U Wave during Supraventricular Tachycardia: Simulation of a Long RP Tachycardia and Hiding the Common Type AVNRT

The main tool for the differentiation of supraventricular tachycardia is the 12‐lead electrocardiogram (ECG). Especially differentiating the atrioventricular nodal reentrant tachycardia (AVNRT) from the atrioventricular reentrant tachycardia (AVRT) due to concealed accessory pathway or from an atrial tachycardia (AT) is very important for catheter setting and ablation approach in an electrophysiological study. In our case we saw the occurrence of a U wave during tachycardia—simulating a pseudo P wave. This mimicked a long RP‐tachycardia, although it was a common type AVNRT.     

阵发性室上性心动过速时ST－T改变的临床意义

回顾性分析经射频消融治疗的４１８例阵发性室上性心动过速（ＰＳＶＴ）发作时的体表１２导联心电图，以了解ＳＴ－Ｔ改变的临床意义。结果表明３０５例房室折返性心动过速（ＡＶＲＴ）中，ＳＴ段下移≥２ｍｍ且持续≥８０ｍｓ和（或）Ｔ波倒置者有１８１例（５９．３４％），明显高于房室结折返性心动过速（ＡＶＮＲＴ）患者（２８／１１３，２４．７８％）Ｐ＜０．００５。并且此差异不能被心率所矫正。此外左游离壁旁道患者ＳＴ段压低多分布在Ｖ３～Ｖ６导联，而右后和左后隔旁道患者倒置的Ｔ波和（或）ＳＴ段压低多发生于Ｉ、ＩＩ、ａＶＦ导联。提示ＰＳＶＴ时心电图上ＳＴ－Ｔ改变可作为区分ＡＶＲＴ和ＡＶＮＲＴ的一个有用指标，并且可能还有粗略的旁道定位价值     

伪冠状T_(Ⅱ,Ⅲ,aVF)波诊断折返性室上性心动过速的价值

回顾性分析 134例射频消融确诊的窄QRS波群室上性心动过速 (SVT)患者窦性心律及心动过速时心电图 ,以评估伪冠状TⅡ、Ⅲ、aVF波在SVT中的鉴别诊断价值。伪冠状TⅡ、Ⅲ、aVF波定义为心动过速时Ⅱ、Ⅲ、aVF导联出现深倒、双支对称T波 ,而窦性心律时T波正常。结果 :10 2例AVRT患者中 2 9例 (2 8% )可见伪冠状TⅡ、Ⅲ、aVF波 ,32例AVNRT患者无 1例有伪冠状TⅡ、Ⅲ、aVF波 (P <0 .0 1)。伪冠状TⅡ、Ⅲ、aVF波诊断AVRT的敏感性为 2 8%、特异性为10 0 %、阳性预测值为 10 0 %。AVRT组中 ,仅后隔旁道可见伪冠状TⅡ、Ⅲ、aVF波 ,伪冠状TⅡ、Ⅲ、aVF波诊断后隔旁道的敏感性为 85 %、特异性 10 0 %、阳性预测值为 10 0 %。结论 :PSVT时伪冠状TⅡ、Ⅲ、aVF波可作为鉴别AVRT和AVNRT的一项心电指标 ,且可用于对旁道的初步定位。逆向心房激动顺序及RP间期可能是产生伪冠状TⅡ、Ⅲ、aVF波的机制     

房室旁道和房室结双径引起的折返性心动过速初次发病年龄和因素分析

探讨房室旁道 (简称房道 )和房室结双径引起的折返性心动过速的初次发病年龄。 15 5例旁道和房室结双径引起的折返性心动过速中 ,房室折返性心动过速 (AVRT) 10 6例、房室结折返性心动过速 (AVNRT) 4 9例。根据年龄和心动过速病程推算初次发病年龄 ,然后比较AVRT和AVNRT、左和右侧旁道并AVRT、显性和隐匿性旁道并AVRT、男性和女性心动过速患者的初发年龄。结果 :AVRT和AVNRT的初发年龄分别为 2 9.9± 12 .85岁和 32 .33±11.84岁 ,组间比较无显著性差异。左、右侧旁道并AVRT、显性和隐匿性AP并AVRT、男性和女性心动过速患者初发年龄比较差异均无显著性 ,均是 30岁左右。结论 :30岁左右是AVRT和AVNRT初次发病的高发年龄段 ,其他年龄段呈散在发病     

