Ultrafiltration Revisited

Ramon G. Duarte, R.N., C.H.N.

Nursing Services in Nephrology, Santa Barbara, Calif.

Dialysis & Transplantation, Vol. 6, No 9, pp. 33-37, September 1977

In response to a need for reaffirming accuracy, simplicity, and understanding of an established concept, three algebraic expressions have been devised to promote the concept of projected fluid removal.

These three formulas were derived from pre-existing axioms and represent not only a review but also a reorganization of a large amount of data that has been available for a number of years.

Use of a dialyzer whose ultrafiltration rate (UFR) versus transmembrane pressure (TMP) is proven to be a straight line graph gives us a great advantage and the opportunity to employ an ultrafiltration care plan that will not over or under ultrafiltrate the patient, but rather will provide a dialysis treatment that will comfortably fulfill his fluid removal needs.

Presented is an ultrafiltration study revealing the result of 20 evaluated dialysis treatments where formulated weight removal and actual fluid removal are shown in comparison and contrast.

The method described utilizes the following formulas:

FL = 454.6 W (1)

Formula one is used in an assay of fluid weight in pounds to be removed and calculates the conversion of fluid weight pounds to fluid volume cubic centimeters. The formula states that FL (fluid loss in ml) equals the product of the conversion factor 454.6 cc/lb and W (fluid gain in pounds)-the projected fluid removal.

fl = FL + IF (2)

The second formula compensates for fluid given to the patient during dialysis and therefore accounts for an intake factor (IF). Fluid loss previously converted is now modified to account for intravenous and oral intake in cubic centimeters giving us fl or the projected fluid removal. Formula two states that projected fluid removal is equal to the sum of the fluid loss as derived from formula one and the oral and intravenous fluids designated IF.

NP = [(fl/dt) / uf] - vr (3)

The third and final formula determines negative pressure setting in one complete balanced equation, incorporating the elements of dialysis time in hours, the ultrafiltration rate of the hollow Fiber kidney type in use, the patient's venous resistance and the projected fluid removal goal as derived from formula two. The formula states that NP (negative pressure) setting is equal to the projected fluid removal to be divided by the total dialyzing time in hours, the result of which divided by the ultrafiltration rate gives us the final quantity from which we subtract the venous resistance.

We are given a patient assessed at a dry weight of 105 pounds. A hollow fiber kidney 2.5 M2 surface area is used, with an ultraflltration rate of 3.0 ml/mmHg/hour. The blood flow is established at 200 cc/ min; the dialysate flow is at 500 cc/min with the dialysate temperature at 38 * centigrade.

On April 10, 1976, the patient arrived at the unit with a predialysis weight of 110.5 pounds, a predialysis blood pressure measuring 180/98 and an assessment of 5.5 pounds of fluid weight to be removed. Dialysis notes reveal a NP setting at 200 mmhg and a venous resistance of 10 mmhg artificially incremented to 100 mmHg using a Harvard clamp just below the venous drip chamber, increasing the total TMP (TMP=NP+VR) to 300 mmhg. During the first hour of dialysis the patient's blood pressure measured 206/96. In one hour and 10 minutes of dialysis, 1044 cc of fluid were removed, resulting in the patient's blood pressure dropping to 70/60, with complaints of headache and dizziness. The negative pressure was decreased to 150 mmhg and the Harvard clamp was removed, bringing the positive pressure of blood to a venous resistance of 10 mmhg. Normal saline 150 cc was administered to titrate the patient's hypotension. Symptoms subsided with the blood pressure leveling at 138/60. The second hour of dialysis proceeded with a TMP of 160 mmhg. The third hour of dialysis revealed a blood pressure of 130/60; because of episodes of nausea and vomiting, the NP was decreased to -50 mmhg and remained so for the duration of the treatment. The TVR (total volume removal) was 1974 cc. The patient's intravenous and oral fluid intake during dialysis equaled 1250 cc, resulting in an AVR (actual volume removal) of 724 cc or oneand one-fourth pounds removed, leaving the patient, after five hours of dialysis, four and one-fourth pounds above his dry weight.

