本文是对以往国际妇产科超声学会(The International Society of Ultrasound in Obstetrics and Gynecology,ISUOG)发表的关于孕中期胎儿心脏超声筛查常规[1] 的修订和更新版本,为产前超声筛查先天性心脏病的最新观点。新的ISUOG 指南与其他指南相一致,推荐将超声筛查胎儿心脏的左右心室流出道观和四腔观列入常规筛查。
先天性心脏病是婴幼儿死亡的主要原因,占活产儿先天性心脏病发病率的4/1 000~13/1 000[2-4]。
1950 年至1994 年,世界卫生组织报道42% 的婴幼儿死于心脏病[5]。在产前超声诊断中心脏结构畸形是最易被漏诊的畸形[6-7]。筛查者的经验,母亲过于肥胖,探头频率不高,腹部瘢痕,孕龄过小,羊水量过少及胎儿体位欠佳等因素均可影响先天性心脏病的产前超声检出率[8-9]。产前胎儿心脏超声筛查可发现胎儿某些心脏畸形,有助于改善某些特殊心脏异常胎儿的预后[10-14]。
胎儿心脏筛查的主要目的是在孕中期最大限度地检出心脏畸形[15]。胎儿心脏超声筛查指南作为常规产前诊断的一部分[16-17],可应用于低风险胎儿的评估,还有助于鉴定胎儿遗传综合征的风险,为产前咨询、产科管理及多学科护理提供有价值的信息。对可疑心脏畸形的胎儿应行胎儿超声心动图筛查,进行更加全面的评价[18]。
一、总论
虽然心脏四腔观和三血管观具有很好的诊断价值,但还应认识到潜在的诊断误区,会影响先天性心脏病的及时诊断[19-21]。某些特殊类型的畸形(如大动脉转位和主动脉缩窄)在四腔观上并无异常表现,有些畸形直至孕晚期才逐渐表现出来,全面细致的心脏筛查能提高筛查效果。在四腔观的基础上加上左右心室流出道观,是提高先天性心脏病诊断率的重要方法。
(一)筛查孕周
尽管很多解剖结构在孕22 周后才能被满意显示,但心脏超声筛查最佳时机是孕18~22 周。一些畸形可能在早孕晚期和中孕早期即被诊断,尤其发现及确定胎儿颈项透明层增厚时[22-27],这些畸形更容易被诊断。
(二)仪器设备
1. 探头:高频探头可提高细小畸形缺陷的显示,但也降低了超声的穿透力。在权衡穿透力和分辨率的前提下,尽可能使用高频率探头。对于孕晚期腹壁较厚的孕妇,谐波成像可提高图像的清晰度[28]。
2. 图像参数:横断面灰阶图像是胎儿心脏超声图像的基础。系统设置应强调高帧频、高对比度、高分辨率、低持久性、单聚焦及窄声窗。
3. 图像放大与动态回放:筛查时应放大图像使心脏占屏幕的1/3~1/2。动态回放功能可实现正常心脏结构的实时评估,在整个心动周期中确定心脏瓣膜的活动。图像放大和动态回放功能的使用有助于对心脏畸形的诊断。
二、胎儿心脏筛查
依据最新的文献报告,目前胎儿心脏超声筛查应包括四腔观和左右心室流出道观[29-37]。
(一)心脏四腔观
1. 心脏位置:胎儿心脏四腔观涉及特殊标准的详细评估,而非几个简单心腔的计数。图1~6 显示四腔观筛查的主要基础。评估心脏的位置,须鉴别胎儿左右两侧,首先确定胎儿左右,确定胃泡和心脏都在胎儿左侧,正常心脏面积不超过胸腔面积的1/3。一些观可能显示胎儿心脏周边小的低回声,会被误诊为心包积液,但这种单独存在的表现通常是正常变异[38-39]。
心脏主要位于左侧胸腔,心脏长轴指向左侧并成角,平均为(45±20)° [40](图1~4)。即使在四腔观显示不满意时[41],也应观察心脏长轴和位置。当胎儿心脏和(或)胃泡不在胎儿左侧时应怀疑心脏位置异常。心轴异常(尤其左右心室流出道异常)时心脏畸形的风险增加。心轴异常还与染色体异常相关。心脏由正常左前位异常移位可能因膈疝或占位性病变所致,如肺囊腺瘤。心脏位置异常也可继发于胎儿肺发育不良或发育不全。心轴移动提示可能存在胎儿腹裂和脐膨出。
