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Ann Thorac Surg 2006;81:29-33
© 2006 The Society of Thoracic Surgeons

Original article: Cardiovascular

Long-Term Neurocognitive Function After Mechanical Aortic Valve Replacement

Daniel Zimpfer, MD a , * , Martin Czerny, MD a , Philipp Schuch, MD a , Richard Fakin, MD b , Christian Madl, MD b , Ernst Wolner, MD, PhD a , Michael Grimm, MD a

a Department of Cardiothoracic Surgery, University of Vienna, Vienna, Austria
b Department of Internal Medicine, University of Vienna, Vienna, Austria

Accepted for publication June 10, 2005.

* Address correspondence to Dr Zimpfer, Department of Cardiothoracic Surgery, University of Vienna, Wahringer Guertel 18-20, Vienna A-1090, Austria (Email: daniel.zimpfer{at}meduniwien.ac.at).


   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
 References
 
BACKGROUND: Mechanical aortic valves are a possible source of microemboli potentially causing cerebral injury. Therefore, the long-term impact of mechanical aortic valve replacement on neurocognitive function is uncertain.

METHODS: In this prospective, contemporary study, we followed 32 consecutive patients (aged 51 ± 8 years; range, 38 to 70; EuroSCORE [European System for Cardiac Operative Risk Evaluation] 4.4 ± 1.7) undergoing isolated aortic valve replacement with a mechanical prosthesis. A cohort of age- and sex-matched patients (n = 28, aged 50 ± 7 years) served as nonsurgical controls. After aortic valve replacement, neurocognitive function was serially reevaluated at 7-day (n = 32), 4-month (n = 31), and 3-year (n = 29) follow-up. Neurocognitive function was measured by means of P300 auditory evoked potentials.

RESULTS: Before the operation, P300 peak latencies were comparable between surgical patients (361 ± 32 ms) and nonsurgical controls (365 ± 33 ms, p = 0.783). In patients undergoing aortic valve replacement, P300 peak latencies were prolonged 7 days after surgery (380 ± 32 ms) as compared with before the operation (361 ± 32 ms, p < 0.0001) and as compared with nonsurgical controls (364 ± 34 ms, p = 0.002). At 4-month (369 ± 30 ms, p = 0.752) and 3-year (370 ± 31 ms, p = 0.825) follow-up, P300 peak latencies normalized as compared with before operation and as compared with nonsurgical controls (4-month follow-up 363 ± 31 ms, p = 0.832; 3-year follow-up 366 ± 32 ms, p = 0.432). We found no difference in patients with different valve types.

CONCLUSIONS: Despite previous assumptions based on the potential occurence of microemboli in patients with mechanical valves, mechanical aortic valve replacement has no adverse long-term impact on neurocognitive function. This finding is only valid for patients with a comparable age range undergoing isolated aortic valve replacement.


   Introduction
Top
 Abstract
 Introduction
Material and Methods
 Results
 Comment
 Acknowledgments
 References
 
Postoperative and long-term neurocognitive deficit after open-heart surgery with cardiopulmonary bypass has turned out as an adverse event in the past, possibly limiting the merits of surgery [1]. Postoperative as well as long-term neurocognitive deficit after coronary artery bypass grafting is well documented throughout the literature. In contrast, the long-term development of neurocognitive function after aortic valve replacement is uncertain.

Neurocognitive deficit after coronary artery bypass grafting is believed to mainly depend on intraoperative damage. In contrast, long-term neurocognitive deficit after aortic valve replacement might turn out to be the result of a combined process of intraoperative damage and cumulative damage caused by microemboli originating from prosthetic cardiac valves.

The aim of the present paper was to objectively measure long-term neurocognitive function in patients after mechanical aortic valve replacement and to compare the findings with nonsurgical controls.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 Acknowledgments
 References
 
Patients
After approval was obtained by the Ethics Committee of the University of Vienna, 32 patients who underwent elective mechanical aortic valve replacement at our department between January and May 2000 gave their written informed consent and were enrolled in this prospective study. Exclusion criteria were a history of one of the following medical conditions: (1) prior stroke with residual deficit, (2) uncontrolled hypertension, (3) carotid artery stenosis of 75% or greater, (4) psychiatric illness requiring treatment, (5) alcoholism, (6) renal disease (defined as a creatinine more than 2.0 mg/dL [177 µmol/L]), and (7) active liver disease.