The patient returned to the dialysis unit after a weekend. His predialysis weight on April 13, 1976, was 114 pounds, nine pounds above his dry weight. The concept of projected fluid removal was used, employing the three algebraic expressions. These were the results: During the first hour of dialysis the patient's blood pressure measured 190/110, the TMP was set at 240 mmhg, and approximately 835 cc were removed. The second hour of treatment the patient's blood pressure remained at 190/110, the TMP was maintained at 250 mmhg, and 750 cc were removed. The third hour of dialysis revealed a minimal blood pressure drop (I70/80) and the patient remained asymptomatic with a fluid removal of 810 cc. During the fourth hour of dialysis, the blood pressure measured 170/80, the TMP was maintained at 260 mmhg, removing 585 cc. The last hour of dialysis revealed a blood pressure drop to 140/80 and the patient complained of a mild headache. The TMP was decreased to 200 mmhg, removing 600 cc of fluid. The TVR for five hours of dialysis totaled 3030 cc. The patient's oral and intravenous fluid intake equaled 550 cc. The AVR totaled 2480 cc or six and one-fourth pounds, leaving the patient with a postdialysis weight of lO7.25 pounds, only two and one-fourth pounds above dry weight. This comparison/contrast of data reveals that projected fluid removal provides a knowledgeable control over ultraflltration and promotes a safe, efficient dialysis treatment, while lessening the patient's discomfort as it may relate to ultrafiltration. For purposes of data collection a fluid assay sheet was designed(Table II). This ultrafiltration profile module reveals areas for recording oral and intravenous intake, and the patient's pre- and postdialysis weights as well as the projected and actual weight losses. More important, it serves as a tool as well as a record for monitoring hourly TMP and calculated fluid removal.


Step One: Convert fluid weight pounds to be removed to fluid volume in cubic centimeters.

Step Two: Accurately account for P.O. and I.V. fluids to be given during dialysis.

(3378 cc is the projected fluid removal for five hours of dialysis compensating for the P.O. and I.V. intake.)

Having used formulas one and two to obtain the projected fluid removal we can now proceed with formula three-a balanced equation that will reveal the negative pressure setting needed to attain our projected fluid removal.

Step Three:

NP = [(fl/dt) / uf] - vr NP = [( 3378 cc/5 hrs) / (3.0 cc/mmhg/hr)] - vr

Venous resistance may not be the only measurement to consider. If the arterial drip chamber is positioned so that it is between the blood pump and the dialyzer, the positive pressure of blood entering the kidney is then measured. This information will allow us to employ delta pressures (averaging the sum of arterial positive pressure of blood and the venous resistance). The employment of delta pressures attains greater accuracy in negative pressure determination.
VR = (Pbi + Pbo) 2

We were able to maintain an even ultrafiltration profile for five hours of dialysis. Total volume removed equaled 3375 cc. Oral and intravenous fluids totaled 650 cc, giving us an actual volume removal of 2725 cc or 5.5 pounds. The projected removal was 6.0 pounds and we were able to come within 8.3% of our projection.

This study encompassed two patient populations, groups A and B, in a total of 20 dialysis treatments where the concept of projected fluid removal was employed. Our most accurate volume removal came within 0.4% of our projected figure. The greatest discrepancy in group A was 17%, where we were one quarter pound off our projection. In group B we were one-half pound off our projection, giving 25% as our greatest discrepancy. The average volume removal came within 7.9% of our projected figure.

The following steps were taken to assure accuracy while conducting the study:

Successful projected fluid removal depends greatly upon the nurse's ability to accurately assess the ultrafiltration needs of the patient, as well as an understanding that ultrafiltration is a dynamic process subject to change by several variables. To judge the concept's accuracy merely by looking at how much weight was projected and how much actual weight loss was attained can be and is misleading if the following questions are not asked: Was there a careful and accurate assessment of the exact P.O. and I.V. intake projectively (remembering that any change or deviation from the projected P.O. and I.V. intake will alter the actual fluid removal)? Does that patient void moderate to large amounts of urine during dialysis or at any time prior to post weight determination? Was a 200 cc/min blood flow maintainable during the majority of dialyzing time? Did venous resistance increase or decrease, changing the TMP? Did monitoring of the patient's vital signs and/or assessment of the patient's homeostasis require a decrement of TMP along with the administration of volume expanders and, if so, can the nurse calculate how these changes will affect the actual volume removal?

The formulas are designed for flexibility. It is ideal to maintain an even ultrafiltration rate but, if the patient's assessment warrants a blood flow or TMP decrement, it should be instituted. Ultrafiltration is a complex and dynamic process. The formulas are tools-the key to successful projected fluid removal is the nurse who knows the patient, the equipment, and the principles of ultraflltration.

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