Evaluation of Cerebral Microembolic Signals in Patients with Mechanical Aortic Valves

摘要

Background and Purpose: Microembolic Signals (MES) are frequently observed in Transcranial Doppler (TCD) recordings of patients with Mechanical Heart Valve (MHV).We hypothesized that number of MES produced by MHV could be reduced with oxygen inhalation, if gaseous bubbles are the underlying cause.Methods: All consecutive patients with St Jude aortic valves visiting the cardiology clinic were refered to the neurosonology unit, Valie Asr Hospital, Khorasan during August 2003 to August 2004. TCD monitoring of MES was performed with an ultrasound device (Vingmed 800 Oslo,Norway) and a 2 MHz probe. The MES counts were recorded during 30 minutes breathing room air and thereafter 30 minutes breathing through a facial mask with reservior bag (6 liter O2 per minute). The criteria of MES detection were characteristic chrip sound, unidirectional signal, random appearance within cardiac cycle and intensity increase ?3dB above background. The MES counts in two periods of monitoring were compared with paired T test and significance was declared at P < 0.05.Results: Twelve patients (8 females and 4 males) were investigated. Oxygen ventilation caused a significant decrease of MES counts in the patients in comarison to breathing room air, P = 0.001. Thus MES in patients with MHV are mainly gaseous bubbles caused by blood agitation with MHV.Conclusion: The quantity of MES in patients with MHV is not related to the risk of thromboembolic complications in these patients. Introduction Thromboembolism is a major cause of morbidity in patients with Mechanical Heart Valve (MHV) and Microembolic Signals (MES) are commonly observed in TCD recording of these patients1. Since at present the precise nature of MES in patients with MHV is unknown, different etiologies are under debate including local activation of coagulation system by the MHV, local increase of platelet aggregation and gaseous cavitation bubbles2. A major drawback of TCD technology is its failure to provide conclusive information concerning the underlying embolic material1. This issue is important in the evaluation of individual risk profiles and adequate management strategies. No correlations have been found between MES count and duration after MHV replacement, valve position, cardiac rhythm, intensity of anticoagulation and history of neurological deficit3,4. Assumption of gaseous bubbles as embolic material could explain this dicripancy, since microbubbles remain asymptomatic by imploding or crossing over to venous circulation through the capillary bed4. Patients with MHV had higher MES counts in common carotid artery than middle and anterior cerebral arteries because gaseous bubble emboli are bound to implode with time5. This study was carried out to evaluate the influence of oxygen ventilation on MES counts in patients with mechanical aortic valves. Patients and Methods All consecutive patients with St Jude mechanical valves in aortic position were referred from the cardiology clinic to the neurosonology unit, Valie Asr Hospital, Khorasan during August 2003 to August 2004. Presence of more than 70% extracranial internal carotid artery stenosis, poor transtemporal window and intolerance to oxygen ventilation for 30 minutes served as exclusion criteria. The protocol entailed 30 minutes of TCD monitoring while the patient was breathing room air and 30 minutes while breathing 6 liters per minute oxygen through a facial mask with a reservior bag (fractional inspiratory O2 = 60%)6. This facial mask was placed over mouth and nose and held in place by an examiner, providing downward pressure with thumb to ensure a tight seal6. Patients were instructed to breath normally, avoid hypo or hyperventilation and immediately give notice if breathing become uncomfortable or other inspiratory or cardiac complaints occurred. MES monitoring was performed with an ultrasound device (Vingmed 800, Oslo, Norway) and a 2MHz probe in 50-58 mm depth of right middle cerebral artery through transtemporal window. M