2. Objectives
• To discuss cardiac cycle
• To discuss electrical activity of heart
• To discuss arterial waveform and jvp wave
• To discuss Cardiac output
• To discuss blood pressure regulation
21. Automaticity Of The Heart
• It is the ability of the heart to initiate its beat
continuously and regularly without any external
stimulus.
• Purely Myogenic, not dependent on nerve
supply
• Due to specialized excitatory and conductive
tissue in heart
• It has ability to generate impulse because of
intrinsic ability of self excitation.
26. Physiology of coronary circulation
• Right coronary has greater flow in 50%
individuals, left has greater flow in 20% and
both has equal flow in 30%.
• Resting coronary blood flow: 225- 250ml/ min,
i.e. ~75 ml/100gm/ min
▫ 4- 5% of cardiac output
27. • Resting myocardial oxygen consumption: 8-
10ml/ 100gm/ min, i.e. ~ 10% of total body
oxygen consumption
The large coronary arteries lie predominantly on
epicardial surface of heart- offer little resistance to
flow
28. • Coronary arterioles- ramify throughout
myocardium- impose resistance to flow, regulate
blood distribution to myocardium
• Arthrosclerosis characterized as coronary artery
disease involves epicardial coronary arteries and
not the coronary arterioles.
29. • At rest heart extracts 70- 80% of the oxygen
from blood, resulting in coronary venous oxygen
saturation ~30%.
• So oxygen supply can be increased significantly
only increasing blood flow
36. • Pressure difference between aorta and RV
greater during systole, so flow not appreciably
reduced during systole in RV
• Blood flow to the left ventricle occurs
predominantly during diastole when cardiac
muscles relax
▫ Diastole is shorter when heart rate is high, so left
ventricular coronary flow is reduced during
tachycardia
▫ Sub-endocardial portion of LV most prone to
ischemic damage- most common site of
myocardial infarction
39. • Good auto-regulation between 60 and 200
mmHg perfusion pressure helps to maintain
normal coronary blood flow whenever coronary
perfusion pressure changes due to changes in
aortic pressure.
40. • Tone of the small intra-myocardial arterioles regulate
diastolic vascular resistance, allowing the matching of
oxygen supply with metabolic demand over a wide range
of perfusion pressures.
• The difference between auto-regulated, baseline flow,
and blood flow available under conditions of maximal
vasodilation is termed coronary vascular reserve,
▫ normally 3 to 5 times higher than basal flow.
• As epicardial stenosis becomes more pronounced,
progressive vasodilation of these resistance vessels
▫ allows preservation of basal flow, but at the cost of reduced
reserve.
41. Chemical factors
• Balance between local metabolic needs of
myocardium (esp O2) and magnitude of
coronary blood flow
▫ Present even in denervated heart
▫ Reflects local release of vasodilator substances
▫ Most potent- adenosine. Others- prostacycline,
NO
42. Neural factors
• Coronary arterioles contain
▫ α receptors: vasoconstriction, mainly in epicardial
arteries
▫ β receptors: vasodilation, mainly in intramuscular
arteries
(Activity in noradrenergic nerves cause vasodilation
secondary to metabolic changes, else it causes
vasoconstriction)
▫ H1 receptors: mediate vasoconstriction
▫ H2 receptors: mediate vaso-dilation
• Parasympathetic nerve fibres sparse, so little direct
effect on flow, which is vaso-dilatory
43. Collateralization
• Progressive ischemic coronary artery disease
results in the growth of new vessels (termed
angiogenesis) and collateralization within the
myocardium.
• Increases myocardial blood supply by increasing
the number of parallel vessels, thereby reducing
vascular resistance within the myocardium.
44. Blood Pressure
• Blood pressure is the product of cardiac output
and systemic vascular resistance
• Blood pressure is maintained in our body by
different mechanism
• Short term
• Intermediate term
• Long term
52. Contractility
• It is the intrinsic ability of the heart.
• Also called “Ionotropism”.
(Other character of heart muscle Dromotropism,
Chronotropism and Lusiotropism)
Related to intracellular calcium( actin, myosin,
tropomyosin, troponin c)
53. CONTD
• There are many types of indices of contractility
1) Isovolumetric contraction phase-
dp/dtContractilityintial length of cardiac muscle
2) Ejection Phase indice:
EF= EDV-ESV/ESV
3) Load dependent indice: time varying indice,
ration of change in pressure over volume which varies
in different phase of cycle.
***The auto regulatory range is not usually used practically
Activation of sympathetic nerves innervating the coronary vasculature causes only transient vasoconstriction mediated by α1-adrenoceptors. This brief (and small) vasoconstrictor response is followed by vasodilation caused by enhanced production of vasodilator metabolites (active hyperemia) due to increased mechanical and metabolic activity of the heart resulting from β1-adrenoceptor activation of the myocardium. Therefore, sympathetic activation to the heart results in coronary vasodilation and increased coronary flow due to increased metabolic activity (increased heart rate, contractility) despite direct vasoconstrictor effects of sympathetic activation on the coronaries. This is termed "functional sympatholysis."
