Măsurarea intervalului atrioventricular în evaluarea cordului fetal – dificultăți în examinarea de rutină

Doppler ultrasound techniques are essential tools in the noninvasive assessment of fetal cardiovascular function, particularly for measuring atrioventricular (AV) time intervals. These measurements provide important information about the heart’s function and can aid in the diagnosis of fetal arrhythmias, especially heart blocks. 

1. Fetal atrioventricular (AV) time intervals

The atrial contraction (A wave) and ventricular systole (S wave) are separated by specific time intervals in the heart’s electrical and mechanical cycle. In a Doppler study, these intervals are analyzed to assess fetal cardiac function and are often measured across different anatomical structures such as:

• Left Ventricular Inflow and Outflow Tracts (LV In/Out)
• Superior Vena Cava and Ascending Aorta (SVC/AA)
• Pulmonary Artery and Pulmonary Vein (PA/PV).

Each of these regions is evaluated using Doppler flow velocities and time intervals between different phases of the cardiac cycle (e.g., atrial contraction and ventricular ejection), which can give insights into the fetal heart’s hemodynamic function.

2. Doppler techniques for measurement

Doppler ultrasound utilizes the Doppler effect, where the frequency of sound waves is altered by the motion of blood cells. By analyzing these frequency shifts, Doppler ultrasonography provides information about the velocity and direction of blood flow. This is particularly useful for assessing the timing and relationship between ventricular and atrial activity, which helps evaluate fetal cardiovascular health.

A. Left Ventricular Inflow and Outflow Tracts (LV In/Out)

• Inflow tract: blood flow from the left atrium to the left ventricle.
  • Measurement: using Doppler across the mitral valve (for left ventricular inflow), assesses the time interval between atrial contraction (A wave) and ventricular systole (S wave).
• Outflow tract: blood flow from the left ventricle to the ascending aorta.
  • Measurement: Doppler at the aortic valve assesses the time between the onset of ventricular systole and the opening of the aortic valve (early systolic ejection phase).
• Doppler data interpretation: the AV time intervals here can provide important information about diastolic function, systolic function, and the coordination between the atria and ventricles.
• The mitral-aortic flow is the most commonly used as the five-chamber view is part of the standard examination. However, it has its downsides: at high heart frequency, the E and A waves tend to fuse, and it is no longer possible to measure the AV interval. Certain authors also describe that it has a low rate of detection of first degree AV block^(1-3).

B. Superior Vena Cava and Ascending Aorta (SVC/AA)

• SVC flow: the superior vena cava is evaluated to determine venous return to the heart. Doppler is applied to measure the flow velocities in the SVC, particularly useful in assessing the atrial contraction phase.
• AA flow: in the ascending aorta, Doppler ultrasound is used to assess the ejection phase of the left ventricle.
• Time intervals: the relationship between the SVC and ascending aorta Doppler signals helps assess the timing of atrial and ventricular contractions, and it can be useful in detecting abnormalities such as fetal arrhythmias.
• Sometimes difficult to obtain in longitudinal views, we can try to obtain the results on a slightly tilted image of the three-vessels view – or on an “approximative” short axis view. Anuwutnavin and colleagues have revealed that SVC/AA obtained AV time intervals had the least fetal heart rate influence⁽⁴⁾. Andelfinger also recommends the SVC/AA as the best approach for AV time measurements⁽⁵⁾.

C. Pulmonary Artery and Pulmonary Vein (PA/PV)

• Pulmonary artery flow: Doppler of the pulmonary artery helps assess the systolic time interval, including how well blood is pumped by the right ventricle.
• Pulmonary vein flow: Doppler measurements from the pulmonary veins provide data on diastolic filling and the pressure gradient between the lungs and left atrium.
• Time intervals: the relationship between the pulmonary artery and vein Doppler signals can help assess the filling pressure and the cardiac cycle timing, which is essential for understanding the functioning of the fetal heart in relation to lung circulation.
• There are studies that described the PA/PV as the easiest to obtain data about AV relations, because it is easy to obtain good images on the four-chamber view, using color Doppler and then pulsed Doppler. It is described to have a low dependence on fetal position and good intra- and interobserver reproducibility^(6-8).

