Anatomy of the Beating Heart: The Four Chambers Unveiled

In cardiac imaging, the 4-chamber view is essential for assessing the main structures of the heart. This perspective allows a detailed evaluation of the heart, including its location, size, orientation, movement, and rhythm. In a typical arrangement known as levoposition, two-thirds of the heart is located on the left side of the chest. The heart's main axis is angled towards the left, and the apex also points in that direction, a positioning termed levocardia. This leftward orientation is critical for ensuring the heart's normal alignment and functionality ⁶.

The four-chamber view of the heart can be readily acquired during standard obstetric sonography. The four cardiac chambers become visible by adjusting the transducer in a cephalad direction from the tomographic plane utilized for measuring the abdominal circumference. This technique facilitates its use as an effective screening method for congenital heart disease. Based on our experience, this view can be successfully obtained in over 95% of cases scanned between 18 weeks of gestation and full term³.

A study was conducted to evaluate the effectiveness of the four-chamber view of the heart as a screening tool for obstetrical sonographers in identifying congenital heart disease in fetuses. The study examined 1,022 pregnancies, during which 74 cases of structurally abnormal hearts were identified. 71 cases, or 96%, were detectable using the four-chamber view. This method demonstrated a sensitivity of 92% and a specificity of 99.7% in detecting congenital heart disease. The positive predictive value was calculated at 95.8%, while the negative predictive value was 99.4%. Based on these findings, the research suggests that the four-chamber view is an effective screening method for congenital heart disease and recommends its incorporation into standard obstetric ultrasound examinations ².

After 11 weeks, it is expected that the heart will display a fully developed, symmetric four-chamber structure, with all chambers being easily identifiable and well-formed. During the period from 8 to 14 weeks, there may be observable changes in the cardiac axis as the heart continues to develop and adjust within its surrounding structures ⁸.

Heart's Position

In the fetal stage, the heart occupies approximately one-third of the chest cavity (Figure 1).

Figure 1

Orientation and Alignment

The heart's orientation at a 45-degree angle (Figure 2) within the thoracic cavity is pivotal for its optimal function and efficiency. This specific positioning ensures effective pumping and distribution of blood throughout the body. Figure 2

Adjacent to the heart, the descending aorta (Figure 3^a) is situated to the left of the spine, playing a critical role in transporting oxygenated blood from the heart to the lower parts of the body.Figure 3^a

Figure 3^bThe descending aorta is typically found on the left front side of the spine, while the inferior vena cava (IVC) is situated on the right front side of the aorta. If an ultrasound reveals the descending aorta on the right side of the spine, this might suggest the presence of a right aortic arch or another anomaly.

In fetal anatomy, the heart is examined through three principal views (Figure 4): apical, basal, and axial (lateral), each obtained by adjusting the ultrasound probe's angle. These perspectives are essential for assessing various aspects of the fetal heart, ensuring proper development, and identifying potential anomalies early on ¹. The fetal heart needs to be assessed from at least two perpendicular planes, which depend on the structure's size, thickness, and its orientation to the ultrasound beam ⁸.

View

How to obtain

Benefits

Apical

1. Place the transducer in a way that the apex of the heart is directed towards it.

2. Ensure that the interventricular septum is properly aligned with the path of the ultrasound beam.

- Optimal AV valve assessment

- Clear ventricular chamber view

- Detailed atrial structure analysis

- Longitudinal ventricular evaluation

- Atrioventricular function insights

Basal

1. To begin, place the transducer in a position that allows you to visualize the upper part of the heart.

2. Ensure that you adjust the transducer so that the interventricular septum is aligned perpendicularly at a 90-degree angle to the path of the ultrasound beam.

- Enhances atria visualization, including the foramen ovale

- Highlights the atrioventricular valves’ clarity

- Shows atrial septal defect clearly

- Improves focus on atrial structures

- Aids in valve assessment

Axial

1. Begin by positioning the ultrasound transducer so the beam crosses the fetal chest area.

2. Adjust the angle so it is perpendicular to the interventricular septum to capture the best image.

- Assesses septum.

- Evaluates myocardial wall thickness.

- Examines chordae tendineae connections.

- Checks septal thickness.

- Assesses ventricular contractility effectively.

