isovolumic ventricular contraction

CHAPTER II: CARDIAC MECHANICS Asst. Prof. Dr. Emre Hamurtekin EMU Faculty of Pharmacy

1. 2. 3. 4.

CARDIAC CYCLE CARDIAC OUTPUT DETERMINANTS of CARDIAC OUTPUT CARDIAC WORK

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• Cardiac cycle can be divided into seven phases: 1. Atrial systole 2. Isovolumic ventricular contraction 3. Rapid ventricular ejection 4. Reduced ventricular ejection 5. Isovolumic ventricular relaxation 6. Rapid ventricular filling 7. Reduced ventricular filling

• Atrial systole is initiated by …………………... atrial excitation • Atrial systole follows the crest of P wave on the ECG. • Atrial contraction forces a small additional blood into the venticular chamber (atrial kick). • Ventricular systole begins with isovolumic ventricular contraction. • In isovolumic ventricular contraction, when intraventricular pressure rises, mitral valve closes. • In isovolumic ventricular contraction, aortic valve is still held closed by higher aortic pressure.

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• In Rapid Ventricular Ejection phase, aortic valve finally opens and blood exits the ventricle. • In this phase, atrium relaxes and the blood starts to fill the atrium. • In Reduced Ventricular Ejection phase, ejection velocity decreases (reduced ejection). • At the end of this phase, aortic valve is finally closed. • Once the aortic valve is closed, Isovolumic Ventricular Relaxation period starts. • In this period, LV volume is lowest.

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• Once intraventricular pressure drops below atrial pressure, mitral valve opens. • Blood in the atrium starts to move into the ventricle in Rapid Ventricular Filling phase. • In Reduced Ventricular Filling (diastasis) phase, atrium and ventricle are both fully relaxed. • Arterial pressure continues to fall as blood flows into capillary beds. • This phase typically disappears when HR increases.

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LV does not empty completely during systole. ESV is around 50 ml. EDV - ESV = SV (stroke volume). SV is the amount of blood transferred from LV to the arterial system during systole. In healty person SV should be > 60 ml. EF (ejection fraction) = SV \ EDV (normally about 55% - 75%. EF is an important measurement of cardiac efficiency. EF is used clinically to assess cardiac status in patients with heart failure.

• CO (L/min) = HR x SV • HR is established on the SA node and is controlled by ANS. • SV is dependent on, • LV preload • LV afterload • Contractility

• • • • • •

Preload: Muscle length before contraction begins. Preload is related with the volume of blood entering the chamber (EDV) Afterload: The load against which a myocyte must shorten. The principal component of afterload is arterial pressure. Contractility: measure of a muscle’s ability to shorten against a afterload. Contractility equates with the cytoplasmic free Ca concentration.

Ability of a muscle cell to develop force (contractility)

(+) inotropic agents

(-) inotropic agents

- Epinephrine - Norepineprine - Digoxin

- β- blockers - Ca channel blockers

Contractility equates with intracellular free Ca concentration

SNS

norepinephrine

PKA activation

β-1 receptors

cAMP

PKA activation

L - type Ca channels, Ca release channels, SERCA

• Heart performs two kinds of work: I. II.

Internal work External work

• Internal work: • Expended in «isovolumic contraction» • The force necessary to open the aortic and pulmonary valves • Accounts for ˃ 90% of total cardiac workload.

• External work (pressure-volume work): • Expended in transferring blood to the arterial system against a resistance. • Accounts for ˂ 10% of total cardiac workload.