Spectral Doppler imaging of the renal vessels facilitates the assessment of fetal arrhythmias

The different approaches in fetal cardiac rhythm assessment include M-mode, spectral Doppler, magnetocardiography and tissue Doppler techniques. As fetal magnetocardiography is not widely available and tissue Doppler imaging has only just been introduced in the diagnosis of fetal arrhythmias, M-mode and spectral Doppler techniques are used in most centers to evaluate the relationship between atrial and ventricular contractions. Although neither of these techniques provides detailed electrophysiological information, the mechanical consequences of the electrical events can be evaluated and interpreted. M-mode was one of the first echocardiographic modalities employed in fetal rhythm evaluation and is still the favored technique in most centers. However, optimal fetal positioning and sufficient image resolution is required in order to achieve interpretable tracings. If image quality is unfavorable, or if cardiac function is poor, it can be exceedingly difficult to visualize small movements of the atria and ventricles by this method, and spectral Doppler techniques have to be used1, 2. A common approach in rhythm assessment with spectral Doppler imaging is to place the wide pulsed-wave Doppler gate between the left ventricular outflow tract and the mitral valve so that both the aortic and the mitral Doppler signals are detected. Alternatively, the Doppler gate can be placed simultaneously over the superior vena cava and ascending aorta, inferior vena cava and descending aorta, azygos vein and descending aorta or a branch pulmonary artery and vein1-4. Numerous factors may influence the success of all these approaches. The intracardiac spectral Doppler technique requires clearly discernable A- and E-waves as well as an optimal angle of insonation that is dependent on the fetal position. Likewise, Doppler interrogation of the low-velocity superior vena cava atrial systolic flow is dependent on an optimal angle of insonation and a low set wall motion filter that renders the signal susceptible to contamination. Simultaneous assessment of descending aorta and inferior vena cava can rarely be obtained at an optimal angle of insonation and the Doppler gate has to be opened wide, resulting in poor signal quality in most cases. The close proximity of the azygos vein and descending aorta allows avoidance of some of these shortcomings. However, this approach is feasible only in fetuses with left isomerism and interruption of the inferior vena cava, where the azygos vein is markedly enlarged and, again, an optimal angle of insonation is rarely obtained4. Although concomitant Doppler signals of the pulmonary artery and vein can usually be obtained at an optimal angle of insonation, the atrial systolic flow in the pulmonary veins is antegrade in the absence of elevated left atrial pressure. Owing to this low pulsatility, determining the timing of the onset of atrial systole can be difficult1, 2. In our institution, spectral Doppler interrogation of the renal vessels has evolved to become the preferred method of fetal rhythm assessment over the past few years. The advantages of these target vessels are obvious: (1) renal arteries and veins run parallel and in close proximity to each other; (2) an optimal angle of insonation can almost always be obtained for both renal arteries and veins; (3) with an optimal angle of insonation the Doppler gate can be set wide, which facilitates the achievement of a clear signal in the presence of fetal movements; and (4) venous pulsations with reversed or absent flow during atrial contraction are almost invariably present in the renal veins. When assessing the fetal heart rhythm with spectral Doppler imaging of the renal vessels we propose the use of a low pulse repetition frequency and that the wall motion filter should be set low (comparable to the Doppler presets best chosen for assessment of the pulmonary veins). The renal vessels are visualized in the color flow mode either in a frontal or a transverse section of the fetal abdomen. The left or right renal vessels are assessed, depending on the side for which an optimal angle of insonation can be obtained (comparable to the assessment of peak systolic velocity in the middle cerebral artery). A wide Doppler gate is positioned over the renal vessels between the hilus of the kidney and the origin of the renal artery at the descending aorta. The waveform will then depict the arterial pulsations, as well as the reversed A-waves of the venous pulsations if present, on one side, and the venous S-waves and D-waves on the opposite side of the Doppler spectrum. Figure 1 illustrates three different types of arrhythmia assessed by spectral Doppler imaging of the renal vessels in a fetus with long QT syndrome that displayed the wide spectrum of arrhythmias typically associated with this kind of disease. Spectral Doppler images of the renal vessels in a fetus with long QT syndrome at 25 weeks of gestation. (a) Sinus bradycardia with a ventricular frequency of 89 beats per minute (bpm). Venous (reversed) atrial contractions (denoted by ‘a’) and ventricular contractions (V) are displayed in the positive Doppler spectrum and demonstrate a 1 : 1 atrioventricular conduction. Venous S- and D-waves are displayed in the negative Doppler spectrum. (b) Bigeminal premature atrial contractions resulting in a ventricular frequency of 92 bpm. Every regular atrioventricular conduction (‘a’ followed by ‘V’ in the positive Doppler spectrum) is followed by a supraventricular extrasystole (SVES) that acts against closed atrioventricular valves resulting in ‘cannon waves’ and a non-compensatory pause of the ventricular contractions (V). (c) A singular SVES in an otherwise unchanged fetal heart rhythm with a ventricular frequency of 170 bpm. SVES occurs during the end of systole (against closed atrioventricular valves) and results in a ‘cannon wave’ with marked reversal of flow in the systemic veins, while the physiological A-waves (denoted by ‘a’) display an absent flow. The SVES is followed by a non-compensatory pause of the ventricular contractions (V). Venous S- and D-waves are displayed in the negative Doppler spectrum. We believe that the assessment of fetal heart rhythm by spectral Doppler of the renal arteries is easy to achieve, reproducible and readily interpretable for most fetal cardiologists. It might therefore become an established tool in fetal rhythm diagnostics. C. Berg christoph.berg@ukb.uni-bonn.de*, A. Geipel*, U. Gembruch*, * Abteilung für Geburtshilfe und Pränatale Medizin, Zentrum für Geburtshilfe und Frauenheilkunde, Rheinische Friedrich-Wilhelms-Universität, Sigmund-Freud-Strasse 25, 53105 Bonn, Germany