Cardiac Cycle Heart Failure Notes

Semester 2

Case 4: The Downward Spiral

• What is the cardiac cycle?

[1] 1 single cardiac cycle = The period between the start of one heartbeat & the beginning of the next.

It includes alternating periods of contraction & relaxation. For any 1 chamber in the heart, the cardiac cycle can be divided into 2 phases: Systole & Diastole.

Systole = Chamber contracts & pushes blood into an adjacent chamber or into an arterial trunk.

Diastole = Chamber fills with blood & prepares for next cardiac cycle.

FLUIDS MOVE FROM HIGHER PRESSURE TO LOWER PRESSURE!

Valves between adjacent chambers help ensure that blood flows in the required direction. The correct pressure relationships are dependent on the careful timing of contractions. Pacemaking & conducting systems normally provide required spacing between atrial & ventricular systoles.

Phases of Cardiac Cycle

This is an example for a heart rate of 75/minute. When heart rate increases, all phases of the cardiac cycle are shortened. The greatest reduction occurs in the length of time spent in diastole.

Although pressures are lower in the right atrium & right ventricle, both sides of the heart contract at the same time, and they eject equal volumes of blood.

In more detail:

Atrial systole – As atria contract, atrial pressures push blood into ventricles through open right & left AV valves. At start of atrial systole, ventricles are already 70% full due to passive blood-flow during the end of the previous cardiac cycle. As atria contract, atrial pressures provide remaining 30% by pushing blood through open AV valves. At the end of atrial systole, each ventricle contains the maximum amount of blood that it will hold in this cardiac cycle. This quantity = END-DIASTOLIC VOLUME (EDV). EDV = 130ml in an adult who is standing at rest.

Ventricular systole – As atrial systole ends, ventricular systole begins. As ventricular pressure rises above pressure in the atria, AV valves shut. Ventricles are contracting isometrically (Tension produced doesn’t exceed resistance, so muscle length doesn’t change) but ventricular pressure isn’t yet high enough to force open the semi-lunar valves. This is the period of ISOVOLUMETRIC CONTRACTION. All heart valves are closed, volumes of ventricles remain constant & ventricular pressure rises. Once P_ventricles > P_{arterial trunks}, semi-lunar valves open & blood flows into the pulmonary & aortic trunks (beginning of VENTRICULAR EJECTION). Isotonic contraction (Muscle cells shorten & tension production remains relatively constant) occurs. After reaching a peak, ventricular pressures gradually near the end of ventricular systole. During ventricular ejection, each ventricle will eject 70-80ml of blood, the STROKE VOLUME. Stroke volume at rest is about 60% of the end-diastolic volume. This is the EJECTION FRACTION. This can vary in response to changing demands on the heart. As end of ventricular systole approaches, ventricular pressure falls rapidly. Blood in aorta & pulmonary trunk now starts to flow back toward the ventricles & this movement closes the semi-lunar valves. As backflow begins, pressure in aorta. When valves close, pressure begins to rise as elastic arterial walls recoil. (This small temporary rise produces a valley in the pressure tracing called a DICROTIC NOTCH. Amount of blood remaining in ventricle when semi-lunar valve closes = END-SYSTOLIC VOLUME. It’s about 50ml at rest: 40% of the end-diastolic volume.)

Ventricular diastole – Lasts for the 430ms remaining in the current cardiac cycle & continues through atrial systole in the next cycle. All heart valves are closed. Ventricular myocardium is relaxing. P_ventricular > P_atrial, so blood can’t flow into ventricles. This is the period of ISOVOLUMETRIC RELAXATION. Ventricular pressures rapidly over this period because elasticity of connective tissues of the heart & fibrous skeleton helps re-expand the ventricles toward their resting dimensions. When P_ventricular < P_atrial, AV valves are forced open. Blood now flows from the atria into the ventricles. Both atria & the ventricles are in diastole, but ventricular pressures continue to as ventricular chambers expand. Throughout this period, pressures in ventricles are so far below those in the major veins that blood pours through the relaxed atria & on through the open AV valves into the ventricles. (Passive mechanism: Primary method of ventricular filling).

The relatively minor contribution that atrial systole makes to ventricular volume explains why individuals can survive quite normally when their atria have been so severely damaged that they can no longer function. In contrast, damage to one or both ventricles can leave the heart unable to maintain adequate bloodflow through peripheral tissues & organs. A condition of heart failure then exists.

Heart Sounds

Auscultation is a simple & effective method of cardiac assessment. A stethoscope is used to listen for normal & abnormal heart sounds. There are 4 heart sounds – S₁, S₂, S₃, and S₄.

• S₁ = “Lubb” marks the sound of the start of ventricular contraction & is produced as the AV valves close. Lasts a little longer than S₂.

• S₂ = “Dupp” occurs at the beginning of ventricular filling when the semi-lunar valves close.

• S₃ & S₄ are usually very faint sounds & are seldom audible in healthy adults.

S₃ = Blood flowing into ventricles. S₄ = Atrial contraction.

Heart murmurs: If valve cusps are malformed or there are problems with the papillary muscles or chordate tendinae, the valves may not close properly. Regurgitation then occurs in ventricular systole. The surges, swirls, & eddies that accompany regurgitation create a rushing, gurgling sound known as a heart murmur. Minor heart murmurs are common & inconsequential.

• What is heart failure?

[2, 3] Heart failure is a complex syndrome that can result from any structural or functional cardiac disorder...

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