CHAPTER II: CARDIAC MECHANICS

Asst. Prof. Dr. Emre HamurtekinEMU Faculty of Pharmacy

1. CARDIAC CYCLE2. CARDIAC OUTPUT 3. DETERMINANTS of CARDIAC OUTPUT4. CARDIAC WORK

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• Cardiac cycle can be divided into seven phases:1.Atrial systole2.Isovolumic ventricular contraction3.Rapid ventricular ejection4.Reduced ventricular ejection5.Isovolumic ventricular relaxation6.Rapid ventricular filling7.Reduced ventricular filling

• Atrial systole is initiated by …………………... • 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 isovolumic ventricular contractionventricular 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.

atrial excitation

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• In Rapid Ventricular Ejection 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 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 Isovolumic Ventricular Relaxation 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 Rapid Ventricular Filling phase.

• In Reduced Ventricular Filling (Reduced Ventricular Filling (diastasisdiastasis) ) 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 EDV - ESV = = SV (stroke volume).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 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 CO (L/min) = HR x x SVSV• HR is established on the SA node and is controlled by ANS.• SV is dependent onSV is dependent on,

• LV preload• LV afterload• Contractility

• PreloadPreload: Muscle length before contraction begins. • Preload is related with the volume of blood entering the

chamber (EDVEDV)• AfterloadAfterload: The load against which a myocyte must shorten.• The principal component of afterload is arterial pressure.• ContractilityContractility: 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

(-) inotropic agents(-) inotropic agents

-β- blockers-Ca channel blockers

Contractility equates with intracellular free CaCa concentration

SNS norepinephrine ββ-1 receptors -1 receptors cAMPcAMP PKA activationPKA activation

PKA activationPKA activation L - type Ca channelsL - type Ca channels, , Ca release channelsCa release channels, SERCA , SERCA

• Heart performs two kinds of work:I. Internal workII. External work

• Internal workInternal work:• Expended in «isovolumic contraction»• The force necessary to open the aortic and pulmonary

valves• Accounts for ˃ 90% of total cardiac workload.

• External work External work (pressure-volume work)(pressure-volume work):• Expended in transferring blood to the arterial system

against a resistance.• Accounts for ˂ 10% of total cardiac workload.