Figure 4

Heart Structure and Functionality

Septal Structures and Valves (IVS+IAS+AV Valves): The heart's crux, or central region, is defined by the intersection of the interventricular septum (IVS), interatrial septum (IAS), and atrioventricular (AV) valves (Figure 5). This region is pivotal in directing blood flow within the fetal heart, allowing communication between chambers necessary for the preferential distribution of oxygenated blood ⁴.

Figure 5

Four Chamber Heart ¹

⅓ of fetal chest (30%)

Apex points to Lt (Levocardia)

45 degrees

Positioned in Lt Chest (Levoposition)

Descending Ao (Lt of spine)

IVS+IAS+AV valves = Crux (center of the heart, Intact)

No linear insertion of atrioventricular valves

Tricuspid valve(TV) - more apical

TV=Rt Ventricle

Mitral valve(MV) = Lt Ventricle

Introduction to Atrioventricular Valves

The primary role of the AV valves is to maintain unidirectional (inflow) blood flow. During fetal circulation, these valves open to allow blood to pass from the atria to the ventricles and close to prevent backflow as the ventricles contract. This ensures that oxygen-rich blood is efficiently distributed, which is vital for the developing fetus.

In the fetal heart, there are two main atrioventricular valves:

Tricuspid Valve¹

(3) leaflets: Anterior, posterior, and septal

Embryology: The leaflets of the tricuspid valve, which are the flaps that help control blood flow, form from two main sources. One part comes from the endocardial cushion tissues, which are special tissues inside the heart. The other part comes from the heart muscle itself, called the myocardium. The myocardium that helps form the valve is from two places: the tricuspid gully complex and the supraventricular crest, which is an area that is developing within the heart.

Function: The tricuspid valve is like a gatekeeper that directs blood from the upper right chamber (right atrium) to the lower right chamber (right ventricle) of the heart. This ensures that blood moves in the right direction for efficient circulation.

Mechanism: In a developing fetus, the lungs aren't used for breathing yet. Instead, blood is mostly directed away from the lungs. The tricuspid valve helps guide blood that moves from the right ventricle into the pulmonary artery. However, much of this blood bypasses the lungs by taking a shortcut through the ductus arteriosus to the aorta, allowing it to circulate throughout the body.

Importance: Essentially, it keeps oxygen-rich blood coming from the mother’s placenta separate from the oxygen-poor blood coming from the baby's body tissues, which helps the developing fetus receive the right amount of oxygen.

Mitral Valve (Bicuspid Valve) ¹

(2) Leaflets: Anterior & Posterior

Embryology: Valve starts forming from special tissues known as endocardial cushions located at the meeting point of the heart's upper and lower chambers. During the early weeks of fetal development, roughly between weeks 5 and 8, these cushions change shape and combine to create the mitral valve leaflets. The papillary muscles and chordae tendineae, which help the valve open and close properly, develop alongside the leaflets. They originate from the heart's muscular wall and are essential for the valve's proper function

Function: The mitral valve is one of the four critical heart valves responsible for regulating blood flow. It is positioned between the left atrium and the left ventricle.

Mechanism: The mitral valve comprises two flaps, or leaflets, that operate in a coordinated manner. During diastole, the heart's relaxation phase, these leaflets open to permit blood to flow from the left atrium into the left ventricle. Conversely, during systole, the heart's contraction phase, the leaflets seal securely to prevent the backflow of blood.

Importance: This precise regulation of blood flow is essential for effective circulation, especially in fetal development. The robust pumping action of the left ventricle is essential for distributing oxygen-rich blood throughout the body, ensuring all tissues receive the necessary nutrients and oxygen to function properly.

Assessing Heart Valves⁴

When examining heart valves, it is essential to ask several key questions to ensure a comprehensive assessment:

How Freely Do They Move?

- Assess the mobility of the valves. This involves examining whether the valves are opening and closing smoothly, which is essential for maintaining efficient blood flow. Restricted movement may indicate stenosis or other malfunctions.

Is the Crux in the Heart Intact?

- The crux (Figure 6), where the four chambers of the heart come together, should be intact and properly functioning. Any abnormalities here could affect the overall function of the heart and compromise circulation.

Figure 6

Where Does the Tricuspid Valve Insert on the Septum with the Mitral Valve?