Electrocardiographic differentiation of typical atrioventricular node reentrant tachycardia from atrioventricular reciprocating tachycardia mediated by concealed accessory pathway in children

The value of the electrocardiogram (ECG) in children with supraventricular tachycardia (SVT) is unclear. The noninvasive differentiation of typical atrioventricular node reentrant tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT) mediated by concealed accessory pathway conduction is clinically important, as it helps in counseling and potentially facilitates ablation procedures. One hundred forty-eight ECGs showing narrow QRS complex SVT were obtained from children before successful radiofrequency catheter ablation. An initial 102 ECGs were analyzed by 3 blinded observers to assess the utility of various electrocardiographic findings. No electrocardiographic criteria were found to discriminate between SVT mechanisms on 1- to 3-channel Holter/event recorder tracings (n = 32); their interpretation mainly (55%) resulted in an incorrect SVT diagnosis. On 12-lead ECGs (n = 70), the 2 arrhythmias were accurately diagnosed in 76% of patients; 5 findings were found to be discriminators of tachycardia mechanism. Predictors of AVRT were visible P waves in 74% of cases (sensitivity 92%; specificity 64%), RP intervals of > or =100 ms in 91% (sensitivity 84%; specificity 91%), and ST-segment depression of > or =2 mm in 73% of cases (sensitivity 52%; specificity 82%). Pseudo r' waves in lead V(1) and pseudo S waves in the inferior leads during tachycardia predicted AVNRT in 100% of cases (sensitivity 55% and 20%, respectively; specificity 100% for both). Based on these results, we developed a new diagnostic 12-lead electrocardiographic algorithm for pseudo r'/S waves, RP duration, and ST-segment depression during tachycardia. Two observers tested the algorithm in 46 (21 AVNRT; 25 AVRT) additional cases; they correctly diagnosed the SVT mechanism in 91% and 87%, respectively. Thus, the stepwise use of diagnostically relevant 12-lead electrocardiographic parameters helps to more accurately differentiate mechanisms of reentrant SVT.     

Differentiating atrioventricular nodal reentrant tachycardia from tachycardia via concealed accessory pathway

Studies analyzing the diagnostic value of 12-lead electrocardiographic criteria differentiating slow-fast atrioventricular nodal reentrant tachycardia (AVNRT) from atrioventricular reentrant tachycardia (AVRT) due to concealed accessory pathway have shown inconsistent results. In 97 patients (50 with AVNRT, 47 with AVRT) 12-lead electrocardiograms (ECGs) were recorded during sinus rhythm and tachycardia (QRS <120 ms). The ECGs were blinded for diagnosis and patient and analyzed independently by 2 electrophysiologists. The studied criteria differentiating AVNRT from AVRT included pseudo-r'/S, the presence of a retrograde P wave, RP interval, ST-segment depression >/=2 mm with the number and location of the affected leads, QRS amplitude, and cycle length alternans.     

His束逆传不应期心室早搏刺激鉴别间隔隐匿性旁道与房室结折返性心动过速

探讨His束逆传不应期心室早搏刺激法临床应用时的注意事项。 37例慢 快型房室结折返性心动过速(AVNRT)和 2 0例间隔部隐匿性快旁道引起的房室折返性心动过速 (AVRT)患者 ,成功消融前在常规行腔内电生理检查的同时均行His束逆传不应期心室早搏刺激法。结果 :心动过速时His束逆传不应期内心室早搏刺激 ,37例AVNRT患者心房激动无明显提前 ,2 0例AVRT心房被提前激动超过 2 0 (30～ 5 0 )ms ,其中 9例终止心动过速 ,此方法鉴别AVRT和AVNRT特异度、灵敏度较高。结论 :心动过速时His束逆传不应期行心室早搏刺激法鉴别诊断AVRT和AVNRT操作简单、准确性高。但行此法检查时要求有持续发作的心动过速 ;能够标测出清晰的His束电位 ;心室早搏刺激最好与His束电位同步发放 ;反复多次重复检测以进一步提高诊断的正确性     

Differentiation of Narrow QRS Complex Tachycardia Types Using the 12‐Lead Electrocardiogram

