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 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
When the pressure in the left ventricle rises above the pressure in the aorta and the right ventricle pressure rises over the pressure in the pulmonary trunk, blood is ejected from the heart. At that time, the increased ventricular pressure forces blood into the semilunar valves, which must overcome the afterload before opening.
b. Massive bleeding
Massive bleeding, or hemorrhage, stresses the circulatory system and reduces tissue blood flow, cardiac effort, and blood pressure. The heart's reaction to bleeding depends on sympathetic nervous system activation and preload and afterload reduction.
c. The drug dopamine
Dopamine treats hypotension. Due to peripheral vasoconstriction, excessive dosages of this positive inotropic drug increase afterload and worsen heart failure.
d. Running a sprint
Running requires four times the left ventricle's usual cardiac output to oxygenate muscles. The left ventricle's afterload and preload drops considerably.
e. Hyperthyroidism with increased secretion of thyroid hormones
Increased levels of thyroid hormones that affect the heart are associated with hyperthyroidism. There is an increased risk of stroke due to increased heart rate and contractility caused by thyroid hormones.
f. Massive myocardial infarction (cardiac tissue death)
The rapidly failing ventricle (myocardial infarction) is dilated by the influx of blood from upstream, which then calls on the preload reserve to restore stroke volume at the expense of an increase in ventricular radius (afterload) and left atrial pressure.
g. Hypothermia
Heart rate drops significantly and stroke volume increases dramatically with hypothermia. Lower temperatures increase stroke volume preload, afterload, and mean arterial pressure.
h. Emotional distress
The heart's contractility and output are both enhanced by stress. Emotional arousal leads to an increase in heart rate, contractility, stroke volume, and cardiac output through increased sympathetic activation and the release of epinephrine and norepinephrine.
—————————
References:
- Brooks, D. P., Chapman, B. J., & Munday, K. A. (1984, October). The effect of hypothermia on the cardiovascular system and the pressor actions of angiotensin II. Journal of Thermal Biology, 9(4), 243-246. doi:10.1016/0306-4565(84)90003-2
- McDonough, K. H., Giaimo, M., Quinn, M., & Miller, H. (1999, March). Intrinsic myocardial function in hemorrhagic shock. Shock (Augusta, Ga.), 11(3), 205-10. doi:10.1097/00024382-199903000-00009
- Osuna, P. M., Udovcic, M., & Sharma, M. D. (2017). Hyperthyroidism and the Heart. Houston Methodist Hospital, 13(2), 60-63. doi:10.14797/mdcj-13-2-60
- Sovari, A. A., Kocheril, A. G., & Baas, A. (2021, October 17). Cardiogenic Pulmonary Edema Medication: Preload Reducers, Afterload Reducers, Inotropic Agents. (G. K. Sharma, Editor) Retrieved from Medscape: https://emedicine.medscape.com/article/157452-medication
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). Wiley.
- Ziegelstein, R. C. (2007, July 18). Acute emotional stress and cardiac arrhythmias. Journal of the American Medical Association, 298(3), 324-329. doi:10.1001/jama.298.3.324