植込み後慢性期に体位変換に伴う閾値変動によりペーシング不全を来した1例

A Case of Pacing Failure Associated with Postural Change

要旨　症例は慢性心房細動を伴う68歳女性．5年前に僧帽弁狭窄症に対して弁置換術，左房縫縮術を施行した．術後に徐脈性心房細動を認めたためVVIペースメーカ植込み術(心室中隔，screw-in)を行った．VVI 50bpmの設定でほぼペーシング依存であった．今回ジェネレータ交換のため入院，術後に一定の体位に伴うペーシング不全を認めた．体位を変えて閾値の測定を行ったところ，仰臥位で0.75V/0.4msec，左側臥位で0.75V/0.4msec，右側臥位で1.5V/0.4msec，おがみ姿勢で1.75V/0.4msecと体位変換に伴う閾値変動を認めた．透視下に各体位でのリードを確認したところ，右側臥位でリードはたわみの消失，牽引が認められ，中等度の三尖弁逆流もあるためリード先端と心筋の接触が変動することが原因と考えられた．キャプチャーマネージメント機能により出力設定を調節する場合，本例のように閾値変動を来す症例ではペーシング不全につながる可能性があるので注意を要する．

　A 63-year-old woman who had undergone mitral valve replacement for mitral valve stenosis, received a VVI pacemaker implantation for atrial fibrillation with slow ventricular response. The screw-in lead was located at the right ventricular septum. Five years later, she was admitted again for a generator exchange without symptoms. Though the pacing output was set as 3.0V/0.4msec after the operation, the electrocardiographic monitor showed pacing failure during sleep in the right lateral position. Telemetry data showed fluctuation of threshold due to postural change;the threshold was 0.75V/0.4msec in the supine position, 0.75V/0.4msec in the left lateral position, but 1.5V/0.4msec in the right lateral position. The X-ray showed the decrease of lead flexure in the right lateral position. The possible mechanism of fluctuation of the pacing threshold was the minimal change of lead contact the ventricle and the traction force by tricuspid valve regurgitation. Clinicians should recognize the possibility of pacing failure due to postural change.