ongestive heart failure and diuretics
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ongestive heart failure and diuretics
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[ID] => 554883
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[post_date] => 2024-12-23 18:41:55
[post_date_gmt] => 2024-12-23 23:41:55
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Congestive heart failure (CHF) is caused by the inability of the heart to pump blood and maintain proper blood flow throughout the body. As a result of poor pumping action, blood can pool in various areas of the body, and excess extracellular fluid (ECF) accumulates in the areas where the blood pools. The accumulation of excess ECF can occur in various parts of the body, depending on which chambers of the heart are not pumping effectively.
Treatment of generalized edema primarily relies on the administration of natriuretic agents, or diuretics, that diminish sodium reabsorption at different sites in the nephron, thereby increasing urinary sodium and water losses. However, the effectiveness of diuretics can be compromised by chronic renal failure (CRF), which is a distinct medical condition but can often be linked to CHF through a complex interplay of hemodynamic changes and shared risk factors.
The following experiments were conducted to examine the effectiveness of diuretics in patients with CHF plus or minus comorbid CR
Experiment 1
Researchers measured the effectiveness of various modes of delivery of furosemide, a loop diuretic that blocks the Na–K–2Cl symporter in the loop of Henle, preventing the active transport of these ions into the interstitium. They compared a group of patients receiving continuous intravenous doses of furosemide to patients receiving intermittent oral doses of furosemide. For each patient, researchers measured the amount of Na+ in the urine per hour.
Figure 1 Urinary sodium measurement in patients receiving continuous (intravenous) vs. intermittent (oral) doses of furosemide
Experiment 2
Researchers examined sodium excretion versus the diuretic concentration in normal patients as well as patients with CHF or CRF. They constructed dose–response curves for loop diuretics illustrating the fractional Na excretion (FENa) as a function of loop diuretic urine concentration for all patient groups.
Figure 2 Dose–response curves for three patient groups
Information and Figure 2 adapted from Lamiere, Norbert. Renal Mechanisms of Diuretic Resistance in Congestive Heart Failure. Kidney Dial, 2023. https://www.mdpi.com/2673-8236/3/1/5
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[question] => How might the renin–angiotensin–aldosterone system (RAAS) contribute to the symptoms of CHF described in the passage?
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[answer] => 1
[description] => Reason for Correct Answer:
The passage describes the buildup of extracellular fluid that results from CHF, and the fact that diuretics are used to help remove extra fluid from the body.
The renin–angiotensin–aldosterone system (RAAS) is primarily activated in response to low blood pressure or reduced blood flow to the kidneys. When the kidneys sense a decrease in blood pressure or blood flow, they release an enzyme called renin into the bloodstream.
The release of renin leads to a series of events, including the conversion of angiotensinogen to angiotensin I, which is then converted to angiotensin II by ACE enzymes. Angiotensin II is a potent vasoconstrictor that increases peripheral resistance and blood pressure, and it also causes the secretion of aldosterone, which increases sodium and water retention in the kidneys.
CHF results in decreased blood pressure and decreased perfusion of the kidneys, which would activate RAAS, leading to various effects including sodium and water retention (aldosterone-mediated). This fluid retention can exacerbate the fluid buildup already present in CHF patients.
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[each_answer] => A. RAAS activation in CHF is caused by decreased blood flow to the kidneys that results from reduced cardiac output, leading to the increased retention of sodium and water.
)
[1] => Array
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[each_answer] => B. RAAS activation in CHF is caused by excessive blood flow to the kidneys, leading to reduced sodium and water retention.
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[2] => Array
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[each_answer] => C. CHF symptoms are exacerbated by RAAS inhibition due to heightened sodium and water retention caused by decreased cardiac output.
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[each_answer] => D. Damage to the kidneys caused by decreased renal perfusion results in RAAS activation resulting in vasodilation and lowered blood pressure.
)
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Where in the loop of Henle does furosemide block the ion symporters?
