20.2
Insuficiência Cardíaca II: Fisiopatologia
Insuficiência Cardíaca Sistólica e Mecanismos Compensatórios
A insuficiência cardíaca sistólica (também denominada IC com fração de ejeção reduzida —…
A fisiopatologia da insuficiência cardíaca sistólica começa com o ventrículo esquerdo bombeando um volume reduzido de sangue.
Os barorreceptores no arco aórtico e nos seios carotídeos detectam essa diminuição, fazendo com que o sistema nervoso simpático libere epinefrina e norepinefrina, que aumentam a frequência cardíaca e a contratilidade.
A estimulação simpática também causa vasoconstrição na pele, trato gastrointestinal e rins.
A perfusão renal reduzida devido ao baixo débito cardíaco e à ativação simpática desencadeia a liberação de renina.
A renina converte o angiotensinogênio em angiotensina I. A angiotensina I é então convertida em angiotensina II pela enzima conversora de angiotensina nos pulmões.
A angiotensina II aumenta a pressão arterial e a pós-carga e estimula a liberação de aldosterona, causando retenção de sódio e líquidos.
Além disso, os peptídeos natriuréticos, liberados das câmaras cardíacas superdistendidas, promovem vasodilatação e diurese, mas geralmente são insuficientes.
Eventualmente, o aumento da carga de trabalho no coração diminui a contratilidade miocárdica, levando à dilatação ventricular, hipertrofia, remodelação e morte precoce das células miocárdicas, resultando em insuficiência cardíaca diastólica.
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Q1: What triggers the sympathetic nervous system response in systolic heart failure?
Baroreceptors in the aortic arch and carotid sinuses detect reduced blood pressure from decreased cardiac output. This triggers the sympathetic nervous system to release epinephrine and norepinephrine, which increase heart rate and contractility to support the weakened myocardium and restore blood pressure.
Q2: How does reduced renal perfusion activate the renin-angiotensin-aldosterone system?
Low cardiac output and sympathetic activation cause vasoconstriction in the kidneys, reducing renal perfusion. This triggers renin release, which converts angiotensinogen to angiotensin I. Angiotensin-converting enzyme in the lungs then converts angiotensin I to angiotensin II, a potent vasoconstrictor that raises blood pressure and afterload.
Q3: What role does aldosterone play in fluid overload during heart failure?
Angiotensin II stimulates the adrenal cortex to release aldosterone, which causes renal tubules to retain sodium and fluid. This increases blood volume and preload, contributing to the fluid overload characteristic of heart failure and further burdening the weakened heart muscle.
Q4: Why are natriuretic peptides insufficient to counteract heart failure progression?
Natriuretic peptides like BNP and ANP are released from distended cardiac chambers to promote vasodilation and diuresis. However, their effects are typically insufficient to counteract the adverse effects of prolonged sympathetic activation, renin-angiotensin-aldosterone system activation, and other neurohormonal mechanisms driving heart failure.
Q5: How does increased cardiac workload lead to ventricular dilation and hypertrophy?
As myocardial contractility decreases, end-diastolic blood volume increases, stretching myocardial fibers and causing ventricular dilation. The heart responds to increased workload by thickening its muscle, a process called ventricular hypertrophy. These structural changes constitute ventricular remodeling, which further impairs cardiac function.
Q6: What is the vicious cycle of heart failure and how does it progress?
Low cardiac output triggers compensatory mechanisms that increase the heart's workload, worsening the condition. Neurohormones like angiotensin II promote myocardial hypertrophy and fibrosis, causing cell death and loss of contractility. This cycle eventually leads to diastolic heart failure, where a stiff ventricle resists filling and decreases cardiac output.
Q7: What adverse effects result from prolonged sympathetic nervous system activation?
Prolonged sympathetic activation causes vasoconstriction in the skin, gastrointestinal tract, and kidneys, reducing perfusion to vital organs. This triggers renin release and activates the renin-angiotensin-aldosterone system, leading to increased blood pressure, fluid retention, and elevated preload and afterload that further stress the failing heart.