Preload, Afterload, Contractility

Two patients can both have dyspnea and poor oxygenation yet require opposite hemodynamic interventions. A patient with acute pulmonary edema and an adequate blood pressure may improve when venous return and ejection resistance are reduced. A patient with a possible right ventricular infarction, hypotension, and clear lungs may deteriorate if preload is removed because the failing ventricle depends on filling to generate output. The bedside question is not simply whether the heart is failing. Ask which part of the cardiac output equation is limiting forward flow: insufficient ventricular filling, excessive resistance to ejection, or inadequate myocardial force. Cardiac output equals heart rate multiplied by stroke volume, and stroke volume reflects the interaction of preload, afterload, and contractility. Assess blood pressure, perfusion, congestion, and likely ventricular dependence before choosing which variable to change.