Background: Previous studies have shown that only 80% of narrow QRS supraventricular tachycardia (SVT) types can be differentiated by standard 12‐lead electrocardiographic (ECG) criteria. This study was designed to determine the value of some new ECG criteria in differentiating narrow QRS SVT. Methods and Results : 120 ECGs demonstrating paroxysmal narrow QRS complex tachycardia (QRS s 0.11 ms and rate > 120 beats/min) were analyzed. Forty atrioventricular reciprocating tachycardia (AVRT), 70 atrioventricular nodal reentrant tachycardia (AVNRT), and 10 atrial tachycardia defined with electrophysiologic study (EPS) consisted the study group. Eight surface ECG criteria were found to be significantly different between tachycardia types by univariate analysis. P waves separate from the QRS complex were observed more frequently in AVRT (70%) and atrial tachycardia (80%). Pseudo r’deflection in lead V¹, pseudo S wave in inferior leads, and cycle length alternans were more common in AVNRT (55, 20, and 6%, respectively). QRS alternans was also present during AVRT (28%). ST‐segment depression (≧ 2 mm) or T‐wave inversion, or both, were present more often in AVRT (60%) than in AVNRT (27%). During sinus rhythm, manifest preexcitation was observed more often in patients with AVRT (42%). When a P wave was present, RP/PR interval ratio > 1 was more common in atrial tachycardia (90%). By multivariate analysis, presence of a P wave separate from the QRS complex, pseudo r’deflection in lead V¹, QRS alternans, preexcitation during sinus rhythm, ST‐segment depression > 2 mm or T‐wave inversion, or both, were independent predictors of tachycardia type. Conclusions: Several new ECG criteria may be useful in differentiation of SVT types. Prediction of mechanism prior to EPS may provide additional benefits concerning the fluoroscopic exposure time and cardiac catheterization procedure. A.N.E. 2002;7(2):120–126     

体表心电图对阵发性窄QRS波心动过速鉴别诊断及定位的价值

目的:通过分析152例阵发性窄QRS波心动过速（NQRST）患者体表心电图（ECG）的6种指标,探讨ECG对NQRST鉴别诊断及定位的价值。方法:选取152例NQRST患者,其中94例为房室结折返性心动过速(AVNRT),42例为房室折返性心动过速（AVRT),16例为房性心动过速(AT)。上述患者电生理机制均经腔内电生理检查所证实,对比分析每位患者窦性心律及心动过速发作时体表心电图在心率、ST-T改变、QRS波电交替、R-P′/P′-R相似文献   

Detection of concealed left sided accessory atrioventricular pathway by P wave signal averaged electrocardiogram

Objectives. The purpose of this study was to examine whether P wave signal-averaged electrocardiogram (P-SAECG), which detects subtle changes in P wave, detects the concealed accessory atrioventricular pathway (AP).Background. It is difficult to differentiate atrioventricular reciprocating tachycardia (AVRT) due to the AP from atrioventricular nodal reentrant tachycardia (AVNRT) when the ventricular preexcitation is absent on 12-lead electrocardiograms. By electrophysiological studies, the anterograde conduction in the concealed AP is shown to be blocked near the AP-ventricular interface during sinus rhythm.Methods. P-SAECG during sinus rhythm was performed in 20 normal volunteers (control), 21 patients with AVRT due to the concealed AP, 19 with AVNRT, 22 with paroxysmal atrial fibrillation (PAF), and 7 with automatic atrial tachycardia (AT). The filtered P wave duration (FPD) and AR20 (power spectrum area ratio of 0–20 to 20–100 Hz) were measured and repeated in AVRT, AVNRT and AT groups at one week after catheter ablation.Results. The anterograde conduction in the concealed left-sided AP was confirmed in all cases by an electrophysiological study. The FPD in AVRT group was more prolonged than that in controls or AVNRT group. Although the FPD was similar between AVRT and PAF groups, AR20 differentiated between the two groups. Ablation of the concealed AP shortened FPD in AVRT group but that of the slow pathway or the atrial focus did not shorten in the AVNRT or AT groups, respectively. The changes in FPD after ablation were correlated with those in the duration of atrial activity by an electrophysiological study (r = 0.67).Conclusions. Our findings suggest that P-SAECG detects the concealed left-sided AP, providing a clinical tool in noninvasively assessing atrial activation patterns.     

