Valve Dynamics

Observing the dynamic nature of the AV valve during the cardiac cycle and throughout valvulogenesis, in conjunction with flow patterns, provides the opportunity to analyze the functional characteristics of heart valves. Here, we present diagrams of valve motions during the cardiac cycle along with a discrete representation of transvalvular blood flow (Sec. 5.1.3).

During the initial stages of chamber formation, the heart is little more than a tube with two bulges. As blood enters the atrium, the atrium expands and the AV canal slides towards the ventricle (Fig. 5.5 J, N). At the onset of diastole, the orifice diameter

increases and blood is expelled from the atrium into the ventricle (Fig. 5.5K, O). During later stages of diastole, the AV orifice remains expanded and recoils towards the atrium (Fig. 5.5L, P). At the beginning of systole the orifice constricts, although not completely,

to prevent the ventricular blood volume from re-entering the atrium (Fig. 5.5I, M). The morphological constraints imposed by the AV canal dimensions and closing mechanics prevent the heart from maintaining unidirectional flow during systole.

Figure 5.5. Valve dynamics and blood flow in 36 hpf embryos. (A-D) Sequence of confocal scans during one cardiac cycle. Outlines of each image are provided (E-H). The gray box (E) indicates the location of blood flow characterization. Sequential images are superimposed upon each other to show cardiac cushion motions (I-L) along with timestamps. The black arrows indicate the motion of the cushion. (M-P) Binary representation of flow during one cardiac cycle. Vertical black line indicates the time point images (E-H) were recorded. See text for details of dynamics.

At 72 hpf, atrial filling commences with the valve cushions in close proximity to each other, but still separated by a short distance (Fig. 5.6I). Atrial filling occurs

synchronously with ventricular systole. Blood fills the atrium, sliding the ECs towards the ventricle, while allowing a small volume of blood to flow backwards through the small opening from the ventricle to the atrium (Fig. 5.6J,N). During diastole, the valve cushions separate and move back towards the atrium, allowing blood to pass from the atrium to ventricle (Fig. 5.6K,O). The end of diastole features the ECs once again approaching each other, but not making complete contact (Fig. 5.6L). Although not evident in the discrete blood flow representation, the heart is becoming a more efficient pump, allowing a smaller blood volume to regress from the ventricle to the atrium. However, further valve development and advanced dynamics are still required to prevent retrograde flow altogether.

Figure 5.6. Valve dynamics and blood flow in 72 hpf embryos. (A-D) Sequence of confocal scans during one cardiac cycle. Outlines of each image are provided (E-F). Sequential images are superimposed upon each other to show cardiac cushion motions (I-J) along with timestamps. The black arrows indicate the motion of the cushion. (M-P) Binary representation of flow during one cardiac cycle. Vertical black line indicates the time point images (E-H) were recorded. See text for details of dynamics.

By 84 hpf, maturing leaflet dynamics continue to improve cardiac pumping efficiency. As mentioned previously, valve leaflets are hinged on the atrial side and free moving on the ventricular side. During atrial filling and ventricular systole, the valve

leaflets approach each other and appear to make contact, but not cohesively enough to prevent retrograde flow (Fig. 5.7J, N). The atrial volume increases while the ventricular volume decreases, and the valve plane moves towards the ventricle. During diastole, the leaflets separate and the valve plane recoils towards the atrium (Fig. 5.7K, L, O, P). Decreased retrograde flow across the valve is evident during the later stages of systole and is present because the valve leaflets are not yet capable of completely sealing the atrium from the ventricle.

Figure 5.7. Valve dynamics and blood flow in 84 hpf embryos. (A-D) Sequence of confocal scans during one cardiac cycle. Outlines of each image are provided (E-F). Sequential images are superimposed upon each other to show cardiac cushion and leaflet motions (I-J) along with timestamps. The black arrows indicate the motion of the cushion and leaflet. (M-P) Binary representation of flow during one cardiac cycle. Vertical black line indicates the time point images (E-H) were recorded. See text for details of dynamics.

By 120 hpf, valve leaflets are fully functional and retrograde flow between the ventricle and atrium is no longer evident (Fig. 5.8M). At the onset of atrial filling, valve leaflets make contact at the atrial hinge while the free edges (ventricular portions) of the valves are still separated (Fig. 5.8I). Blood continues to enter the atrium, the chamber expands, and the contact point between the leaflets moves towards the free edge (Fig. 5.8J). At the later stages of atrial filling, the valve plane moves towards the ventricle (Fig. 5.8K). During diastole, the leaflets separate and allow blood flow from the atrium to ventricle (Fig. 5.8L, P). The ventricular volume increase, the atrial volume decreases, and the valve plane recoils towards the atrium (Fig. 5.8I, M). The leaflets are long enough to make contact throughout systole, preventing retrograde flow across the valve region.

Figure 5.8. 120 hpf valve dynamics and blood flow. (A-D) Sequence of confocal scans during one cardiac cycle. Outlines of each image are provided (E-F).

Sequential images are superimposed upon each other to show valve leaflet motions (I-J) along with timestamps. The black arrows indicate the motion of the leaflet. (M- P) Binary representation of flow during one cardiac cycle. Vertical black line

indicates the time point images (E-H) were recorded. See text for details of dynamics.

5.5 Frequency and Flow