Final answer:
Cardiac output, calculated by stroke volume multiplied by heart rate, is the key parameter for assessing heart failure. Stroke volume is influenced by preload, contractility, and afterload, while cardiac output is affected by factors that regulate heart rate and contractility.
Step-by-step explanation:
The cardiac parameter commonly used in assessing a patient with heart failure is cardiac output. Cardiac output is the total volume of blood pumped by the ventricles per minute and is a critical measure of heart performance. It can be calculated by multiplying stroke volume (SV), the amount of blood pumped by each ventricle with each heartbeat, by heart rate (HR), the number of heartbeats per minute.
Positive factors affecting heart contractility, and thereby potentially increasing cardiac output, include sympathetic stimulation, catecholamines like epinephrine and norepinephrine, thyroid hormones, and increased calcium ion levels. Negative factors that can decrease contractility include parasympathetic stimulation, abnormal potassium levels, decreased calcium levels, anoxia, and acidosis.
Stroke volume is affected by preload, contractility, and afterload. Preload is the stretch of the ventricles at the end of diastole, contractility is the force of the contraction, and afterload is the resistance the left ventricle must overcome to circulate blood. Variations in blood flow and pressure evoke cardiac responses to maintain adequate circulation and blood pressure, such as changes in stroke volume and heart rate to adjust cardiac output as necessary.