[value] => Array
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[answer] => 4
[description] => Reason for Correct Answer:
The passage states that furosemide blocks the Na-K-2Cl symporter in the loop of Henle, preventing the active transport of these ions into the interstitium. Active transport of these ions occurs in the thick ascending limb of the loop of Henle. Here is a review of the parts of the loop of Henle:
Figure 1
Figure 2- Thin descending limb: This first limb is permeable to water but not to ions like sodium. Here, water is passively reabsorbed due to the higher osmolarity of the surrounding interstitial fluid.
- Thin ascending limb: The thin ascending limb is still permeable to water but also to some ions, such as sodium and chloride. As the filtrate ascends, these ions are reabsorbed into the interstitial fluid, diluting the tubular fluid.
- Thick ascending limb: The thick ascending limb contains the sodium–potassium–chloride (Na–K–2Cl) symporter, responsible for actively transporting sodium, potassium, and chloride ions out of the tubular fluid and into the interstitial fluid. This segment is impermeable to water, preventing its reabsorption.
The apical side of a cell refers to the side that faces the interior of a tubular structure or a body cavity. In the case of the renal tubules, such as the thick ascending limb of the loop of Henle, the apical side faces the tubular lumen where urine is formed. The basolateral side, on the other hand, faces the interstitial fluid and blood vessels.
Furosemide inhibits the sodium–potassium–chloride (Na–K–2Cl) symporter responsible for actively transporting sodium, potassium, and chloride ions from the tubular fluid into the renal cells of the thick ascending limb. By blocking this symporter on the apical side, furosemide prevents the reabsorption of these ions back into the cells, which would then be transported to the interstitium and eventually into the bloodstream.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. In the descending loop of Henle on the basolateral side
) [1] => Array ( [each_answer] =>B. In the descending loop of Henle on the apical side
) [2] => Array ( [each_answer] =>C. In the ascending loop of Henle on the basolateral side
) [3] => Array ( [each_answer] =>D. In the ascending loop of Henle on the apical side
) ) ) [2] => Array ( [quiz_unique_key] => 1403770772 [question] =>The concentration of ions is responsible for the movement of water across cellular membranes. In what way is concentration and flow of water affected by the use of furosemide?
[value] => Array ( [answer] => 2 [description] =>Reason for Correct Answer:
Water flows toward regions of higher salt concentration.
The loops of Henle extend into the medulla, as shown below. These loops play a significant role in establishing the osmotic gradient between the renal tubules and interstitium of the medulla.
https://commons.wikimedia.org/wiki/File:Structure_of_the_mammalian_kidney..jpg
Over time, the loss of transport of ions in the loop of Henle will lead to decreased salt levels in the medulla, which will help favor water retention in the tubules.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. Salt levels increase in the medulla, producing a gradient favorable for the retention of water in the tubules.
) [1] => Array ( [each_answer] =>B. Salt levels decrease in the medulla, producing a gradient favorable for the retention of water in the tubules.
) [2] => Array ( [each_answer] =>C. Salt levels decrease in the cortex, producing a gradient favorable for the retention of water in the tubules.
) [3] => Array ( [each_answer] =>D. Salt levels increase in the renal tubules, producing a gradient favorable for the retention of water in the cortex.
) ) ) [3] => Array ( [quiz_unique_key] => 1403770772 [question] =>If the left ventricle of the heart is inefficient at pumping blood, where in the body would significant quantities of excess extracellular fluid (ECF) first accumulate?
[value] => Array ( [answer] => 1 [description] =>Reason for Correct Answer:
Excess ECF occurs in areas where blood has pooled because the heart cannot pump the blood effectively.
Blood flows through the heart in the following order: vena cava (cranial and caudal), right atrium, right ventricle, pulmonary artery, the lungs, pulmonary vein, left atrium, left ventricle, and then aorta.
If the left ventricle is not able to pump blood effectively, then the areas before the left ventricle will pool blood and accumulate excess ECF. The pooled blood applies pressure against the pulmonary capillaries in the lungs, and as a result, the ECF is forced across the capillary bed into the alveoli air sacs, which fill with ECF rather than air.
