when the crit-line is showing or displaying a positive graph, at zero or above, does this correlate to a certain extra volume of fluid that the pt is able to remove, and if so, does each percent mean say example, one percent equals 250ccs or is there no relationship, how can the graph show more fluid in the vascular system as the system will only hold so many liters, how can the "system" be over filled?

A Crit-Line blood volume (BV) tracing that "goes positive" or shows an INCREASE in blood volume is a rather common occurence in the fluid overloaded patient.

Each patient begins at zero or starting point on the BV graph. A blood volume increase greater than zero during the treatment (i.e. Hct decrease)shows that more fluid is currently present in the intravascular space than there was at the beginning of treatment.

The patient is refilling faster that the dialysis machine is removing fluid and has actually increased the volume of the intravascular compartment.

How?Why?

BV can increase due to
the saline prime
an increase in the sodium level of the blood (sodium modeling)or a dialysate sodium level greater than the current level of the blood.
a response to the initiation of dialysis

In many cases this increase in circulating blood volume is caused by an increased osmolarity of the circulating blood due to increased sodium levels. Even if the vascular system was "full" this sodium difference osmotically forces fluid into the vascular space. In an already overloaded patient this extra fluid can actually cause the heart to begin to fail - evidenced by FALLING blood pressure. So much fluid that pressure can not be maintained. In most cases the patient would benefit from more aggressive UF. Further investigation may show that sodium modeling may not be necessary or may need to be altered.

Regardless of the reason, a positive trending graph is an indiction that the patient not only has fluid, but is shifting it into the intravascular space at a rate faster than removal is occuring.

The BV graph shows percent change - not an absolute volume - therefore no direct correlation can be made to actual volume.