Ann Thorac Surg 2002;74:412
© 2002 The Society of Thoracic Surgeons
Invited commentary
Christopher I. Blauth, FRCSa
a Cardiothoracic Centre, Guy’s & St. Thomas’ Hospital, London SE1 7EH, England UK
e-mail: ciblauth{at}aol.com
The vulnerability of the brain during operations on the heart has always been recognized. In the 50 years of open heart surgery multiple etiologies of cerebral injury have been identified, both procedure-related and patient-related, and mitigating strategies continue to occupy the attention of surgeons, anesthesiologists, and perfusionsists. Diagnosis and the audit of major neurological events present little difficulty. More problematic is the measurement of cognitive impairment. As doctors we recognize the central importance of cognitive function on the quality of life and its influence on survival. But the complexity of the human brain and the massive diversity of its functions and compensatory mechanisms has confounded numerous attempts to devise measures reliable and sensitive enough to guide the selection of therapeutic interventions.
Measurement of auditory-evoked potentials has the superficial appeal of apparent simplicity, objectivity, and reproducibility, and therefore suitability for studying patients undergoing cardiac operations. Nevertheless, like neuropyschological tests, the results are heavily dependent on patient concentration and performance, and may also be influenced by electrode number and placement, and the physical condition of the patient, together with metabolic and pharmacological variables. Although auditory evoked potentials are known to be abnormal in overt clinical syndromes of neurological, hematological, metabolic, and toxic disorders, the significance of abnormal auditory potentials in subjects without such overt disorders remains unknown and speculative.
The embolic potential of prosthetic heart valves is well known, and ultrasound Doppler studies of the middle cerebral artery in prosthetic valve recipients suggest ongoing asymptomatic microemboli events. Just as we do not really know what the cumulative effect of these microemboli adds up to in the longer term, we do not really know the long-term clinical or social cost of isolated deficits in neuropsychological tests or more specific measures like auditory-evoked potential after cardiac surgery. However, we are not surprised when another group of researchers informs us that aortic valve replacement has a greater effect on the brain in the short term than coronary bypass surgery.
A major advantage of both cranial Doppler and auditory-evoked potential studies is their relative independence of the learning effects which reduce the reliability of multiple repeat neuropsychological tests. This advantage would best be explored in longitudinal studies over several years including comparison groups of patients undergoing nonvascular major surgery, and patients having percutaneous cardiological interventions. Longer-term data correlated to survival and quality of life scores might eventually identify the significance of these measures.
