Fluids and Electrolytes

Fluid Resuscitation

Figure 1: Electrolyte Composition of Commonly Used Intravenous Fluids

Figure 2: Electrolyte makeup of common bodily fluids

Diuresis and Fluid Removal

How I think about diuresis in the PICU:

1) Why am I diuresing this patient? Typically it is to improve pulmonary edema, pleural effusions, lung compliance, etc and to facilitate improvements in respiratory status and deescalation of support (ventilator settings, coming off mechanical ventilation, etc). Basically, dry lungs are happy lungs. Diuresis can also sometimes help with intraabdominal pressure (ie ascites), heart failure (leading to pulmonary or hepatic congestion), etc.

2) Set a fluid goal for the day. How much do you want to take off? A general rule I use is 20 cc/kg/day as a baseline for moderate diuresis. The circulating blood volume for most of our patients is 70-80 cc/kg and so this will take off about 25% of the circulating blood volume. Remember that your goal is to remove interstitial or "third-space" fluid, not actually intravascular volume but this is the only way we can get at that fluid, by reducing intravascular hydrostatic pressure. This is described by the Starling equation, which describes net filtration out of the capillary as a balance between hydrostatic and oncotic forces. The reflection coefficient (sigma) describes the propensity of a substance to stay in the vasculature (for example, sodium is 1.0, meaning it stays within the vasculature perfectly, vs. mannitol is closer to 0.75-0.9, meaning it eventually leaks out of the vessels to some degree, pulling water with it back to the interstitium. 

General rule: ~ 20 cc/kg/day

If you are relatively hypotensive, have concerns for worsening AKI, want to be more ginger, etc: 10 cc/kg/day

If you are relatively hypertensive, are really up against a wall from a respiratory standpoint with high settings etc: 40 cc/kg/day

These are general guidelines that can then be adjusted based on how the patient is doing (lung compliance, blood pressure, renal function, etc)

3) You've chosen a fluid goal. Now choose a starting diuretic regimen. What you choose (ie lasix 1 mg/kg IV q8h)  is much less important than how you monitor and adjust throughout the day to achieve your goal. Typical starting regimens and other considerations include:

    a) Lasix 0.5 mg/kg IV or PO (IV thought to be twice as potent) either as a single spot dose up to as frequently as q6 hours (lasix- "lasts six" hours). In adult sized patients, smaller doses such as 10-20 mg may be sufficient in a lasix naive patient. 1 mg/kg may be more appropriate for patients who are not lasix naive. 

    b) Consideration of addition of Diuril (chlorthiazide) to augment diuresis. The distal convoluted tubule will try to counteract the loop diuretic, lasix, you've given, by reabsorbing sodium. Diuril poisons the sodium channel's reabsorption, leading to a synergistic effect on diuresis. This can be dose 5 mg/kg up to q6 hours given shortly after the lasix (in order to have that synergystic effect). In some circumstances, double dose diuril at 10 mg/kg is utilized as well.

    c) Consideration of a lasix infusion- this can be helpful to have a steady dose of diuretics to avoid swings with higher drug levels and thus "dumping" of urine which can occasionally have hemodynamic effects like hypotension. Lasix infusions are dosed mg/kg/hr. Hence, 0.2 mg/kg/hr would be the equivalent of 1.2 mg/kg IV q6 (a relatively moderate/high dose). A typical starting dose might be 0.05-0.15 mg/kg/hr which would be equivalent to 0.3 mg/kg-0.9 mg/kg IV q6 and then adjusted based on response. 

    d) Consider how often you need to check electrolytes (primarily potassium as well as monitoring renal function). With aggressive diuresis, I typically plan to check at least q12h if not more frequently. 

    e) Consider 25% albumin. As seen by the Starling equation, oncotic pressure also helps determine how much fluid moves out from the interstitium back into the vasculature. At serum albumin levels <2.5 (or occasionally <3), one could consider 0.5 mg/kg of 25% albumin (max 25 g) IVq6 for 4 doses to run for several hours before each dose of lasix/diuril in order to "pull fluid" from the interstitium and then remove it via the diuretics. Exogenous albumin will eventually leak into the interstitium (ie radiolabeled albumin can be seen to do so) and thus it is only potentially effective to augment diuresis. Evidence for this approach is limited but experientially, does seem to augment diuresis. 

    f) If your response to diuretics is poor, consider how your renal function is, ensure you have a sufficient level of chloride (typically want >90 as lasix works on the Na K 2Cl channel) , or consider an alternative agent like bumetanide, which although is in the same class, we sometimes see a different effect. 

4) Again, MONITOR your patient's fluid status and adjust throughout the day to meet the predetermined fluid goal from A. This is key. 

5) Sometimes it is worth diuresing until you see a little hypotension and even accepting that and using a vasoactive agent to facilitate further diuresis. Similarly, sometimes with bad lung disease, we will diurese until we see biochemical evidence of some intravascular depletion and potentially even AKI (BUN elevated, bump in creatinine etc), recognizing we are making a conscious choice to lean on the kidneys to help the lungs. 

6) Once you've achieved the goal of your diuresis (ie get off the ventilator etc), reassess whether you still need the same (or any) level of diuresis/fluid removal.

Starling Equation describing net filtration out of the capillary (by convention) as determined by hydrostatic and oncotic pressures

The nephron and site of action for various diuretics

Sodium Disorders

Figure 3: Expected Changes for Common Sodium Disorders

NEJM Review Article (Adrogue NEJM 2000) 

Potassium Disorders

Figure 4: EKG changes in hypo and hyperkalemia

Magnesium Disorders

Phosphorous Disorders

Calcium Disorders

Figure 5: Calcium homeostasis

Electrolyte Derangement Chart

Courtesy of Richard Pierce, MD


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McNab, Sarah et al. The Lancet , Volume 385 , Issue 9974 , 1190 - 1197

9) FEN Basics Powerpoint