In the simplest form, this is the formula for finding the flow rate of an infusion.
You need to know the volume of the liquid flowing into the patient and divide this by
the time it takes. The volume will be measured in units of some variations of litres, millilitres,
or microlitres. And time will be measured in hours or minutes, or could theoretically
be calculated in seconds... But this would not be a time unit that would be very practical
in nursing. What is important to remember, however, is the units put into the one formula
are compatible. That means that if the flow rate is measured in mls per hour, the volume
must be measured in terms of mls and the time must be in terms of hours.
If the flow rate is in microlitres per second, the time will be in seconds, and the volume
will be put into the formula in microlitres. If the flow rate is in terms of drops per
minute, the volume would have to be in drops. Mls and drops are both ways to measure volume,
but they are still different units. To have flow rates in drops per minute, you would
have to convert the mls value to a value of drops before you put it in. And likewise,
the time needs to be in minutes, so if you have time in hours, you would have to convert
it before you put it into the formula.
The formula, as it is, gives you a way to calculate the flow rate if you already have
the volume and the time. However, you can calculate any of the three factors as long
as you have the other two. The formula can be rearranged so that you calculate either
the volume or the time, if these are the missing quantities. The triangle on the left hand
side of the screen may help you derive any other helpful formulae. We can see how flow
rate can be achieved using volume on time. If we want to calculate volume and we are
given the flow rate and the time, we would multiply these together… And if we need
to find the time after we are given the volume and flow rate, then time equals volume divided
by flow rate. Let’s put this formula to use. Here we have
a problem whereby we know the volume of the liquid to be infused into the patient and
we know the flow rate. The missing factor here is the time, the volume is 100 mls, and
the flow rate is 25 mls per hour, and the formula for time is volume divided by flow
rate. Plugging those values in, 100 mls divided by 25 mls per hour, the mls will cancel.
Notice here we have a fraction within the fraction and in that the 25 ml per hour is
in the denominator. So the denominator of the denominator, that is the hours here, gets
flipped up and multiplies out the numerator. 100 divided by 25 is 4 and the unit now at
the top is hours. In our next problem, we need to find the time again, but this time
we are given the volume in mls and the flow rate in drops per minute. What is more, we
for the time in terms of hours. Unfortunately, we cannot have hours and minutes in the same
formula, so we will have to get our answer for time in minutes first, and then at the
very end we can turn the minutes into hours, if that’s what’s needed. Similarly, the
mls and the drops cannot exist together in the same formula, so one of them will have
to change. We could turn 100 mls into 2000 drops, or we could change the 10 drops into
half a ml. Either of these will bring us to the same answer.
I will turn the 10 drops into mls, so we have mls in the formula. This allows for the mls
to be cancelled. The minutes at the bottom of the denominator get flipped to the top
again, and minutes is clearly the unit that our answer will be in. 100 divided by a half
is 200, that is 200 minutes, and 200 minutes is three-and-a-third of a hour, or three hours
and 20 minutes.