1. 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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When there is high blood pressure, the sympathetic nervous system activity is inhibited, which causes the contractile force of the cardiac muscle to be increased. Moreover, due to the high pressure in blood vessels, the heat needs to pump blood harder to overcome resistance, which increases the afterload (Afterload, 2015); hence, the increase in stroke volume and cardiac output.
b. Massive bleeding
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During massive bleeding, the circulating blood volume is decreased, which causes the amount of blood that returns to the heart to decrease. This accounts for low preload and afterload, reduced stroke work, and reduced cardiac output (O’Keefe & Singh, 2022).
c. the drug dopamine
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The mechanism of action of the drug dopamine is it produces positive chronotropic and inotropic effects on the myocardium, which leads to an increase in stroke volume, heart rate, and cardiac contractility (Dopamine: Uses, Interactions, Mechanism of Action | DrugBank Online, n.d.). Intermediate infusion rates of dopamine increase electrical conductivity in the heart and stimulate myocardial contractility, which leads to an increase in cardiac output.
d. Running a sprint
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When running a sprint, or exercise generally, the activation of skeletal muscle and respiratory pumps are stimulated, which increases the venous return. The increase of venous return causes preload and contractility to increase as well, hence increasing stroke volume, and consequently, the cardiac output.
e. Hyperthyroidism with increased secretion of thyroid hormones
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Hyperthyroidism with increased secretion of thyroid hormones results in an increased cardiac output because the thyroid hormones raise the heart rate along with the cardiac contractility (Klein and Ojama, 2001). The increase in contractility results in a greater stroke volume since it reduces end-systolic volume; thus, increasing the cardiac output (Froelicher & Myers, 2007).
f. Massive myocardial infarction (cardiac tissue death)
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There is a reduced cardiac output in massive myocardial infarction because of the loss of viable myocardium, which causes systolic dysfunction, caused by loss of intrinsic inotropy, which means there is a decrease in contractility (Klabunde, 2015). Moreover, the afterload increases, which results in decreased stroke volume and cardiac output.
g. Hypothermia
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During hypothermia, there is a reflex adrenergic vasoconstriction of the skin and increased cardiac contractility (Yamada, 2021). The increased cardiac contractility causes the stroke volume to increase; hence, increasing the cardiac output.
h. Emotional distress
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Emotional distress stimulates the sympathetic nervous system activity which increases the heart rate. Moreover, norepinephrine and epinephrine enhance the heart’s pumping effectiveness, increasing the heart rate and contractility, which causes the stroke volume to increase; therefore, increasing cardiac output.
References:
Afterload. (2015, May 8). Learn the Heart. https://www.healio.com/cardiology/learn-the-heart/cardiology-review/topic-reviews/afterload
Dopamine: Uses, Interactions, Mechanism of Action | DrugBank Online. (n.d.). DrugBank. https://go.drugbank.com/drugs/DB00988
Froelicher, V. F., & Myers, J. (2007). The Physiologic Response to the Exercise Test. Science Direct. https://www.sciencedirect.com/science/article/pii/B978032303302250004XKlein I.,
Klabunde, R. E. (2015). Pathophysiology of Heart Failure. Cardiovascular Physiology Concepts. https://www.cvphysiology.com/Heart%20Failure/HF003
Marieb, E., & Hoehn, K. (2018). Human Anatomy & Physiology (11th ed.). Pearson
Ojamaa K. (2001). Thyroid hormone and the cardiovascular system. N. Engl. J. Med. 344 501–509. 10.1056/nejm200102153440707
O’Keefe, E., & Singh, P. (2022, September 26). Physiology, Cardiac Preload. National Library of Medicine - National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/books/NBK541109/
Tortora, G. J., & Derrickson, B. H. (2012). Principles of Anatomy and Physiology (13th ed.). Wiley.