2. 胎心率:正常胎心率在120~160 次/min。孕中期正常胎儿可能存在轻微短暂的心动过缓。持续心动过缓,尤其心率持续低于110 次/min[42] 可能存在心脏传导阻滞,需经胎儿心脏学专家进行实时评估。在孕晚期反复出现胎心率减速可能胎儿缺氧。偶发心率异常与胎儿心脏结构异常的风险无直接关系,常可自行恢复。伴发严重节律异常,需行超声心动图筛查[43-44]。增加多普勒听诊次数并有针对性扫查以排除心包积液,确定胎儿心脏观的正常。轻微的心动过速(> 160 次/min)可能是胎动时胎心的正常变异。而持续心动过速(≥ 180 次/min)则需要进一步评估,以除外存在胎儿缺氧或严重的快速型心律失常[45]。
3. 四腔观结构:正常情况下两侧心房大小基本一致,卵圆孔瓣开放于左心房内。房间隔下缘为原发隔。心脏十字交叉处房间隔下段与室间隔上段连接,并有两侧房室瓣附着。通常可观察到肺静脉进入左心房,当仪器条件允许时建议至少观察到两条肺静脉进入左心房。
调节束接近心尖部,为一条明显的肉柱横穿右心室腔,有助于鉴别形态学右心室。左心室心尖部光滑,构成心脏心尖部。正常情况下两心室大小相近,无室壁增厚表现。孕晚期可能出现左右心室腔轻微不对称,属于正常变异,但在孕中期发现明显左右心不对称应进一步筛查[46]。左心梗阻性病变,如主动脉缩窄,左心发育不良,是造成左右心室腔不对称的重要原因[47-48]。
室间隔缺损诊断较困难,应仔细检查室间隔,从心尖部到十字交叉处。当声束垂直于室间隔时易于观察室间隔;当超声束平行于室壁时可能将回声失落误认为室间隔缺损。当超声图像不能提供足够的侧向分辨率,尤其当胎儿大小和位置不利于筛查时,小的室间隔缺损(1~2 mm)诊断非常困难。这些小缺损临床意义不大,常可在宫内自然闭合[49-50]。二尖瓣及三尖瓣分别自如开启、关闭。三尖瓣隔叶在室间隔的附着点较二尖瓣更靠近心尖部。房室瓣排列异常是心脏异常的重要超声表现,如房室间隔缺损。
(二)左右心室流出道观
左右心室流出道观是胎儿心脏超声筛查的必要部分。主动脉和肺动脉这两条血管的筛查非常重要,包括血管与心室的连接关系、大小和位置关系及动脉瓣的开放程度。如不能确定,推荐行进一步评估。要求至少应观察到流出道从相应的心室发出,血管的大小径线相似,起始部互相交叉。
还可以在左右心室流出道观上增加三血管观和三血管气管观。与单纯四腔观相比,增加左右心室流出道筛查可提高严重心脏畸形的诊断率[31,33,51-52]。左右心室流出道观有利于圆锥动脉畸形的诊断,如法洛四联征、大动脉转位、右心室双出口和共干畸形[34-37,53-58]。
1. 超声筛查技术:从胎儿腹部(标准的腹围切面)横向向头侧进行移动扫查,从心脏四腔观到纵隔,连续评估各个不同的观:左心室流出道观,右心室流出道观,三血管观,三血管气管观(图7~12)[59]。在理想的条件下筛查,左右心室流出道观较容易获得。但由于机器等条件的影响,并不是每一个受筛查者在常规筛查过程中都能获得这些观。
从心脏四腔观开始,将探头向胎儿头侧滑动(或转动角度)就能获得左心室流出道观和右心室流出道观(图13,14),获得主动脉和主肺动脉在起始处的交叉观,还可显示肺动脉分叉处。另外,描述评估胎儿流出道另一种方法是旋转技术[32]:从心脏四腔观开始,将探头向胎儿的右肩旋转。当声束与室间隔垂直时更容易进行此项筛查,但需要一些技巧。左心室流出道观,在扫查过程中可显示完整的升主动脉。
