse neurologic events after isolated CABG have remainedconstant despite various attempts to reduce their incidence.In a large-scale multi center study 6.1% incidence of serious adverse neurologic events was reported in a survey of 2,108 patients undergoing isolated CABG.¹² Threepercent of these patients had a perioperative stroke, whereasa further 3.1% had prolonged unconsciousness, seizures, or encephalopathy.Advanced age and duration of CPB were the strongest correlatingfactors for neurologic complications. In another large prospectivestudy, Newman and colleagues observed that 3.2% of theirpatients after isolated CABG had unfavorable neurologic consequences.In their model they identified age, history of neurologic disease,diabetes, history of peripheral vascular disease, redo CABG,and unstable angina as major correlating preoperative variablesfor adverse neurologic outcomes which are also the factors identified in our study at locum most responsible for the adverse neurologic outcome after cardiopulmonary bypass grafting especially in the elderly population. Bowlesand colleagues reported a significant reduction in cerebralmicro emboli in the off-pump group compared with the on-pumpgroup. More importantly, by using transcranial Doppler studies,they illustrated that although surgical manipulation does accountfor some emboli during CABG, the vast majority of emboli (84%)occurred while the patient was receiving CPB with no surgicalmanipulation identified. Stump and Newman showed thatthe embolic load during aortic cannulation / decannulation orcross-clamp application / removal is similar to the load seenwhen applying and removing the side-biting clamp for proximalanastomoses. These two studies support our finding that elderly patients with heavily calcified aorta when cannulated and cross clamped and whom aorta is roughly manipulated during application of side-biting clamp bear manifold increased risk of contracting neurological consequences. Cerebral embolization with macro- andmicro-emboli has been shown to be the most common mechanism involved. Using transcranial Doppler, Stump and colleagues clearly demonstrated that emboli could be detected during instrumentationof the aorta and heart.With the introduction of off-pump CABG, there exists a potentialto reduce the incidence of adverse neurologic events which seems to be a new horizon in the future cardiac surgery especially regarding CABG in the elderly. Off-pumpcoronary artery bypass procedures do not use aortic cannulationand cross-clamping and thereby avoid injury to the aorta and thusdislodgement of any atheroma during bypass. Bearing these facts in mind, many surgeons has started constructing minimal top ends directly to the aorta by making maximum piggybacks to internal mammary artery (IMA) or the venous grafts where off-pump surgery is not possible and the elderly patients have atheromatous calcified aorta. However, the potential exists for an increased risk ofinjury to the aortic wall during partial clamping of a tenseaorta, especially if the aorta is diseased.

CONCLUSION

We found several factors to be important in predicting the occurrence of a permanent neurological deficit after CABG. These included the presence of diabetes mellitus, increased age, mural thrombi, presence of severe aortic calcification and postoperative arrhythmias. In addition, we determined that the role of these individual factors is additive to the risk of perioperative neurologic outcome when more than one factors are present. Consideration of these factors should prove helpful to the surgeon or the cardiologist while selecting patients for CABG.

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