- Analyze the anatomical placement of the tricuspid valve. This includes understanding its insertion on the septum relative to the mitral valve. In a normally developed fetal heart, the tricuspid valve connects to the heart's wall at a lower point compared to the mitral valve. This difference in attachment is known as differential insertion of the atrioventricular valves (DIAVV). In a histology section resembling the four-chamber view seen in an ultrasound, the septal leaflet of the tricuspid valve was connected to the lower portion of the membranous septum. Meanwhile, the mitral valve was connected to the upper part of the membranous septum ⁷. Misplacement or structural anomalies can indicate congenital or acquired heart conditions.

Figure 7

Linear Insertion?

- Examine if the valves exhibit a linear insertion. This regularity ensures proper valve function. Linear insertions of Av Valves (Figure 6) might suggest developmental issues affecting how the heart valves operate under physiological conditions.

Figure 8

Regurgitation Present?

- Check for any signs of regurgitation, where blood flows backward due to improper valve closure. This can lead to decreased cardiac efficiency and, over time, may cause additional stress on the heart.

Key to Development: Identifying Morphological Markers in Fetal Heart Chambers¹

Left Ventricle	Gives APEX Smooth, thicker wall Bullet shaped Bicuspid atrioventricular valve=Mitral (leaflets: Anterior & Posterior) No moderator band(s) (2) papillary muscles-free ventricular wall Ant leaflet fibrous cont. w/AV
Left Atrium	Most Posterior chamber Closest to the Descending Aorta and spine “Drainage” (4) Pulmonary Veins(PV) (4ch-Rt & Lt inferior PV, 5ch-Rt & Lt superior PV) Foramen Ovale Flap (Blood flows from R-L) Left atrial appendage: finger-like & narrow Coronary Sinus
Right Ventricle	Most Anterior Chamber Crescent shaped Heavily trabeculated Moderator band near the apex “TRI” Atrioventricular valves (leaflets: Ant/Post/Septal) TV inserts more apically on the septum than MV (3) Papillary muscles Chordae tendineae from the TV insert into RV wall
Right Atrium	Receives the SVC, IVC & coronary sinus. Rt atrial appendage (pyramidal shape) Chiari network (net-like strands seen at the level of TV annulus within) SA node (superiorly, just under the SVC) AV node (on the floor near coronary sinus opening) Lining: Pectinate muscles Crista terminalis ridge separates the posterior (smooth)wall from anterior (coarse) wall IVS inserts near the interatrial septum with valve opening-Eustachian valve

Figure 9

Coronary sinus ⁴, positioned along the rear side of the left atrium and extends into the left atrioventricular groove (Figure 9).

Drainage Pathway:
Normally, it empties into the right atrium, often near the site where the atrioventricular valves attach.

Best seen at:
4-chamber view

Importance:
The presence and accurate positioning of the coronary sinus are vital for optimal cardiac development and operation. It serves as a reference point for identifying other cardiac structures, such as the atrioventricular node.

Anomalies:
A dilated coronary sinus, which might mimic an atrioventricular canal defect, may be linked with other heart anomalies such as a persistent left superior vena cava.

Figure 10

Foramen ovale ⁴, a special pathway channeling oxygen-rich blood from the right upper heart chamber to the left one (Figure 10).

Skipping the Lungs:
Before birth, oxygen and nutrients are obtained from the mother's placenta. The foramen ovale provides a shortcut for the oxygenated blood from the placenta, bypassing the lungs, so that vital organs like the brain receive the oxygen they need.

Connection Between Heart Chambers:

The foramen ovale is an opening between the heart's upper chambers. Because of pressure differences, most of the blood that enters the right chamber is directed through the foramen ovale into the left chamber, rather than being pumped to the lungs through the right ventricle.

Pulmonary veins ¹, are assessed to confirm their proper connection to the left atrium. This perspective offers a clear view of the heart's chambers and valves, particularly focusing on the left atrium where the pulmonary veins drain.
Detailed Explanation:

1. Identifying the Pulmonary Veins (Figure 11):
- The pulmonary veins typically appear as entering the posterior wall of the left atrium in the 4CV.

2. Normal Connection:
- Usually, two inferior pulmonary veins are visible—one from the right and one from the left—entering the left atrium.

