How our Heart Failure Occur
How
our Heart Failure Occur
Heart failure occurs when the heart is unable to put out an amount of blood that is adequate for the needs of the tissues. It can be acute and associated with sudden death, or chronic. The failure may involve primarily the right ventricle (cor pulmonale), but much more commonly it involves the larger, thicker left ventricle or both ventricles. Heart failure may also be systolic or diastolic.
In systolic failure, stroke
volume is reduced because ventricular contraction is weak. This causes an
increase in the end-systolic ventricular volume, so that the ejection raction
falls from 65% to as low as 20%. The initial response to failure is
activation of the genes that cause cardiac myocytes to hypertrophy, and
thickening of the ventricular wall (cardiac remodeling). The incomplete
filling of the arterial system leads to increased discharge of the sympathetic nervous
system and increased secretion of renin and aldosterone, so Na+ and water are
retained. These responses are initially compensatory, but eventually the
failure worsens and the ventricles dilate. output may be elevated in absolute
terms but still be inadequate to meet the needs of the tissues (high-output
failure).
Treatment of congestive heart failure is aimed at improving cardiac
contractility, treating the symptoms, and decreasing the load on the heart.
Currently, the most effective treatment in general use is inhibition of the
production of angiotensin II with angiotensin-converting enzyme (ACE)
inhibitors. Blockade of the effects of angiotensin II on AT1 receptors
with nonpeptide antagonists is also of value. Blocking the production of
angiotensin II or its effects also reduces the circulating aldosterone level
and decreases blood pressure, reducing the afterload against which the heart
pumps. The effects of aldosterone can be further reduced by administering
aldosterone receptor blockers
.
.
Reducing venous tone with nitrates or
hydralazine increases venous capacity so that the amount of blood returned to
the heart is reduced, lowering the preload. Diuretics reduce the fluid
overload. Drugs that block β-adrenergic receptors have been shown to decrease
mortality and morbidity. Digitalis derivatives such as digoxin have classically
been used to treat congestive heart failure because of their ability to
increase intracellular Ca2+ and hence exert a positive inotropic
effect, but they are now used in a secondary role to treat systolic dysfunction
and slow the ventricular rate in patients with atrial fibrillation.
Events of the cardiac cycle at a heart rate of 75
beats/min.
The phases of the cardiac cycle
identified by the numbers at the bottom are as follows:
1, atrial systole;
2, isovolumetric ventricular contraction;
3, ventricular ejection;
4, isovolumetric ventricular
relaxation;
5, ventricular filling. Note that
late in systole, aortic pressure actually exceeds left ventricular pressure.
However, the momentum of the blood keeps it flowing out of the ventricle for a
short period. The pressure relationships in the right ventricle and pulmonary
artery are similar. Atr. syst., atrial systole; Ventric. syst., ventricular
systole.
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