Adequate cardiac output is necessary in order to supply oxygen and other nutrients to all body tissues. Cardiac output is influenced by stroke volume and heart rate. Factors affecting the stroke volume includes PRELOAD, CONTRACTILITY and AFTERLOAD.
In the following conditions, indicate what factor/s influence the cardiac output: PRELOAD, CONTRACTILITY, AFTERLOAD, HEART RATE. Explain the why the factor/s affects cardiac output in 1 to 2 sentences.
a. High blood pressure - It is more difficult for blood to travel efficiently throughout the body when arteries are narrow and less elastic. This causes the heart to work harder, thereby increasing preload, contractility, afterload, and heart rate.
b. Massive bleeding - The reduction in blood volume during blood loss causes a fall in central venous pressure and cardiac filling, which leads to decreased overall cardiac output and arterial pressure. The body handles this situation through increasing heart rate and contractility and decreasing the preload and afterload.
c. Drug Dopamine - At moderate doses, the positive chronotropic and inotropic effects that dopmaine brings on the myocardium results to an increase in heart rate and contractility. Higher doses, however, exacerbate heart failure by increasing afterload and producing additional workload on the heart.
d. Running a sprint - Since the muscles need more oxygen during exercise, the body needs triple to quadruple the value of the heart’s cardiac output, so the heart typically beats faster (increased heart rate) and enhances contractility along with it. On the contrary, preload and afterload decrease as exercise intensities increase.
e. Hyperthryoidism with increased secretion of thyroid hormones - Hyperthyroidism causes hemodynamic changes, such as increased preload and contractility and decreased systemic vascular resistance that causes increased cardiac output.
f. Massive myocardial infarction (cardiac tissue death) - Losing viable myocardium impairs global cardiac function, which leads to reduced cardiac output and contractility.
It decreases heart rate but increases contractility.
g. Hypothermia - In severe hypothermia, resistance in the coronary vascular bed is reduced and flow continues despite very low aortic pressure. This results in increased cardiac output despite reduced heart rate.
h. Emotional distress - When experiencing stress, the body releases cortisol and adrenaline, which cause the heart to beat more rapidly and the blood vessels to narrow to help push blood to the center of the body. With this, cardiac output and contractility are increased.