Chapter 44: Agents for Treating Heart Failure

Agents for Treating Heart Failure delves into the agents used for treating heart failure (HF), a complex syndrome characterized by the heart's reduced ability to pump blood effectively, leading to systemic congestion. HF often stems from conditions such as coronary artery disease, chronic hypertension, cardiomyopathy, or valvular disease, which impair the cardiac muscle's contractile unit, the sarcomere, specifically by disrupting the effective movement and utilization of calcium ions necessary for actin and myosin interaction. The body attempts to compensate for decreased cardiac output through mechanisms like sympathetic nervous system stimulation and activation of the renin–angiotensin–aldosterone system, which initially increase heart rate and blood volume but ultimately escalate the heart’s workload, causing muscle enlargement known as cardiomegaly. Clinical manifestations are distinct depending on the primary side of failure: left-sided HF results in pulmonary congestion, leading to rapid, shallow breathing (tachypnea), discomfort breathing (dyspnea), difficulty breathing when supine (orthopnea, described by the number of required pillows), and potentially blood-tinged sputum (hemoptysis) or life-threatening pulmonary edema. Conversely, right-sided HF causes systemic venous backup, presenting as peripheral edema in gravity-dependent areas, elevated jugular venous pressure (JVP), liver congestion, and increased nighttime urination (nocturia) due to fluid shifts. Initial pharmacological interventions aim to reduce cardiac workload using vasodilators (like ACE inhibitors and nitrates to decrease preload and afterload), diuretics, and specific beta-blockers. The chapter then details the focused cardiotonic agents (inotropic drugs) and the newer class of hyperpolarization-activated cyclic nucleotide–gated (HCN) channel blockers. Digoxin, a classic cardiac glycoside, works by increasing intracellular calcium, resulting in a positive inotropic effect (stronger contraction) and a negative chronotropic effect (slower heart rate), thereby improving cardiac output and renal perfusion. Caution is stressed due to its narrow safety margin and risk of toxicity, especially in older adults and children, requiring careful dose checking and monitoring for symptoms like vision changes or arrhythmias; the antidote is digoxin immune Fab. Phosphodiesterase inhibitors, such as milrinone, are reserved for short-term, severe HF unresponsive to other treatments because they significantly increase calcium via increased cyclic adenosine monophosphate (cAMP) but are associated with potentially fatal ventricular arrhythmias. Finally, ivabradine, the first HCN blocker, functions by slowing the heart’s pacemaker, the sinus node, to reduce heart rate and improve ventricular filling time without affecting muscle contractility or causing the systemic effects of beta-blockers; this agent is indicated for stable, chronic HF in patients meeting specific criteria, and major adverse effects include bradycardia and visual disturbances known as luminous phenomena.