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
Hypertension increases the aortic pressure which is countered by increasing left ventricular pressure; that builds to overcome the elevated arterial blood pressure and blood is ejected from the heart. As afterload increases, it makes ejection more difficult and decreases stroke volume and cardiac output.
b. Massive bleeding
Hemorrhage is a stress on the circulatory system that lowers the perfusion pressure for tissue blood flow while simultaneously lowering the heart's workload and blood pressure. Both an increase in the heart's sympathetic nervous system activation and a decrease in the heart's preload and afterload control the heart's response to bleeding.
c. The drug dopamine
Dopamine is used to treat hypotension and low cardiac output, wherein it enhances the cardiac contractility and heart rate. However, high doses increase afterload due to peripheral vasoconstriction.
d. Running a sprint
When running a sprint, the muscles need more oxygen and there is an increase in heart rate to allow more blood to flow to the rest of the body, which also increases the stroke volume. The major causes of increased stroke volume during exercise are increased myocardial contractility and increased venous return to the heart.
e. Hyperthyroidism with increased secretion of thyroid hormones
Along with its cellular effects, hyperthyroidism also affects the heart's hemodynamics, increasing preload and contractility; while decreasing systemic vascular resistance raises cardiac output.
f. Massive myocardial infarction (cardiac tissue death)
The acutely failing ventricle—myocardial infarction—is distended by accumulating upstream blood volume that recruits preload reserve, restoring stroke volume at the expense of increased ventricular radius (hence afterload) and left atrial pressure.
g. Hypothermia
Hypothermia is the lowering of body temperature below 35° C, which results in an associated increase in ventricular contractile force. Hypothermia also caused a significant increase in stroke volume and a significant decrease in heart rate with no change in cardiac output.
h. Emotional distress
During emotional distress, the body releases adrenaline, a hormone that temporarily causes the heart rate to speed up and the blood pressure to rise, which will then have an increase in contractility and cardiac output.
REFERENCES:
BENDER HW, AUSTEN WG, TSUNEKAWA T, EBERT PA. Effects of Hypothermia on Myocardial Contractile Force. Arch Surg. 1963;87(3):464–467. doi:10.1001/archsurg.1963.01310150100023
Cardiac Output. (n.d.). MyHealth Alberta. https://myhealth.alberta.ca/Health/Pages/conditions.aspx?hwid=tx4080abc
Ertek, S., & Cicero, A. F. (2013). Hyperthyroidism and cardiovascular complications: a narrative review on the basis of pathophysiology. Archives of medical science : AMS, 9(5), 944–952. https://doi.org/10.5114/aoms.2013.38685
How Does Everyday Stress Impact Your Heart? | UPMC. (2014, November 18). UPMC HealthBeat. https://share.upmc.com/2014/11/how-does-stress-impact-heart/
McDonough KH, Giaimo M, Quinn M, Miller H. Intrinsic myocardial function in hemorrhagic shock. Shock. 1999 Mar;11(3):205-10. doi: 10.1097/00024382-199903000-00009. PMID: 10188774.
Reddi, B., Shanmugam, N., & Fletcher, N. (2017, May 1). Heart failure—pathophysiology and inpatient management. BJA Education, 17(5), 151-160. https://doi.org/10.1093/bjaed/mkw067
Sovari, A. A., Kocheril, A. G., & Baas, A. S. (2020, July 23). Cardiogenic Pulmonary Edema Medication. Medscape. https://emedicine.medscape.com/article/157452-medication#4