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 include PRELOAD, CONTRACTILITY and AFTERLOAD.
In the following conditions, indicate what factor/s influence the cardiac output: PRELOAD, CONTRACTILITY, AFTERLOAD, HEART RATE. Explain why the factor/s affects cardiac output in 1 to 2 sentences.
a. High blood pressure
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Narrower, less elastic arteries make it difficult for blood to properly circulate throughout the body. This increases the heart's workload. High blood pressure hence raises afterload and decreases cardiac output. The resistance of high blood pressure to systolic contractions increases afterload and end-systolic volume. If the end systolic volume rises, the stroke volume falls.
b. Massive bleeding
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Severe blood loss leads to a decrease in central venous pressure and heart filling as a result of a reduction in blood volume. Consequently, cardiac output, arterial pressure, and stroke volume are decreased. As the arterial pressure of the heart lowers, its preload reduces due to less blood, and the heart's contractility increases to compensate. The autonomic nervous system will increase heart rate in an effort to maintain cardiac output.
c. The drug dopamine
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The drug dopamine has positive chronotropic and inotropic effects on the myocardial, leading to an increase in heart rate, cardiac contractility, and blood pressure via vasoconstriction, which results in an increase in stroke volume.
d. Running a sprint
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The increase in blood supply to muscles during sprint running necessitates an increase in cardiac output. This increases venous return, end-diastolic volume, preload, and the effort of contraction necessary to eject the end-diastolic volume, resulting in an increase in stroke volume.
e. Hyperthyroidism with increased secretion of thyroid hormones
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Hyperthyroidism accompanied by increased thyroid hormone secretion raises the heart rate and stroke volume. These responses increase cardiac output, systolic blood pressure, blood circulation, cardiac contractility, and preload.
f. Massive myocardial infarction (cardiac tissue death)
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Massive myocardial infarction is caused by diminished or halted blood flow to a portion of the heart muscle. Loss of viable myocardium compromises global cardiac function, leading to decreased cardiac output, increased contractility, and elevated arterial pressure.
g. Hypothermia
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During a decrease in core body temperature (hypothermia), the skin undergoes a reflex adrenergic vasoconstriction (noradrenalin). During hypothermia, the increase in systemic arterial blood pressure attributes to the concurrent increase in cardiac output. The increased cardiac output results in a heightened heart rate.
h. Emotional distress
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Increased sympathetic nervous system activity occurs from emotional distress. An increase in SNS stimulation has an effect on cardiac function, resulting in an increase in heart rate, stroke volume, and cardiac output. Peripheral vasoconstriction also affects the blood vessels. Increase in cardiac output and peripheral vasoconstriction result in an increase in systolic blood pressure.
References:
Tortora, G. J., & Derrickson, B. (2014). Principles of anatomy & physiology. 14th edition. Danvers, MA, Wiley.
British Journal of Anaesthesia Education. (2019, January 17). Oxford Academic. Retrieved November 12, 2022, from https://academic.oup.com/bjaed/article/4/6/185/314696
CV Physiology | Hemorrhagic Shock. (n.d.). Cardiovascular Physiology Concepts. Retrieved November 12, 2022, from https://www.cvphysiology.com/Blood%20Pressure/BP031
Dopamine: Uses, Interactions, Mechanism of Action. (n.d.). DrugBank. Retrieved November 12, 2022, from https://go.drugbank.com/drugs/DB00988
Frishman, W. (n.d.). Thyroid hormone and cardiovascular disease. PubMed. Retrieved November 12, 2022, from https://pubmed.ncbi.nlm.nih.gov/9489964/
How High Blood Pressure Can Lead to Heart Failure. (2022, March 4). American Heart Association. Retrieved November 12, 2022, from https://www.heart.org/en/health-topics/high-blood-pressure/health-threats-from-high-blood-pressure/how-high-blood-pressure-can-lead-to-heart-failure