Nonsurgical Controls
For nonsurgical controls, we screened patients admitted to the department of internal medicine for routine medical checkup. The same exclusion criteria used in patients undergoing aortic valve replacement were applied on control subjects. Patients were contacted by the study coordinator. Patients were informed by the study coordinator about the planned tests as well as the frequency of reexamination. All patients serving as controls had to give their written and informed consent. Tests were not performed as part of another study. Of the patients contacted, 28 gave their written and informed consent and were enrolled.

Neurocognitive Testing
Neurocognitive testing and physical examinations were completed preoperatively, 7 days, 4 months, and 3 years after surgery. All examinations were performed individually by the same experienced investigator. Neurocognitive testing consisted of cognitive P300 evoked potentials. To avoid any influences due to biorhythm, all investigations were performed in the afternoon under comparable conditions. Special care was taken to ensure that patients were free from narcotics and sedatives (in the perioperative tests) for at least 2 days before testing.

Cognitive P300 Evoked Potentials
Cognitive P300 evoked potentials have previously been used to measure neurocognitive function in various metabolic disorders, patients undergoing heart transplantation, and patients undergoing open-heart surgery [2–6]. Cognitive P300 evoked potential are a valid marker of central nervous system activity [22, 23]. Cognitive P300 auditory evoked potentials are the result of an activation of a widespread network of cortical structures, including association areas in the parietal, temporal, and prefrontal cortex, as well as the hippocampus [7]. As a result of the involvement of these brain regions in the P300 generation, P300 can be used as a general indicator for neurocognitive function [8–10]. Cognitive P300 auditory evoked potentials have been shown to be abnormal in patients with magnetic resonance imaging–proven cerebral lesions after cardiac surgery. Cognitive P300 evoked potentials were recorded with Ag/AgCl electrodes on a Nicolet 2000 (Nicolet, Madison, Wisconsin). The P300 evoked potentials were generated after a binaurally presented tone discrimination paradigm (odd-ball paradigm) with frequent (80%) tones of 1,000 Hz and rare (20%) target-tones of 2,000 Hz at 75 dB HL. Filter bandpass was 0.01 to 30 Hz. Active electrodes were placed at Cz (vertex) and Fz (frontal), respectively, and referenced to linked earlobe A1/2 electrodes (10/20 international system) [11]. During the paradigm, the subjects were instructed to keep a running mental count of the rare 2,000 Hz target tones. To verify attention, P300 recordings with a discrepancy of greater than 10% between the actual number of stimuli and the number counted by the subjects were rejected and repeated. The P300 evoked potential recording resulted in a stable sequence of positive and negative peaks. Latencies (ms) of the cognitive P300 peak were assessed. To confirm reproducibility, two sets of P300 measurements were recorded in all patients.

Follow-Up
In addition to neurocognitive testing, patients were studied by means of echocardiography, electrocardiography, blood tests, and clinical investigations at all points of follow-up. Echocardiography was used to assess functional state of heart valves. Persistent atrial fibrillation was defined as presence of atrial fibrillation at baseline and 3-year follow-up.

Anesthesia and Surgical Procedure
Patients were premedicated with midazolam. Additionally midazolam in 1-mg increments was administered intravenously as needed for general anesthesia with midazolam, ethmidate, fentanyl, and pancuronium. Patients were ventilated with oxygen in air; ventilation was set to a tidal volume of 8 mL/kg and a respiratory rate of 12 breaths per minute, positive end-expiratory pressure 5. The transesophageal echocardiography (TEE) probe was placed after anesthetic induction in all patients. The TEE views used to assess regional wall motion abnormalities included the transesophageal four- and two-chamber views and the transgastric short- and long-axis views.

Surgical access was gained through a median sternotomy. All patients underwent mildly hypothermic cardiopulmonary bypass (CPB [35°C]) with intermittent cold blood cardioplegia with a hot shot before opening the cross clamp. The CPB circuit consisted of a hollow-fiber oxygenator (Bard HF 5701; CR Bard, Havorhill, Massachusetts) and a lining system primed with Ringer lactate, mannitol, heparin, and apoprotein. Flow during CPB was maintained at 2.5 L · min–1 · m– 2. Blood cardioplegia was maintained at 4:1 ratio. Hematocrit was kept above 20% with packed red blood cells if necessary. Perfusion pressure during CPB was kept above 50 mm Hg with phenylephrine if necessary. Before opening of cross-clamp as well as weaning from cardiopulmonary bypass, careful deairing was performed through the apex of the heart and the ascending aorta under continuous inflation of the lungs, which was vigorously controlled by TEE monitoring. Heparin was antagonized with protamin sulfate until preoperative activated clotting time was achieved. Mean arterial pressure after CPB was kept above 60 mm Hg with volume and vasoactive drugs as appropriate. Intensive care unit treatment was performed according to institutional standards.

