心不全 I: はじめに

Heart failure is a clinical syndrome where structural or functional cardiac disorders impair the heart's ability to pump sufficient blood to meet the body's metabolic needs. It typically results from myocardial damage that disrupts cardiac output, which depends on preload, afterload, contractility, and heart rate. Changes in these factors impair ventricular function, leading to reduced tissue perfusion and impaired gas exchange.

Cardiac output is determined by stroke volume multiplied by heart rate, with stroke volume influenced by preload, afterload, myocardial contractility, and heart rate. Preload affects ventricular filling, afterload increases cardiac workload, and contractility determines the force of myocardial contraction. Alterations in any of these factors can impair ventricular function and lead to heart failure.

Common causes include coronary artery disease, where ischemia and myocardial infarction damage heart cells and reduce contractility. Hypertension increases afterload, causing myocardial hypertrophy and impaired filling. Cardiomyopathies, particularly dilated cardiomyopathy, cause myocardial necrosis and fibrosis. Valvular heart disease and systemic conditions like chronic kidney disease and cardiac arrhythmias also contribute significantly.

Hypertension increases afterload, which elevates cardiac workload and causes myocardial hypertrophy as the heart compensates. Over time, this hypertrophy impairs the heart's ability to fill properly and reduces its pumping efficiency. Eventually, these changes lead to heart failure as the myocardium can no longer maintain adequate cardiac output.

Risk factors include age over 60, cigarette smoking, obesity, poorly managed diabetes, hypothyroidism, anemia, and pulmonary embolism. Men over 60 are at higher risk. Additionally, thyrotoxicosis, Paget's disease of bone, hypoxia, acidosis, and electrolyte imbalances can exacerbate myocardial dysfunction and increase heart failure risk.

Cardiomyopathies, particularly dilated cardiomyopathy, cause myocardial necrosis and fibrosis that progressively damage heart tissue. This tissue damage reduces the heart's contractile function and its ability to pump blood effectively. The progressive nature of these changes leads to worsening cardiac output and eventual heart failure.

Systemic conditions like chronic kidney disease cause volume overload that increases cardiac workload. Cardiac arrhythmias such as atrial fibrillation impair myocardial contraction and reduce overall cardiac efficiency. Additionally, hypoxia, acidosis, and electrolyte imbalances exacerbate myocardial dysfunction, all contributing to heart failure development.