Doppler Microembolic Signals in Children With Prosthetic Cardiac Valves
Background and Purpose The aim of this study was the evaluation of the prevalence and counts of Doppler microembolic signals (MES) in children with prosthetic cardiac valves and their comparison to those obtained in corresponding adult patients.
Patients and Methods Nine children and 43 adults with ATS valves implanted in the aortic position were monitored over both middle cerebral arteries with transcranial Doppler ultrasound. MES were identified on-line according to standard criteria. Heart rate and rhythm, valve type, size and duration, patients’ height, International Normalized Ratio, and prevalence of neurological complications were obtained from all study participants.
Results MES prevalence and counts were significantly higher in children compared with adult patients (100% versus 25.5% and 58 [18.5 to 115.5] versus 5.5 [2 to 10.5], median, 95% CI, respectively). No corresponding differences in valve size or duration of valve implant were evident, but children had faster heart rates and were significantly smaller compared with adults. A positive correlation between patients’ size, heart rate, and MES counts was noted.
Conclusions MES counts in children with mechanical prosthetic valves are significantly higher compared with those in corresponding adults. We hypothesize that this is due to (1) the shorter distance between aortic valve and middle cerebral artery, since cavitation bubbles have a short life span and are bound to dissolve with time, and (2) the faster heart rate in children, resulting in a higher number of valve closures per minute.
Detection of intracranial microembolic signals (MES) by means of transcranial Doppler sonography (TCD) in patients with prosthetic cardiac valves was initially described by Rams et al1 in 1991. As of this time, several crucial issues concerning MES in valve patients remain unclear, including potential underlying embolic material and clinical significance of MES.
Although insertion of mechanical prosthetic valves in children remains controversial,2 3 4 several considerations, including the relatively short life span of homografts and porcine valves due to increased calcium metabolism causing early calcification5 6 and childs’ growth potentially leading to mismatch,7 support their use in these patients. Also, degeneration or failure of a previously inserted biological or homograft valve or impossibility to obtain those grafts due to an emergency procedure are clear indications for mechanical valve implantation.
The aim of this study was to evaluate MES prevalence and counts in children with ATS valves and compare them to those in corresponding adult patients.
Patients and Methods
Nine children (age 14±1 years, 7 male) and 43 adult patients (age 65±1 years, 23 male) carrying an ATS valve (a mechanical bileaflet valve) in aortic position were enrolled in this study. Indications for valve replacement in children were severe combined lesions in 3 cases; pure insufficiency, not feasible for reconstruction, in 4 cases; degenerated homografts in 2 cases; and postendocarditis in 1 case.
Both middle cerebral arteries (MCA) were insonated at a depth between 50 and 55 mm for 60 minutes per patient using a pulsed ultrasound machine (Multi-Dop ×4, DWL). High intensity transient signals in the Doppler signal were recognized as MES based on accepted criteria8 and stored on hard disc for further evaluation. In brief, detection criteria were characteristic harmonic sound, intensity at least 3 dB above the background, short duration, and random occurrence in the cardiac cycle. All monitoring sessions were performed by the same examiner. Also, 15 sessions were saved on DAT tapes and reevaluated by a second observer blinded to all clinical details.
Prevalence of neurological complications was evaluated on the basis of a standard neurological questionnaire (transient or permanent motor weakness or sensory, visual, or speech disturbances). A detailed neurological examination was performed in all subjects who reported symptoms suggestive of cerebral ischemia. All patients were stabilized on warfarin at the time of study.
Statistical analysis was performed using two-sample t test for normally distributed and Mann-Whitney U test for non-normally distributed data. Distribution of frequencies was evaluated using the χ2-square test, and Spearman-Rank test was applied to examine potential correlations of non-parametric data. Two-sample t test was used to assess agreement between the two observers. Significance was declared at P<.05.
Mean time between valve replacement and TCD monitoring was 6±1 and 8±1 months in children and adults, respectively (P>.05, two-sample t test). No significant differences in the efficacy of oral anticoagulation or valve size (22.5 [20 to 25] versus 24 [23 to 25], P>.05, Mann-Whitney U test) were evident between the two groups. Children were smaller (153 [139 to 169] versus 169 [166 to 172] cm, P<.05, Mann-Whitney U test) compared with adults. Heart rate was higher in children compared with adult patients (80 [71 to 95] versus 72 [68 to 76] beats per minute, P=.06, Mann-Whitney U test). Cardiac rhythm was sinus in all examined children and 33 adult patients and atrial fibrillation in the remaining cases. Neurological complications occurred in only 1 adult patient (2.3%).
MES prevalence and counts were significantly higher in children compared with adults (100% versus 25.5%, P<.05, χ2 test and 58 [18.5 to 115.5] versus 5.5 [2 to 10.5], P<.01, Mann-Whitney U test). A positive correlation between patients’ height and heart rate with MES counts was noted (MES/pulse r=.4, P>.05, MES/height r=.56, P<.05).
Agreement between the two observers was satisfactory (observer 1, total number of MES 287; observer 2, total number of MES 276; P=.98).
This is the first report of TCD examinations in children with prosthetic cardiac valves. The finding of significantly higher MES counts and prevalence compared with adults, in particular in the absence of corresponding differences in valve size or position, is striking. Interestingly, the recorded counts are also higher than any previously reported group, with the exception of patients with Björk-Shiley monostrut valves.9 10 11 Any attempt to explain this observation requires some assumptions concerning the underlying embolic material in valve patients. Presuming formed underlying material, eg, thrombi or activated platelets, distinct differences in the activity of the anticoagulation system or in platelet function would be expected. However, no such differences have been previously described, and the level of oral anticoagulation was also comparable between the two groups. Two factors could be etiologically involved in this phenomenon assuming a gaseous underlying material: (1) heart rate, since cavitation bubbles are directly dependent on the number of valve actions, and (2) distance between valve and MCA, since bubbles are bound to implode with time so that a shorter distance could result in significantly higher MES counts.
Our results thus support the hypothesis that the underlying embolic material in patients with prosthetic valves is gaseous. The recent reports of Spencer12 and Kaps et al13 concerning alteration of MES counts in prosthetic valve patients under decompression or during inhalation of 100% oxygen lend further support to the above thesis. Still, cavitation bubbles are known to implode within milliseconds and could thus not enter the systemic circulation.14 15 However, it must be stressed that all reports on this phenomenon originate from bench models in which water or normal saline were used as circulating fluids. These in vitro conditions obviously cannot exist in the human heart.16 It appears possible that in vivo interactions between cavitation bubbles and blood contents extend their life span. Also, fluid acceleration and deceleration through closure of the artificial valve could cause bubble formation.16 Such cavitation bubbles could potentially be of bigger size and energetically more stable.
The number of symptomatic patients in our study was too low for definitive statements. Nevertheless, the demonstrated lack of neurological symptoms in the examined children despite high MES counts argues against a direct clinical significance of the detected signals, at least concerning evident neurological sequelae.
In conclusion, significantly higher MES counts in children compared with adults were observed in our study. The lack of evident corresponding clinical or hematological differences between the two groups is consistent with the assumption that greater numbers of cavitation bubbles, caused by the higher cardiac rate and the shorter distance between place of origin and detection site, are responsible for our observation.
- Received April 17, 1997.
- Revision received April 29, 1997.
- Accepted April 29, 1997.
- Copyright © 1997 by American Heart Association
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