Stephen Dolphin wrote

>That being said, the study in Pre-Hospital Immediate Care this issue says
we
>can infuse a bag of Hartmanns
(saline actually)
>through a 16g cannula in 72 seconds if we
>squeeze the bag manually, which is pretty quick. With the trend towards
>controlled hypotension, this will probably be quite quick enough to dribble
>through the teacupful we will now be giving. :-)
>
>Because when they get to A&E, and they start pouring in the blood, I'm sure
>it will go in much faster through a 14g than an 18g, blood being thicker
>than water. All the flow rates for cannulae are given for water.
>
Not in the studies that have been published. The one in PHIC used normal
saline. Stoneham (2) used crystalloid and colloid, flow rates were affected
as predicted by the viscosity of the fluid as predicted by that law which I
cannot remember. As blood is thicker than water (and the others) I agree
that flow rates will probably be less.

Also, the flow rates printed on the cannula may be widly different from
those possible in clinical practice (see PHIC paper)

>I believe there may also be the question of damage to blood cells by
>turbulence and high pressure in the small cannula. I wonder if there has
>been a study of that?

No, this was identified as a problem in the PHIC study. It would be an
interesting project for someone. You would need a lot of time expired blood
and a friendly heamatologist but this is indcreasingly rare thesedays (the
blood I mean!).

I have included the abstract from Stonehams paper on flow rates which
answered many of the questions already raised (reference 2):
different size cannula.
different methods of speeding infusion
different fluids
fluids in different containers.
Those really interested in the subject would find reading the paper
worthwhile.

Reference 1 looked at the effect of the giving set.

Simon

Simon Carley
SpR in Emergency Medicine
Hope Hospital
Salford
England
[log in to unmask]

1. Stoneham MD.
Title
Factors affecting flow through blood administration sets.
Source
European Journal of Anaesthesiology. 14(3):333-9, 1997 May.
Abstract
Factors affecting flow through blood administration sets in vitro were
assessed under gravity-fed and pressurized conditions including an
assessment of the influence of the intravenous (i.v.) cannula and Luer lock
fitting. The fastest gravity fed flow of 4.775 mL s-1 was obtained through
the largest internal diameter (ID = 4.8 mm) blood administration set. Flow
through blood administration sets with ID = 3 mm was approximately 50% of
this. Flow increased over base-line through all the administration sets when
the i.v. cannula was removed (range 18-50%) and increased further over
base-line when the Luer lock fitting was removed from the distal end (range
26-129%), indicating that these are rate-limiting steps in the system. The
Y-type trauma set with the largest diameter tubing facilitated the fastest
flow, although flow through all the Y-type trauma sets produced lower flow
rates than the corresponding blood administration sets, which may reflect
their relative increased length. The ideal blood administration set should
have an internal diameter at least 4 mm and be less than 170 cm in length.



2. Stoneham MD.
Title
An evaluation of methods of increasing the flow rate of i.v. fluid
administration.
Source
British Journal of Anaesthesia. 75(3):361-5, 1995 Sep.
Abstract
I have evaluated in vitro methods of increasing the flow rate of clear
fluids through an i.v. cannula at room temperature. These included, alone
and in combination: increasing the height of a gravity-fed system;
increasing the i.v. cannula diameter, manual compression of the lower drip
chamber and the use of pressure bags. Flow rate was measured using a
uroflowmeter, which was found to be reliable and reproducible. The most
effective methods of increasing flow were the use of a 14-gauge cannula
rather than a 16-gauge cannula, which resulted in a 50% increase, and the
use of a 300-mm Hg pressure bag with automatic adjustable pressure
regulator, which doubled the flow rate. The combination of these two tripled
the overall flow to nearly 600 ml min-1. Manual compression of the drip
chamber, despite producing peak pressures of more than 100 cm H2O, was an
inefficient method of improving flow compared with an external pressure bag.






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