Fluid therapy is beset by many misconceptions, so I talked to Dr. Sean Smarick, a board-certified criticalist at AVETS in Monroeville, Pa., to clarify them.
What is the true maintenance fluid rate?
Maintenance fluids represent the amount of fluids needed to meet the metabolic requirements of a patient. Therefore, deficits from dehydration or anticipation of ongoing losses are typically not included in this amount. Metabolic demands can vary between caged vs. active patients, as well as healthy vs. sick patients.
We often tend to use simple formulas or universal recipes such as 1 ml/lb/hour or 40 ml/kg/day, yet they don’t work for all patients. It is important to realize that these linear formulas tend to underdose small patients and overdose large patients.
This becomes especially significant when the kidneys aren’t functioning properly (fortunately, they often correct for our inaccurate fluid plans) or when there is an increased risk of volume overload, e.g. in patients with heart failure. If we use multiples of maintenance rates (“twice maintenance,” “three times maintenance,” etc.) to address deficits due to dehydration or anticipated ongoing losses, such errors will be multiplied by the same factor.
For patients under 5 kg and over 40 kg, we should calculate their true fluid needs with the help of a maintenance chart or the following formula: 70 x (weight in kg) to the power 0.75.
This sounds complicated. How do you figure that out easily?
A cheap calculator with a square root function is all you need. Multiply the weight (in kg) by itself three times, then hit equal, and calculate the square root twice. This gives you the body weight to the 3/4 power. Then simply multiply by 70.
Alternatively, you can use the following formula: [30 x (weight in kg)] + 70.
It is no coincidence that these formulas are similar to basal metabolic rates.
Both formulas provide the number of ml to be perfused over 24 hours. Yet any fluid therapy plan is just a plan. It begs to be re-evaluated by monitoring the patient’s hydration (mucous membrane moisture, skin turgor) and perfusion (mucous membrane color, CRT, pulse quality, heart rate).
So now we know how much fluid to give. What is the best type?
As far as maintenance fluids are concerned, the type can be just as important as the amount. Once again, functional kidneys will correct our errors, but patients with renal compromise are less likely to deal with our flawed plans of using replacement fluids instead of maintenance fluids. Purely speaking, normal saline (0.9 percent NaCl) is not considered maintenance fluid because it is so rich in sodium. Think about it. We don’t drink salt water when we’re thirsty!
Lactated Ringer’s solution (LRS) is better tolerated as a maintenance solution than Normosol-R, Plasma-Lyte 148 or normal saline because it is buffered and contains less sodium. We get away with using such replacement fluids (the “R” in Normosol R stands for “Replacement”) because healthy kidneys sort things out. In other words, they balance the salt and water ratio.
However, after a few days, even healthy kidneys may not keep up with the sodium load. This free water deficit is often manifested by thirst despite being adequately hydrated. I have seen patients on LRS run to drink mop water because they were so thirsty.
Normosol-M and Plasma-Lyte 56 are considered true maintenance solutions. A compromise may be half-strength saline supplemented with KCl.
Is additional KCl needed in maintenance fluids?
Except for saline, commercially available solutions contain potassium concentrations greater than what is found in plasma. If the patient is not eating and on IV fluids, its potassium levels are often low, so supplementing is considered beneficial. However, we should not exceed 0.5 mEq/kg/hr to avoid potential cardiac complications secondary to hyperkalemia.
Is vitamin B needed in maintenance fluids?
Not routinely. There are conditions where this important water-soluble vitamin may be deficient and the patient may benefit, for example, with short bowel syndrome, malabsorption/maldigestion and chronic renal failure.
When treating shock, what would you consider a “shock dose”?
First, we should remember that in 2012, we treat shock with fluids, not steroids.
Second, shock doses should be given as boluses to rapidly expand the effective circulating volume. It is probably acceptable in some cases to use 90 ml/kg in dogs and 60 ml/kg in cats, but it is much better to titrate to effect.
This means that we should frequently monitor vital parameters to decide which volume to give.
If the parameters are not improving, then we should ask ourselves why. We might be dealing with septic shock, which may not respond to fluids or therapy alone, or there may be ongoing hemorrhage.
Why is SQ dextrose a bad idea?
Dextrose solutions are hyperosmolar, and they cause skin necrosis or sloughing when given subcutaneously. By the way, dextrose solutions such as D5W are not a good choice as replacement fluids. In fact, you can kill a patient by giving a large amount of D5W. Once the glucose has been absorbed, we are left with free water, which is distributed to the various fluid compartments. That water is absorbed by all cells, which can lead to intracellular edema. When it happens in the brain, it can be a deadly proposition.
Does dextrose in IV fluids provide any calories?
It provides an insignificant amount, which is definitely not enough to sustain a patient’s energy requirements.
To read Part 2 of this discussion, click here.