A. Lungs
) [1] => Array ( [each_answer] =>B. Abdominal cavity
) [2] => Array ( [each_answer] =>C. Legs
) [3] => Array ( [each_answer] =>D. Periaortic (the area surrounding the aorta)
) ) ) [4] => Array ( [quiz_unique_key] => 1403770772 [question] =>To relieve the symptoms of an acute CHF exacerbation caused by excessive ECF, what is the better treatment option and why?
[value] => Array ( [answer] => 1 [description] =>Reason for Correct Answer:
The purpose of furosemide is to increase the ion concentrations in the filtrate (i.e. urine) so that more water flows into, or is retained by, the renal tubule and is eliminated from the body.
Figure 1 shows how urinary sodium excretion measurements in patients treated with continuous intravenous furosemide compared with those in patients treated with intermittent oral furosemide. In these groups, more TOTAL sodium excretion is consistent with more effective treatment, and you can roughly approximate the total excretion by looking at the areas under the graphs.
This is the area under each graph:
If you’re having a hard time comparing these, notice they only really differ in these colored areas and overlap in the area shown in gray:
It appears as though the continuous infusion resulted in higher overall levels of sodium excretion over the period of 15 hours assessed. This would allow more water to be retained by the filtrate in the renal tubule, thus making it the most effective acute treatment.
A. Continuous intravenous furosemide, because this increases the amount of Na+ in the filtrate and increases the volume of water retained by the renal tubule
) [1] => Array ( [each_answer] =>B. Continuous intravenous furosemide, because this increases the amount of Na+ in the filtrate and as a result decreases the water absorbed by the filtrate
) [2] => Array ( [each_answer] =>C. Oral doses of furosemide, because these reduce the amount of Na+ in the filtrate and increase the water retained by the renal tubule
) [3] => Array ( [each_answer] =>D. Oral doses of furosemide, because these increase the amount of Na+ in the filtrate and increase the volume of water retained by the renal tubule
) ) ) [5] => Array ( [quiz_unique_key] => 1997864699 [question] =>Which conclusion is LEAST supported by the results of Experiment 2?
[value] => Array ( [answer] => 4 [description] =>Reason for Correct Answer:
Figure 2 in experiment 2 shows the fractional Na excretion (FENa) at different concentrations of furosemide. The FENa is an indication of how well the diuretic is working.
This question has a lot to unpack, so let’s start by examining the results for CHF patients. The graph shows that a higher dose of furosemide was required to get FENa to increase in these patients. It also shows that the upper limit of the FENa was lower in these patients.
Two of these effects are shown here and correspond to Choices A and B (the right shift of this complete curve also indicates decreased sensitivity to furosemide treatment at all doses in these patients, but that is not an answer choice).
Now let’s look at the curve for CRF patients. This curve shows a right shift compared with the normal curve, but the FENa values eventually increase to the same level of around 19% as the dose increases (these patients also display a higher threshold before the diuretic effect, but this is not asked about).
The right shift in the curve could be explained by impaired diuretic secretion in CRF patients, requiring the patients to take larger doses of diuretic to achieve the same effects, as in Choice C. However, CRF patients DO not exhibit a decreased maximal response to treatment, as in Choice D.
A. Patients with congestive heart failure (CHF) exhibit a decreased maximal response to treatment with furosemide.
) [1] => Array ( [each_answer] =>B. The threshold concentration of furosemide required to achieve a diuretic effect is increased in chronic heart failure (CHF) patients.
) [2] => Array ( [each_answer] =>C. Patients with chronic renal failure (CRF) show resistance to treatment owing to impaired diuretic secretion.
) [3] => Array ( [each_answer] =>D. Patients with chronic renal failure (CRF) exhibit a decreased maximal response to treatment with furosemide.
) ) ) ) [total_question] => 6 [correct_answers] => Array ( [554883|1] => A [554883|2] => D [554883|3] => B [554883|4] => A [554883|5] => A [554883|6] => D ) [hide_display_feedback_settings] => [hide_solutions] => )