Actual
blood flow is not always consistent with what is indicated
on the machine blood pump display setting. The greater
the negative arterial pressure, the greater the “negative”
effect on the actual delivered blood flow. Multiple factors
contribute to increased negative pressure, however poor
access flow is most common cause.
-240mm
/ Hg is the upper limit of negative pre pump arterial
pressure accepted in clinical practice. Even with negative
arterial pressure within allowable limits, there is already
a 9% reduction in actual blood flow. The best way to prevent
further reductions in delivered blood flow is to monitor
arterial pressure.
If
actual blood flow delivered is less than expected, clearances
will be negatively affected.
Larger
needles such as 15 gauge size are indicated for blood
flow rates in excess of 400ml / minute to help prevent
the occurrence of high negative arterial pressure –
thus helping to ensure that adequate blood flow rates
are achieved.
Smaller
size needles used in conjunction with high blood flow
rates result in higher negative arterial pressure.
This
graph indicates that as blood flow rates are increased
with the use of smaller bore needles, the negative pre-pump
arterial pressure increases. This has a negative effect
on the delivered blood flow.
