cardiac physiology in brief

1. CARDIAC PHYSIOLOGY

2. • NO conflict of interest

3. DEFINITIONS:
• Cardiac output (Q) is the volume of blood being pumped by the
heart, in particular by a ventricle in a minute.
• It reflects the state of entire circulatory system.
• Q= HR * SV
• Determinants are ➢ heart rate ➢ Contractility ➢ Preload ➢
Afterload

4. • SV: volume of blood pumped by a ventricle per beat
SV= end diastolic volume (EDV) minus end systolic volume
(ESV); SV = EDV - ESV
• EDV = end diastolic volume
amount of blood in a ventricle at end of diastole
• ESV = end systolic volume
amount of blood remaining in a ventricle after contraction
• Ejection Fraction - % of EDV that is pumped by the
ventricle; important clinical parameter
Ejection fraction should be about 55-60% or higher

5. • Cardiac Output is the product of
Stroke Volume(SV) and Heart Rate (HR).
• Stroke volume is determined by three factors:
preload, afterload and contractility

6. Normal Cardiac Output
• Normal resting cardiac output:
Stroke volume of 70 ml
Heart rate of 72 beats/minute
Cardiac output ~ 5 litres/minute
• During exercise, cardiac output may
increase to > 20 liters/minutes

7. What are the Main Determinants of
Cardiac Performance?
• Preload
• Afterload
• Contractility
• Heart Rate
• Synergy of Contraction

8. Preload
• Preload is the initial fibre length.
• Preload is the load on myocardial fibers just prior to contraction.
• The force which fills the heart
• The extent of filling of the heart
• Different definitions exist
End-diastolic pressure
End-diastolic volume
Wall stress in diastole

11. Contractility
• The ability of the heart to contract with a given force and rate
• Represented by dP/dV (or elastance, E)
• Independent of afterload and preload
• Determined by conditions with in the myocyte
Degree of binding between actin and myosin
Calcium is critical

12. Effects of Autonomic Activity on Contractility:
• Sympathetic stimulation
– Release norepinephrine from symp. postganglionic fiber
– Have positive ionotropic effect
– Ventricles contract more forcefully, increasing SV, increasing
ejection fraction and decreasing ESV
Parasympathetic stimulation via Vagus Nerve -CNX
– Releases ACh
– Has a negative inotropic effect
Hyperpolarization and inhibition
– Force of contractions is reduced, ejection fraction
decreased

15. Pressure volume loop
• Pressure–volume loop of the left ventricle.
• During diastole, the ventricle fills and pressure increases from d to a.
• Pressure then rises sharply from a to b during isovolumetric
contraction and from b to c during ventricular ejection.
• At c, the aortic valves close and pressure falls during isovolumetric
relaxation from c back to d.