Heart as a muscle
DR. AMIR SAMADIAN M.D.
DEPARTMENT OF PHYSIOLOGY
SPRING 2020

Clinical Case



Q1



Q2



Q3







Similarities of myocardial and skeletal muscle contraction



myocardial and skeletal muscle contraction



Removal of calcium in cardiomyocytes



Cardiac output (as a measure of cardiac performance)



SYSTOLIC PERFORMANCE OF THE VENTRICLE
 Systolic performance actually means the overall force generated by the ventricular muscle during
systole.
 The heart does 2 things in systole: pressurizes and ejects blood. An important factor influencing
this systolic performance is the number of cross-bridges cycling during contraction.
 The greater the number of cross-bridges cycling à the greater the force of contraction.
 Systolic performance is determined by 3 independent variables:
      - Preload
      - Contractility
      - Afterload

PRELOAD
ØPreload is the load on the muscle in the relaxed state (before it contracts). More specifically,
it is the load or prestretch on ventricular muscle at the end of diastole.

 Applying preload to muscle does 2 things  ;
1.Stretches the muscle (sarcomere) à greater the Preload à greater the stretch of the sarcomere
2.Generates passive tension in the muscle (muscle is elastic à resists the stretch like a rubber
band !à resistant is measured as passive tension)

 The greater the Preload à the greater the stretch or length of sarcomere à the greater the passive
tension àmore cross-bridge cycles to be formed à GREATER FORCE OF CONTRACTION! à increase in SV

FRANK-STARLING MECHANISM



Lenght-tension relationship



What determines the Preload ?



How to measure preload?
 Preload on ventricular muscle is not measured directly; rather, indices are utilized.
      - Left ventricular end-diastolic volume (LVEDV)
      - Left ventricular end-diastolic pressure (LVEDP)
      - Central venous pressure (CVP)
      - Pulmonary capillary wedge pressure (PCWP)
      - Right atrial pressure (RAP)

AFTERLOAD
 Afterload is defined as the “load” that the heart must eject blood against.
 Probably, the best “marker” of afterload is systemic vascular resistance (SVR), also called total
peripheral resistance (TPR).
  However, TPR is not routinely calculated clinically and thus arterial pressure (diastolic, mean,
or systolic) is often used as the index of afterload.

Afterload



CONTRACTILITY (ionotropism)
1.Intrinsic ability of cardiac muscle to develop force at a given muscle length (inotropism).
2.Is related to the intracellular Ca2+ concentration.
3.Can be estimated by the ejection fraction (stroke volume/end-diastolic volume), which is normally
0.55 (55%).
4.
4.Increased dp/dt (change in pressure vs. change in time) = rate of pressure development during
isovolumetric contraction. Contractility affects the rate at which the ventricular muscle develops
active tension, which is expressed as pressure in the ventricle during isovolumetric contraction.

What changes the Contractility ?



Modulation of contractility SNS



Effect of increasing contractility



Heterometric and homeometric autoregulation of contractility



IMPORTANT!



Why are we learning all of these!



Pharmacological approach



Cardiac Output as the measure of performance (courtesy of Prof. Marie Novakova)



Cardiac Reserve (courtesy of Prof. Marie Novakova)



Cardiac Reserve



CARDIAC OXYGEN (O2) CONSUMPTION
 Is directly related to the amount of tension developed by the ventricles.
 Increased by:
      - Increased afterload (increased aortic pressure).
      - Increased size of the heart (Laplace's law states that tension is proportional to the
radius of …….a sphere).
      - Increased contractility.
      - Increased heart rate.

MEASUREMENT OF CARDIAC OUTPUT BY THE FICK PRINCIPLE
 The Fick principle for measuring cardiac output is expressed by the following equation:


 The equation is solved as follows:
      1. O2 consumption for the whole body is measured.
      2. Pulmonary vein [O2] is measured in systemic arterial blood.
      3. Pulmonary artery [O2] is measured in systemic mixed venous blood.




3. CARDIAC CYCLE.
mage result for systole and diastole


Cardiac cycle
 Made of 2 phases :
 1) SYSTOLE
 A. Cardiac contraction(isovolumic contration)
 B. Ejection of the blood out of heart (systolic ejection)
 2) DIASTOLE
 A. Cardiac Relaxation (isovolumic relaxation)
 B. Filling of the heart with blood (Ventricular filling)

Intracardiac pressures



LV Pressure-Volume loop



AUSCULTATION OF HEART



phonocardiography



Physiological heart sounds



Question ?



murmurs






4. POLYGRAPHY






THANKS FOR YOUR ATTENTION!