Anticoagulation Therapy
Anticoagulation therapy was perioperative 2 x 7,500 IE daily low molecular weight heparin dalteparin-natrium (Fragmin; Pharmacia & Upjohn GmbH, Vienna, Austria); on day 5 start with phenoprocoumon (Marcumar; Roche Austria GmbH, Vienna, Austria) life long (targeted international normalized ratio [INR] range: 2.5 to 3.5; targeted INR: 3.0). No change in anticoagulation regime in patients with atrial fibrillation. The INR values were regularly monitored by the patient's general practitioner—all patients were within the therapeutic range throughout the study period.

Statistical Analysis
Data are reported as mean ± SD. The time course of P300 auditory evoked potentials was analyzed by means of two-way analysis of variance (ANOVA). Comparison of P300 evoked potentials was performed using ANOVA after testing for normality of distribution. All p values of serial measurements were corrected (Bonfferoni-Holm). Categorical variables were compared using the {chi}2 test or Fisher's exact test as appropriate. All p values less than 0.05 were considered as significant, two sided. A power analysis was performed before conducting the study. The power analysis was based a power of 0.85 and an alpha of 0.05. The study was analyzed using SPSS, version 12.0 (SPSS, Chicago, Illinois).


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 Acknowledgments
 References
 
Thirty-two patients undergoing isolated aortic valve replacement at our institution were prospectively observed. The baseline characteristics of patients as well as controls are given in Table 1. Patients and controls were comparable with regard to demographic variables. Detailed information about the mechanical valves used is given in Table 2.


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  		Table 1. Patient Characteristics
 

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  		Table 2. Aortic Valve Types
 
Clinical Outcome
We observed no death in the present study. Operative data and clinical outcome are given in Table 3.


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  		Table 3. Operative Data and Clinical Outcome
 
P300 Auditory Evoked Potentials
Before the operation, P300 peak latencies were comparable between surgical patients (361 ± 32 ms) and nonsurgical controls (365 ± 33 ms, p = 0.783). In patients undergoing mechanical aortic valve replacement, P300 peak latencies were prolonged 7 days after surgery (380 ± 32 ms) as compared with before the operation (361 ± 32 ms, p < 0.0001) and as compared with nonsurgical controls (364 ± 34 ms, p = 0.002; Fig 1). At 4-month (369 ± 30 ms, p = 0.752) and 3-year follow-up (370 ± 31 ms, p = 0.825), P300 peak latencies normalized as compared with before operation and as compared with nonsurgical controls (4-month follow-up 363 ± 31 ms, p = 0.832; 3-year follow-up 366 ± 32 ms, p = 0.432).



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  		Fig 1. Serial assessments of cognitive brain function by cognitive P300 evoked potentials. The black line represents patients undergoing aortic valve replacement; the gray line represents age- and sex-matched control subjects. *p < 0.001 compared with preoperative values. {dagger}p = 0.002 between the two groups. (FUP = follow-up; pre-OP = preoperative.)

 
Comparing neurocognitive function in patients with different types of mechanical aortic valves we found no difference. Time course of P300 auditory evoked potentials in patients with different types of mechanical aortic valves is given in Figure 2.



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  		Fig 2. Graph showing serial assessments of cognitive brain function by P300 auditory evoked potentials in patients with mechanical aortic valves: Medtronic Hall mechanical aortic valve (black line), Carbomedics mechanical aortic valve (dashed line), Edwards Mira mechanical aortic valve (dotted line), and On-X mechanical aortic valve (gray line). (FUP = follow-up; pre-OP = preoperative.)

 

   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
Acknowledgments
 References
 
As shown by means of objective testing, mechanical aortic valve replacement has no long-term impact on neurocognitive function.