3. Assessment:
- The 4CV, paired with color Doppler, is used to assess the direction and velocity of blood flow in the pulmonary veins, verifying their normal connection to the left atrium.

4. Importance:
- This evaluation is essential for detecting potential congenital heart defects, especially anomalous pulmonary venous connection, where the veins may drain into the right atrium or other incorrect locations.

Figure 11

Mastering the Evaluation of Four Chamber Measurements ¹

Cardio- Measurement

Normal Parameters

Assessment Indications

Cardio-thoracic area ratio (CTR)

1. Acquire Image: Capture a clear ultrasound image of the fetal heart and chest showcasing a four-chamber view.

2. Measure Heart Diameter: Determine the anteroposterior (AP) distance from the heart's apex to the top of the atrial septum. Optionally, measure the transverse diameter across the heart's widest part.

3. Measure Thoracic Diameter: Record the AP distance from the front thoracic wall to the fetal spine. Similarly, measure the transverse distance across the widest part of the thorax, including ribs and skin.

4. Calculate Ratio: Compute the CTR by dividing the heart's diameter by the thoracic diameter.

(normal <0.34)

TTTS
Anemia
Sacrococcygeal teratoma
CPAM
CHAOS

Cardiac axis

1. Establish the four-chamber view:

Locate the fetal heart in a transverse plane, ensuring the four chambers are visible.

2. Identify the interventricular septum:

This is the wall separating the left and right ventricles of the heart.

3. Draw a line bisecting the chest:

This line runs horizontally through the center of the fetal chest, perpendicular to the spine.

4. Measure the angle:

Determine the angle between the interventricular septum and the bisecting line

(normal 35-65°)

Axis <20°

Rt aortic arch
Situs ambiguous

Axis >65°

Absent pulmonary valve
Aortic stenosis
Tetralogy of Fallot
Pulmonary atresia

Av Valves

Tricuspid (RT) & Mitral (LT) valve annulus measurement

End-Diastolic Phase Identification:

The end-diastolic phase of the cardiac cycle is characterized by the complete opening of the atrioventricular valves, specifically the mitral and tricuspid valves. During this phase, the ventricles are in the process of filling with blood.

Measure in Diastole

(Compare Rt to Lt) TV>MV

Tricuspid Annulus Measurement:
- Use the apical four-chamber view.
- Determine the distance between the anterior and septal leaflets of the tricuspid valve at their attachment points to the annulus.

Mitral Annulus Measurement:
- Also use the apical four-chamber view.
- Measure the span between the anterior and inferior leaflets of the mitral valve at their points of attachment to the annulus.

Key Considerations:
- Hinge Points: Clearly define where the leaflets connect to the annulus for precise measurement.
- Timing: Take measurements during the end-diastolic phase, as this is when the valves are fully open.

Prior to 34 weeks:

TV annulus 0.66-1.2cm

MV annulus 0.63-1.0cm

After 34 weeks:

TV annulus 0.95-1.5cm

MV annulus 0.92-1.3cm

Atrioventricular septal defects
Ebsteins' anomaly
HLHS
HRHS
Aortic atresia/stenosis
Mitral atresia/stenosis
Pulmonary atresia/stenosis
Tricuspid atresia/stenosis

Transverse atrial dimensions

*Largest TRV atrial diameter

Varies w/gestational age

14-38 wks

Mean Left Atrial Width =

3.16 - 14.66 mm

Mean Right Atrial Width =

3.54 - 16.74 mm

AV valve regurgitation

The four-chamber view is the starting point for cardiac evaluation, being the most familiar and frequently utilized perspective. It is the first view taught in fetal echocardiography and standard second-trimester anatomy scans because of its critical role. This essential view is accessible without specialized skills and can be captured at any fetal position from 12 weeks to term. Importantly, the four-chamber view is instrumental in identifying up to 70% of detectable cardiac anomalies prenatally. Understanding the normal markers in the four-chamber view equips you to discern abnormal structures and measurements effectively. Stay tuned for Part 2, where we will focus on identifying irregular markers and measurements within this view.

Part 1 Fetal Cardiac Morphology: Understanding the Four Chambers and Normal Markers

Exploring the Heart: A Journey Through Its Four Chambers