希氏束旁起搏鉴别间隔部隐匿性AVRT与AVNRT的临床价值

目的：探讨希氏束旁起搏鉴别间隔部隐匿性房室旁道与慢一快型房室结折返性心动过速（AVNRT）的临床价值。方法：采用希氏束逆传不应期心室早搏刺激法将61例患者分别诊断为37例慢一快AVNRT和24例间隔部房室折返性心动过速（AVRT）；再对61例患者采用希氏束旁起搏方法进一步检测。结果：采用希氏束旁起搏法检测37例AVNRT患者中有6例未检测成功，其余31例均为逆传房室结图形；24例AVRT患者中4例未检测成功，15例呈逆传旁道／旁道图形，5例呈非逆传旁道／旁道图形。如以逆传旁道／旁道图形为标准，鉴别间隔快旁路引起的AVRT与慢一快型房室结折返性心动过速，敏感性75％，特异性可达1009／6。结论：希氏束旁刺激法对鉴别诊断AVRT与AVNRT有较高的特异性。     

心房起搏时PR与RR间期比值对阵发性室上性心动过速鉴别诊断的意义

为探讨快速心房起搏最短１∶１房室传导时最大ＰＲ间期（ＰＲｍａｘ）与ＲＲ间期比值（ＰＲｍａｘ／ＲＲ）在鉴别阵发性室上性心动过速中的意义，分析比较了２０例房室结折返性心动过速（ＡＶＮＲＴ，有房室结前传跳跃现象者１２例、无跳跃现象者８例）和２０例房室折返性心动过速（ＡＶＲＴ）患者消融前、后快速心房起搏时最短１∶１房室传导的ＰＲｍａｘ／ＲＲ。ＡＶＮＲＴ组消融前、后心房快速起搏时最短１∶１房室传导的ＰＲｍａｘ／ＲＲ为１．１２±０．１２和０．４２±０．０７，两者比较差异有高度显著性，Ｐ＜０．０１；ＡＶＲＴ组为０．５２±０．１６和０．５１±０．１８，两者比较差异无显著性，Ｐ＞０．０５。消融前，ＡＶＮＲＴ组ＰＲｍａｘ／ＲＲ与ＡＶＲＴ组相比有显著性差异（１．１２±０．１２ｖｓ０．５２±０．１６，Ｐ＜０．０１）。ＰＲｍａｘ／ＲＲ＞１诊断ＡＶＮＲＴ的敏感性为９０％、特异性９１％。提示ＰＲｍａｘ／ＲＲ＞１在消融前可用来鉴别ＡＶＮＲＴ与ＡＶＲＴ；对无房室结前传跳跃的ＡＶＮＲＴ，消融后ＰＲｍａｘ／ＲＲ＜１可作为慢径消融成功的指标     

下位法射频消融慢径路改良房室结治疗室上性心动过速23例

用下位法射频消融慢径路改良房室结治疗房室结折返性心动过速(AVNRT)18例,房室折返性心动过速(AVRT)5例.AVNRT中16例为慢—快型,1例快—慢型,1例慢—快型与快—慢型并存,18例慢径路全部阻断成功.AVRT中1例显性预激,4例隐性预激,有5例慢径路和3例房室旁路消融成功.射频放电时21例出现结性心律.无严重并发症出现.AVNRT病人中随仿1—15个月有1例复发,第二次射频成功.认为下位法射频消融阻断慢径路成功率高,并发症少.     

The Utility of a Lewis Lead for Distinguishing Atrioventricular Reentrant Tachycardia from Typical Atrioventricular Nodal Reentrant Tachycardia

Objective The Lewis lead configuration is an alternative bipolar chest lead and it can help detect atrial activity. The utility of the Lewis lead to distinguish orthodromic atrioventricular reentrant tachycardia (AVRT) from typical atrioventricular nodal reentrant tachycardia (AVNRT) by visualizing the apparent retrogradely conducted P waves was investigated. Methods Forty-four patients with paroxysmal supraventricular tachycardia (PSVT) were included in this study. All patients had PSVT documented by an electrocardiogram (ECG) and underwent an electrophysiological study (EPS). During tachycardia, an ECG recording was performed using a Lewis lead with the electrode on the right aspect of the sternum at the second intercostal space instead of the right arm and the electrode on the fourth intercostal space instead of the left arm. The ECG parameters during tachycardia were compared between AVRT and AVNRT. Results Fourteen patients were diagnosed with AVRTs and 30 with typical AVNRTs on EPS. The positive P wave could be seen in the Lewis lead configuration in 9 of 14 patients with AVRTs and 21 of 30 patients with AVNRTs. P waves were more often visible in the Lewis lead configuration than in the standard leads (66% vs. 45%). The RP interval was significantly longer for AVRTs than for AVNRTs (88±17 vs. 154±34 ms, p<0.001), which yields 89% sensitivity and 71% specificity for distinguishing these 2 tachyarrhythmias with a cut-off point of 100 ms. Conclusion A Lewis lead configuration may help to make an accurate diagnosis among the reentrant supraventricular tachycardias prior to procedures, owing to its ability to locate P waves.     