应用胎儿心脏超声筛查技术获得左心室流出道观后,将探头向头侧倾斜,直至超声束几乎垂直于主动脉时可以观察到肺动脉。将探头从右心室流出道观继续向头侧滑动或倾斜,可获得主动脉和肺动脉的其他观。这些观与三血管观和三血管气管观是一致的,其中显示了两个动脉与上腔静脉和气管的关系,这一观中既可显示主动脉弓,又可显示动脉导管弓[53-56]。
2. 左心室流出道观:左心室流出道观可以确定主动脉起源于形态左心室(图15~18)观察室间隔和主动脉前壁之间的连续性,主动脉瓣启闭自如,无增厚。追踪主动脉弓处向颈部方向发出3 条动脉。
三、彩色多普勒
虽然彩色多普勒血流在指南中不是必须筛查的项目,但在胎儿心脏筛查中推荐用于常规筛查[60],彩色多普勒可用于心脏结构成像及显示异常血流类型。彩色多普勒血流技术有助于肥胖孕妇的胎儿心脏结构的评估[61],并能提高低危孕妇中胎儿严重先天性心脏畸形的检出率[37,62]。
四、胎儿超声心动图
当胎儿心脏筛查时不能获得正常心脏四腔观和左右心室流出道观,或有先天性心脏病风险增高的指标而疑有先天性心脏畸形时,应行胎儿超声心动图筛查。这些筛查步骤的具体细节此前曾发布过[18]。在无任何风险因素和心外畸形的胎儿中,产前诊断出的先天性心脏病并不少见,产前胎儿心脏筛查非常重要[58]。然而,医疗保健医师应掌握胎儿超声心动图筛查的指征[63]。如在妊娠11~14 周时胎儿颈项透明层厚度大于3.5 mm,即使后来的测量值在正常范围内[64-67],也提示应行详细的心脏筛查。
胎儿超声心动图筛查应由熟悉先天性心脏畸形的产前诊断专家完成。除基本筛查信息外,还应包括心脏结构和功能的详细分析,进一步描述内脏心房位置、体静脉、肺静脉的连接、卵圆孔、房室连接、心室大动脉连接、大血管的关系和主动脉弓、动脉导管弓的矢状观[68-70]。
其他传统的超声技术也可用于胎儿心脏筛查中,包括多普勒超声测量血流速度,鉴别通过瓣膜和心室的异常流量类型。M 型超声心动图是分析心脏节律、心功能和心肌壁厚度的一种重要方法。新技术,如组织多普勒与容积超声(三维- 时间空间关联成像技术),用于对胎儿心脏解剖结构和功能进行更详细的评估。三维- 时间空间关联成像技术有助于对复杂心脏畸形胎儿诊断评估,包括圆锥动脉干畸形、主动脉弓畸形和异常肺静脉反流。其他超声模式,如斑点追踪目前主要用于研究方面,可能成为评估胎儿心脏功能的一个重要临床方法。总之,在胎儿心脏超声筛查常规中增加左右心室流出道观,必要时增加三血管观和三血管气管观,能弥补单纯胎儿心脏超声四腔观筛查的不足,可明显提高胎儿先天性心脏病产前超声检出率。
(本文图1~26 见光盘)
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[ 译自:International Society of Ultrasound in Obstetrics and Gynecology, Carvalho JS, Allan LD, et al. ISUOG Practice Guidelines (updated): sonographic screening examination of the fetal heart[J]. Ultrasound Obstet Gynecol, 2013, 41(3): 348-359.]
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