Neurocognitive deficit, defined as combination of deficits in memory, learning, concentration, and visual motor response, is an adverse event of open-heart surgery, with an incidence of as high as 80% perhaps the most common adverse event [3, 4, 12–14]. Roach and colleagues [14] reported on a multi-institutional prospective study that that neurocognitive deficit is associated with increased mortality (10%), a twofold increase in hospital length of stay, and a sixfold likelihood of discharge to a nursing home. These are associated with a tremendously increased use of health care resources. From the patient;s view, the impact of neurocognitive deficit is devastating, as it has been shown to reduce subjective working capacity, quality of life, job-related abilities, and productive working status, and to impair car-driving abilities [15, 16]. Summarizing, neurocognitive deficit is a drawback of open-heart surgery, as it may reduce the merits of surgical intervention. Considering the high number of mechanical valves implanted worldwide each year and that mainly younger patients receive mechanical cardiac valves, neurocognitive impairment caused by mechanical cardiac valves has important clinical and economic implications.

By means of P300 auditory evoked potentials and in comparison with age- and sex- matched patients, we have shown that postoperative neurocognitive deficit (7-day follow-up) is reversible in patients undergoing mechanical aortic valve replacement. We have previously reported and discussed these findings in a study comparing neurocognitive function in patients undergoing mechanical and biological aortic valve replacement [17]. Furthermore, we have shown that mechanical aortic valve replacement has no long-term impact on neurocognitive function. Mechanical heart valves have in the past been shown to be the source of microeboli, detected as microembolic signals, entering the cerebral blood circuit. However, the clinical relevance of microemboli is a matter of discussion. Results of previous studies addressing neurocognitive function in patients with mechanical aortic valves are contradictive [18–20]. Studies addressing neurocognitive function in patients with mechanical valves in the past concentrated on establishing a correlation between number of microembolic signals and neurocognitive function. The main limitation of these studies is that patients were examined at single occasions only, no follow-up data were provided, and no control groups were included [18–20]. In contrast to previous studies, we provide long-term data and include a control group. Our findings significantly question the clinical relevance of circulating microemboli, as we found no long-term neurocognitive injury in patients with mechanical valve replacement.

Neurocognitive function was measured by means of P300 auditory evoked potentials. In healthy persons, P300 peak latencies are increased with age; and P300 auditory evoked potentials have been used by us and others to detect neurocognitive disorders after cardiac surgery and have been shown to correlate with magneatic resonance imaging–proven cerebral lesions after cardiac surgery [3–5, 17, 24].The clinical relevance of cognitive P300 evoked potentials is based on their being shown to be related to cognitive impairment rating, rapid evaluation of cognitive function tests, orientation, stimulus evaluation, selective attention, visual pattern recognition, and digit span [2, 3,]. Therefore, P300 evoked potentials are a valid marker of cognitive function [22, 23] We used P300 auditory evoked potentials for several reasons in the present study. In the past, P300 auditory evoked potentials have been shown to be much more sensitive and accurate in detecting neurocognitive deficit than psychometric tests or electroencephalograms [21]. The P300 technique lacks several limitations of psychometric test batteries. Psychometric test batteries are affected by biases such as long performance times (stressing attention), visual impairment, and influence of psychomotor function as well as level of education and learning effects [25, 26]. Moreover, the P300 technique has a very low intraindiviual test-retest variability with a coefficient of variation of below 2%, which further stresses its usefulness for cognitive follow-up studies [3]. All P300 recordings were taken repeatedly (double tracing) to confirm reproducibility of measurements. The high standard deviations of mean P300 peak latencies in patients and age- and sex-matched control subjects are the result of age dependency of cognitive P300 measurements.

Limitations
The present study is limited in that we performed no transcranial doppler measurements. The reason for this is the inability of currently available transcranial doppler systems to differentiate between the size and the nature (particular and gaseous) of emboli. It seems plausible that more severe damage is caused by particular emboli. Therefore, data obtained by the currently available transcranial doppler systems might be misleading. Furthermore, we did not perform magnetic resonance imaging studies. The present data are valid only for elective-surgery patients with a comparable age range undergoing aortic valve replacement with mildly hypothermic cardiopulmonary bypass and can not be extrapolated to patients in different age ranges.

In summary, despite previous assumptions based on the presence of microemboli in patients with mechanical valves, mechanical aortic valve replacement has no long-term impact on neurocognitive function.


   Acknowledgments
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
References
 
We thank Daniela Dunkler, MS (Stat), for the statistical analysis of the work.


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 Acknowledgments
 References
 

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