Question
A 29-year-old man is brought to the emergency room 6 hours after the onset of severe epigastric pain and vomiting. His heart rate is 110/min and blood pressure is 98/72 mm Hg. He is diagnosed with acute pancreatitis, and fluid resuscitation with normal saline is initiated. Which of the following is the most likely immediate effect of fluid resuscitation in this patient?
| A. |
Increase in cardiac afterload
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| B. |
Increase in volume of distribution
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| C. |
Increase in myocardial oxygen demand
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| D. |
Increase in plasma oncotic pressure
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Show Answer
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Correct Answer � C
Explanation
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Answer C) Increase in myocardial oxygen demand
Fluid resuscitation would increase venous return and right ventricular end diastolic volume (EDV).
Increase in myocardial oxygen demand
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Myocardial oxygen demand depends on the heart rate, heart contractility, and wall tension (affected by afterload). An increase in end-diastolic volume, as occurs in fluid resuscitation, causes an increase in cardiac preload by stretching cardiac myocyte sarcomeres, which results in increased myocardial contractility based on the Frank-Starling mechanism. Increased contractility also increases cardiac output (CO) and cardiac work, and hence myocardial oxygen demand as well. Note, however, that fluid resuscitation is beneficial in hypovolemic patients because it also increases myocardial oxygen supply by improving coronary perfusion pressure.
Increase in cardiac afterload
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Cardiac afterload represents the pressure against which the heart must pump during systole.
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A large, rapid infusion of normal saline can lead to transient hemodilution, decreasing blood viscosity, and causing a decrease in afterload, not an increase.
Increase in volume of distribution
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Volume of distribution (VD) is an intrinsic measured property of a drug, not a physiological property of the body.
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Although the infusion of normal saline given to this patient will temporarily increase his total body water, the VD of any particular drug would remain unaffected.
Increase in plasma oncotic pressure
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Plasma oncotic pressure is exerted by proteins (primarily albumin) within blood vessels. A transient increase in plasma oncotic pressure would occur with the infusion of colloids but not with normal saline infusion.
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Normal saline is isotonic to serum and generally redistributes to replenish depleted extracellular fluid, only minimally affecting plasma oncotic pressure.
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A large, rapid infusion of normal saline can decrease plasma oncotic pressure transiently by diluting the concentration of protein intravascularly.
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