aVL导联切迹对慢快型房室结折返性心动过速的诊断价值

目的探讨aVL导联切迹在慢快型房室结折返性心动过速(AVNRT)和顺传型房室折返性心动过速(AVRT)鉴别诊断中的作用。方法对138例经心内电生理检查及射频导管消融术治疗成功的阵发性窄QRS波心动过速患者12导联心电图进行分析,其中AVNRT 74例,AVRT 64例,比较传统指标与单独aVL导联切迹对AVNRT与AVRT鉴别价值。结果 aVL导联切迹在AVNRT出现29例(39.2%),在AVRT出现1例(P<0.01);下壁导联伪s波在AVNRT出现28例(37.8%),在AVRT出现1例(1.6%,P<0.01);V1导联伪r’波在AVNRT出现33例(44.6%),在AVRT出现3例(4.7%,P<0.01)。aVL导联切迹诊断AVNRT的敏感性为39.2%、特异性为98.4%;下壁导联伪s波诊断AVNRT的敏感性为37.8%、特异性为98.4%;V1导联伪r’波诊断AVNRT的敏感性为44.6%、特异性为95.3%,三种判断标准的敏感性、特异性差异无统计学意义(P>0.05)。结论以aVL导联切迹为判断标准,对慢快型AVNRT的诊断特异性强,敏感性较高,可作为房室结折返性心动过速的诊断指标之一。     

快-慢型房室结折返性心动过速的电生理特征和射频消融治疗

报道８例快－慢型房室结折返性心动过速（ＡＶＮＲＴ）的电生理特征及射频消融治疗。其中３例为慢－快型ＡＶＮ－ＲＴ射频消融改良慢径后出现的快－慢型ＡＶＮＲＴ。８例均经消融慢径而成功终止心动过速。平均放电次数３±１．１次、平均放电时间１２０±３０．４ｓ、平均放电功率３０±１１Ｗ。随访６～２４个月，无复发。快－慢型ＡＶＮＲＴ具有以下临床电生理特征：①快径不应期短、慢径不应期长。②心内电刺激无房室结双径路现象。③心动过速能由心房刺激诱发。④心动过速时ＡＨ间期＜ＨＡ间期，冠状窦近端Ａ波最提前。熟悉快－慢型ＡＶＮＲＴ的电生理特征，对于鉴别房性心动过速及右后间隔旁道参与的房室折返性心动过速十分重要，也是指导快－慢型ＡＶＮＲＴ射频消融成功的关键。     

Clinical and electrophysiological characteristics in patients with atrioventricular reentrant and atrioventricular nodal reentrant tachycardia.

AIM: To compare clinical, electrophysiological characteristics and transcatheter ablation results between two groups of patients, one with atrioventricular reentrant tachycardia (AVRT) and the other with atrioventricular nodal reentrant tachycardia (AVNRT). METHODS: The study population consisted of 94 consecutive patients who underwent endocavitary electrophysiological study and radiofrequency (RF) ablation: 46 patients had AVRT due to an accessory pathway with only retrograde conduction while 48 patients had AVNRT. RESULTS: In relation to general and clinical characteristics, differences between the two groups emerged regarding the age of symptom onset (25+/-16 vs 37+/-17 years, p=0.001), the prevalence of heart disease (8 vs 31%, p=0.001) and the correct diagnosis on surface ECG (50 vs 79%, p=0.001). Clinical presentation was quite similar apart from a higher prevalence of fatigue and sweating in the AVNRT group. Transcatheter RF ablation therapy results were similar. CONCLUSIONS: Patients with AVRT have a lower mean age at arrhythmia symptom onset compared with those with AVNRT and have fewer associated cardiac abnormalities. Clinical presentation is quite similar as well as their outcome after ablation. A correct diagnosis by standard ECG is more frequent in AVNRT.     

