October 9, 2013

The Physic of Flow in Relation to Intravenous Access’s Size Selection.

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In emergency setting, the crucial part in managing patient is delivering of fluids or medication through the Intravenous (IV) Access. Many factors will influence the selection of the IV size to serve the purpose of the treatment.

In order to select the proper size of IV access, we need to understand the physic of flow. From the physic, the Hagan-Pouseille equation is derived.

The Hagan-Pouseille equation
To understand the derived formula, we need understand about the concept of flow. It is being defined as the quantity of the fluid that passes a point per unit. The fluid may also exist in form of gas, liquid or vapor.

Flow: (F) = Quantity (Q) / Time (t)

Or also known as Rate of change of mass or volume (∆Q)

There are two type of flow which are 1) Laminar, and 2) turbulent.

In case of flow through IV access, it is represented by the laminar flow and hence will be discussed here. Laminar flow occurs when fluid flows through a tube and the rate of flow is low.

In laminar flow, the molecule of fluids will be moving in a numerous layer with different velocity.

If the mean velocity of the flow is v, then the molecules at the center of tube are twice the velocity (2v) and the flow of molecules at the side of the tube is stationary.

In order for the flow to take place, we need a different pressure gradient (∆P) between the ends of a tube. The greater the pressure gradient, the greater flow would be.

Resistance will occur between the fluid and the opposing vessel wall against the flow. This resistance is constant.

Resistance (R) = ∆P / ∆Q

Factors affecting the flow

1) Tube diameter.

a) Flow is directly proportionate to d^4. If the diameter of the tube is halved, then the flow is reduced to 1/16.

b) This will explain why fluids flow more rapidly through the large bore IV Access and size 14, 16 and 18 gauze branulla is selected during resuscitation.

2) Length

a) If the length is doubled, then the flow is halved. Flow is inversely proportionate to the length of tube.
b) This explains why the fluids flow slower through central line compared to peripheral IV access even though the internal diameter is similar.
3) Viscosity
a) As the viscosity increases, the flow decreases proportionately.

b) Due to this fact, I prefer crystalloid during the initial phase of resuscitation of trauma patient followed by colloid and blood product.

4) Pressure gradient
a) As being explained before, the greater the pressure gradient, the greater flow would be.
b)  The higher the height of the gravity system, the greater flow would be.
c) Manually compressing the lower drip chamber and usage of pressure bag will increase the flow of fluid during the resuscitation.

5) Multi lumen extension
If you attach the multi lumen extension into the large bore IV branulla and run multiple fluids spontaneously, it will reduce the flow up to 76%.

IV Vascular Access in Emergency Department

Pediatric intraosseous line for difficult IV Access in pediatric group. 

EZ-IO Drill and Needle.

Second Generation Bone Injection Gun (B.I.G)

A: Triple Lumen Central Venous Line, B: Single Lumen Central Venous Line, C: Single lumen Central Venous line

24 gauze branulla for neonate age group. It provides about 23 ml/min fluids.

22 Gauze branulla for pediatric size. It provides about 36 ml/ min fluids
20 gauze size branulla. It provides about 61 ml/min fluids

18 gauze size branulla. It provides about 96 ml/min fluids

16 gauze size branulla. It provides about 196 ml/min fluids
14 gauze size branulla. It provides about 300 ml/min fluids
14 gauze size Angiocath. It provides about 210 ml/min fluids. Note that even though the internal diameter is 14 gauze similar to the branulla, but the length is more than branulla, hence reducing the flow of the fluids.
In short, the selection of IV Access

a) Large bore branulla for pediatric size is 22, 20 and 18
b) Large bore branulla for adult size is 18, 16 and 14
c) Most of the antibiotics are irritant to the vessel; hence selecting the smaller size branulla will reduce the complication.
d) Central venous access may be obtained if failed to get peripheral line. It is preferable to use angiocath rather than triple lumen/short line or long line


D.M Levine, A.L Garden, H.T Truong et al, "Influence of multi-lumen extensions on fluid flow through intravenous cannulae", Anaesthesia, The Association of Anaesthetists of Great Britain and Ireland, 2013

M. D. S TONEHAM, "An evaluation of methods of increasing the flow rate of i.v. fluid administration", British Journal of Anaesthesia 1995;75: 361–365.

Anesthesiology note.

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