3. Clinical significance of Doppler AV time interval measurements

The measurement of fetal atrioventricular (AV) time intervals plays a pivotal role in the assessment of fetal cardiac function and the identification of potential conduction abnormalities. These intervals provide valuable insights into fetal cardiac development and are integral to the early detection and management of fetal arrhythmias and conduction disorders, such as congenital heart block. Doppler ultrasound techniques are commonly employed to measure AV time intervals, utilizing specific anatomical landmarks such as the left ventricular inflow and outflow tracts (LV in/out), the superior vena cava and ascending aorta (SVC/AA), and the pulmonary artery and pulmonary vein (PA/PV). Each of these methods offers unique advantages, with varying degrees of sensitivity to factors like fetal heart rate and gestational age. Notably, AV time intervals have been shown to correlate positively with gestational age and negatively with fetal heart rate, further underscoring their clinical relevance in monitoring fetal cardiac health. Among the Doppler techniques, the LV in/out method is highly favored for its reproducibility and ease of obtaining clear Doppler signals. The SVC/AA approach, with its minimal susceptibility to heart rate influences, is particularly useful in assessing the interplay between heart rate and AV intervals. Meanwhile, the PA/PV method, known for yielding the longest AV time intervals, also demonstrates high reproducibility, making it a valuable addition to the toolkit for fetal cardiac evaluation.

4. Challenges and limitations

• Doppler measurements are highly operator-dependent, requiring significant expertise in fetal echocardiography.
• Artifacts and motion can interfere with the accurate measurement of time intervals, particularly in early pregnancy.

Our project

We tried to evaluate how long does it take to obtain cardiac rhythm with measurable AV intervals by three Doppler methods in all cases. We used mitral and aortic Doppler, Ao and SVC Doppler, and pulmonary vein – pulmonary artery Doppler. The target was to obtain good images of all three in the same patient, without prolonging the examination time, and obtain them for 10 consecutive scans of different patients. All examination were done in an ambulatory practice, from 25 September 2024 to 4 October 2024. We used a Voluson E8 ultrasound machine with a RAB-6D probe. All images were obtained by one examiner (AC), but consensus regarding the image quality was made by unanimous vote of all members involved in the project.

Inclusion criteria:

• routine anomaly scan in the second or third trimester
• no heart anomaly
• no known rhythm anomaly
• no other fetal pathology.

First week: 25-27 September 2024

The main challenges were in obtaining good quality images that would also allow the measurements, if needed, without extending the examination time. There were 10 cases examined, with a 60% success rate (Figure 1).

Second week: 30 September – 4 October 2024

Pulmonary vein-artery provided the most challenges. Confidence and image quality increased substantially after the first week. We had 14 cases and, although there were not 10 consecutive good examinations, we had an 86% rate (Figure 2).

Third week: 7-11 October 2024

It took us 32 examinations (including the last 10 consecutive cases) to arrive at proficiency using these three methods (Figure 3).

By the end of our project, we were able to provide good images that would allow the measurement of the AV intervals, in the second and third trimesters (Figures 4-9).

Conclusions

Doppler ultrasound techniques for measuring atrioventricular time intervals in the fetal left ventricular inflow and outflow tracts (LV In/Out), superior vena cava and ascending aorta (SVC/AA), and pulmonary artery and pulmonary vein (PA/PV) are valuable diagnostic tools for evaluating fetal cardiovascular rhythm. These measurements are crucial in detecting early signs of fetal heart disease, allowing for timely interventions and management strategies.

Training with new techniques in everyday practice prepares us for those rare difficult situations. We should not live with the false impression that they are easy, because at the time we will need them, we will not only have to provide good images, but also reliable measurements that matter in the diagnosis.

Autor corespondent: Maria-Cristina Comănescu E-mail: cristinacomanescu85@gmail.com

CONFLICT OF INTEREST: none declared.

FINANCIAL SUPPORT: none declared.

This work is permanently accessible online free of charge and published under the CC-BY.