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
- In high blood pressure, there is an increase in afterload which forces the ventricles to work harder to eject blood. It also causes stroke volume to decrease so that more blood remains in the ventricles at the end of systole. With this, the cardiac output is reduced.
B. Massive bleeding
- The decrease in blood volume causes a decrease in central venous pressure and cardiac filling. This results in decreased cardiac output and decreased arterial pressure through the Frank-Starling mechanism. The body raises heart rate and contractility to make up for volume loss. The heart's response to hemorrhage is governed by increased sympathetic nervous system activation as well as decreased preload and afterload.
C. The drug dopamine
- Dopamine has positive chronotropic and inotropic effects on the myocardium, resulting in increased heart rate and cardiac contractility. The hemodynamic effect of dopamine is dose-dependent. Low doses are linked to renal and splanchnic vasculature dilation, which improves diuresis. Heart rate and cardiac contractility are enhanced at moderate doses. Meanwhile, high doses cause peripheral vasoconstriction, which increases afterload.
D. Running a sprint
- Your body may require three to four times your normal cardiac output while running because it causes your muscles to demand more oxygen. Your heart typically beats more quickly during exercise so that more blood can leave your body. The heart rate rises proportionally to the workload until it reaches near-maximal levels. Increased myocardial contractility and increased venous return to the heart are the main contributors to increased stroke volume in humans during exercise. With a rising heart rate, contractility rises. This allows greater ejection of blood at the end of systole and shortens systole, allowing more time for diastolic filling of the ventricles.
E. Hyperthyroidism
- Hyperthyroidism occurs when the thyroid gland produces an excessive amount of thyroid hormone. Thyroid hormones have a significant impact on the heart and cardiovascular system. Resting heart rate, myocardial contractility, and preload are all increased in hyperthyroidism. As a result, cardiac output increases. In addition to making your heart beat more quickly, hyperthyroidism can also lead to atrial fibrillation in which the upper chambers of the heart beat erratically.
F. Massive myocardial infarction (cardiac tissue death)
- Myocardial infarction reduces cardiac output or contractility, resulting in sympathetic nervous system activation. However, a serious complication of acute myocardial infarction which is the low-output syndrome, is largely caused by changes in ventricular preload and afterload. In many cases, the heart rate increases as well, but certain conditions and medications can cause it to remain constant or even slow down.
G. Hypothermia
- There is a reflex adrenergic vasoconstriction (noradrenalin) of the skin in hypothermia. Due to increased bradycardia, cardiac output falls below a core temperature of 34˚C. Hypothermia reduces heart rate while increasing ventricular contractile force.
H. Emotional distress
- Emotional distress causes an increase in heart rate, an increase in blood pressure, and the release of stress hormones such as epinephrine. Acute emotional stress can cause left ventricular contractile dysfunction, myocardial ischemia, or cardiac rhythm disturbances. All of this adds to the workload on the heart. During times of stress, cardiac output increases.
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