Fluids and Diuretics

A Tactical Framework for the Wards

When you write for fluids or diuretics, you’re not just ordering medications — you’re adjusting the patient’s internal ocean. Every liter has direction and intent. Your job is to understand where the water is, where it needs to go, and what signal you’re sending to the kidney.

Step 1: Start With a Volume Assessment

Every fluid or diuretic decision begins with this question: What is the patient’s current effective circulating volume?

• Signs of low volume: orthostatic hypotension, flat neck veins, dry mucous membranes, cool extremities, low urine sodium (<20 mmol/L).

• Signs of high volume: JVD, pulmonary rales, edema, ascites, weight gain, low urine sodium but high total body sodium.

• Signs of normal or ambiguous volume: requires context — think SIADH, euvolemic hyponatremia, or mild diuretic overuse.

If you can’t tell the volume status, get baseline data: daily weights, urine output, serum and urine electrolytes.

Step 2: Define the Purpose of the Fluid

Fluids are prescribed for one of three reasons. Always know which you’re doing.

• Resuscitation – restore perfusion (shock, hypotension, sepsis).

• Maintenance – replace ongoing insensible and metabolic losses when NPO.

• Replacement – correct specific losses (vomiting, diarrhea, drains, diuretics).

Resuscitation:

• Use balanced crystalloids (Lactated Ringer’s or Plasma-Lyte) unless the patient is hyperkalemic or hypercalcemic.

• Normal saline is fine short-term but causes hyperchloremic acidosis in large volumes.

  Boluses are typically 500–1000 mL at a time, guided by clinical response and urine output.

Maintenance:

• Few patients in the hospital need maintenance fluids. If they are eating and drinking, they likely don’t need maintenance fluids. Additionally, most ICU patients get plenty of fluid via IV antibiotics and other IV medications.

• For adults, start around 25–30 mL/kg/day (≈2 L/day).

• Use balanced crystalloids or D5½NS if they need some glucose.

• Add KCl if NPO >24 hours and renal function is stable.

Replacememnt

• Match the composition of the fluid lost (e.g., gastric losses need chloride replacement with NS + KCl; diarrhea may need bicarbonate equivalents).

Step 3: When to Hold Fluids

• Do not automatically order maintenance fluids if the patient is eating, drinking, or edematous. Excess fluid is the most common iatrogenic harm in hospitalized patients.

• When in doubt, hold fluids and reassess in 6–8 hours with weights and I/Os.

Step 4: Approach to Diuretic Therapy

Diuretics are tools to remove pathologic water, not simply urine generators. Use them deliberately.

1. Identify the target.

   1. Is the patient fluid overloaded (heart failure, cirrhosis, CKD) or do they need a diagnostic trial to assess renal response?

2. Choose the agent and route.

   1. Loop diuretics (furosemide, bumetanide, torsemide)

      1. Work in the thick ascending limb by blocking Na-K-2Cl reabsorption.

      2. Most potent natriuretics; they increase distal sodium delivery.

      3. Oral-to-IV conversion: furosemide 40 mg PO ≈ 20 mg IV ≈ torsemide 20 mg PO ≈ bumetanide 1 mg IV.

      4. Torsemide and bumetanide have better bioavailability and more predictable absorption in gut edema.

   2. Thiazide and thiazide-like diuretics (hydrochlorothiazide, metolazone)

      1. Work in distal convoluted tubule; weak as single agents for volume removal but powerful when added to a loop (“sequential nephron blockade”).

      2. Use metolazone 2.5–5 mg PO 30 minutes before the loop diuretic for synergy.

   3. Potassium-sparing diuretics (spironolactone, eplerenone, amiloride)

      1. Work in the distal nephron to block sodium reabsorption and potassium secretion.

      2. Used for chronic conditions like cirrhosis and heart failure, not for acute volume removal.

   4. Osmotic diuretics (mannitol)

      1. Rarely used outside neurocritical care for intracranial pressure control.
         

Step 5: Interpret the Response

1. After giving a loop diuretic, evaluate the response within 2–3 hours.

   1. Adequate response: ≥1 mL/kg/hr urine output or >2 L net negative over 24 hours.

   2. Poor response: could indicate underdosing, poor absorption (edematous gut), or true diuretic resistance.

2. If response is inadequate:

   1. Double the dose of the loop (logarithmic, not linear response).

   2. Switch to IV if on PO.

   3. Add thiazide-like agent for sequential blockade.

3. Restrict sodium intake (<2 g/day).

4. Check spot urine sodium 2 hours after loop administration:

   1. Urine Na < 50 mmol/L means the loop is underdosed or not reaching the tubule.

   2. Urine Na > 50 mmol/L means effective delivery, continue same plan.
      

Step 6: Prevent and Manage Complications

1. Electrolyte losses:

   1. Monitor sodium, potassium, magnesium, and chloride daily.

   2. Replace proactively; magnesium deficiency prevents potassium correction.

2. Overdiuresis:

   1. Watch for rising bicarbonate, rising creatinine, or orthostasis.

   2. If hypovolemia develops, temporarily hold diuretics, give small balanced-crystalloid boluses, and reassess.

3. Metabolic alkalosis:

   1. Contraction alkalosis results from volume and chloride loss. Treat with isotonic saline or acetazolamide if volume repletion isn’t possible.

Step 7: Integrate Diuretics and Fluids

Fluids and diuretics are not opposites; they are levers on the same system. In heart failure, you may use hypertonic saline plus loop diuretics to pull water out while replenishing chloride to restore diuretic responsiveness. In AKI recovery, you might use gentle diuresis while replacing urine losses with isotonic fluids to avoid swings in sodium.

Step 8: Monitor Quantitatively

• Daily weights (1 kg ≈ 1 L water).

• Strict I/Os – measure urine accurately.

• Serial chemistries – monitor Na, K, Cl, bicarbonate, and creatinine daily.

• Urine sodium – tells you whether the kidney is responding to diuretic therapy.

Step 9: Know When to Escalate

Call nephrology when:

1. Diuretic resistance persists after loop + thiazide combination.

2. Rising creatinine with ongoing congestion (possible cardiorenal syndrome).

3. Severe electrolyte abnormalities.

4. Need for ultrafiltration or dialysis for refractory volume overload.
   

Step 10: Core Physiologic Principles

1. Fluids and diuretics are signals to the kidney about pressure and salt — not just water in or out.

2. Always clarify purpose before prescribing.

3. Evaluate response within hours, not days.

4. Diuretic effect is logarithmic: doubling the dose is how you escalate, not adding small increments.

5. Replace what you remove — if you’re pulling sodium and chloride, the patient may need solute back.

6. The most important fluid in the hospital is the one you don’t give unnecessarily.
   

Quick Reference Equivalents

• 1 kg = 1 liter of water.

• 1 g NaCl = 17 mEq sodium.

• 100 mL of 3% saline = 3 g NaCl = 51 mEq sodium.

• Furosemide 40 mg PO ≈ 20 mg IV ≈ torsemide 20 mg PO ≈ bumetanide 1 mg IV.

• Fluid and diuretic management is controlled physiology. Every liter carries intent, every milligram of loop diuretic tells the nephron something. Think about what you want the kidney to do — conserve, excrete, or rebalance — and send the right signal.