Adenosine induced PR jump on surface ECG to differentiate atrioventricular nodal re-entrant tachycardia from concealed accessory pathway mediated tachycardia: a bedside test

Objective: To evaluate the efficacy of single dose intravenous adenosine in differentiating atrioventricular nodal re-entrant tachycardia (AVNRT) from concealed pathway mediated atrioventricular re-entrant tachycardia (AVRT) using surface ECG at the bedside.     

Validation of Standard and New Criteria for the Differential Diagnosis of Narrow QRS Tachycardia in Children and Adolescents

To establish an appropriate treatment strategy and determine if ablation is indicated for patients with narrow QRS complex supraventricular tachycardia (SVT), analysis of a standard 12-lead electrocardiogram (ECG) is required, which can differentiate between the 2 most common mechanisms underlying SVT: atrioventricular nodal reentry tachycardia (AVNRT) and orthodromic atrioventricular reentry tachycardia (OAVRT). Recently, new, highly accurate electrocardiographic criteria for the differential diagnosis of SVT in adults were proposed; however, those criteria have not yet been validated in a pediatric population.All ECGs were recorded during invasive electrophysiology study of pediatric patients (n = 212; age: 13.2 ± 3.5, range: 1–18; girls: 48%). We assessed the diagnostic value of the 2 new and 7 standard criteria for differentiating AVNRT from OAVRT in a pediatric population.Two of the standard criteria were found significantly more often in ECGs from the OAVRT group than from the AVNRT group (retrograde P waves [63% vs 11%, P < 0.001] and ST-segment depression in the II, III, aVF, V1–V6 leads [42% vs 27%; P < 0.05]), whereas 1 standard criterion was found significantly more often in ECGs from the AVNRT group than from the OAVRT group (pseudo r′ wave in V1 lead [39% vs 10%, P < 0.001]). The remaining 6 criteria did not reach statistical significance for differentiating SVT, and the accuracy of prediction did not exceed 70%. Based on these results, a multivariable decision rule to evaluate differential diagnosis of SVT was performed.These results indicate that both the standard and new electrocardiographic criteria for discriminating between AVNRT and OAVRT have lower diagnostic values in children and adolescents than in adults. A decision model based on 5 simple clinical and ECG parameters may predict a final diagnosis with better accuracy.     

QRS Complex Voltage Changes Associated with Supraventricular Tachycardia

INTRODUCTION: The aim of this study was to evaluate the changes in ventricular complex voltage associated with narrow QRS supraventricular tachycardia (SVT). METHODS AND RESULTS: One hundred forty-five patients undergoing catheter ablation for SVT, 85 with AV nodal reentrant tachycardia (AVNRT) and 60 with AV reentrant tachycardia (AVRT) due to a concealed accessory pathway, were studied. Four consecutive tachycardia beats and four consecutive sinus beats were analyzed, excluding the last tachycardia complex and the first sinus one. For each of the 12 leads, the QRS complex voltage was measured, and the results of four beats were averaged both in SVT and in sinus rhythm (SR). The sum (sigma) of the QRS voltages measured in the 12 leads during SVT (sigmaSVT) and SR (sigmaSR) were calculated, as well as the QRS axis during SVT and SR. QRS complex voltage was significantly increased during SVT, with respect to SR, in leads II, III, aVR, aVF, and V2 to V6. In addition, sigmaSVT was significantly greater than sigmaSR. Only lead V1 showed a significant voltage decrease during SVT. These voltage changes were almost identical in patients with AVNRT and patients with AVRT. No relationship was found between tachycardia rate and QRS voltage variation. The QRS axis showed a significant shift during SVT, from 55.8 degrees to 64.5 degrees. CONCLUSION: QRS voltage increase occurs in reentrant SVT, independent of the underlying reentrant circuit. The phenomenon likely depends on tachycardia-related reduced ventricular filling. This could result in displacement of the heart in such a way that the left ventricle becomes closer to the precordial electrodes (proximity effect). Alternatively, decreased intracavitary blood mass could diminish the intracardiac short-circuiting of potentials, resulting in augmented transmission of cardiac vectors to the body surface.