What causes veins to collapse when an IV is administered?
Veins are characterized in their anatomy by three layers:
1. The intima or inner lining with endothelial cells.
2. The muscular layer, essentially the middle in a cross-section of the vessel.
3. The externa or outer covering.
The endothelial lining is supplied with nerves as is the muscular layer. If the intima is stimulated by mechanical, thermal or chemical means the muscular layer will contract. The degree to which this occurs depends on the strength and duration of the stimulus, the nature of the stimulus, the age of the vein and other factors.
When a vein is cannulated with a needle and catheter apparatus (an angiocath), such as occurs during placement of an "IV' the entire wall thickness is penetrated with a needle. This causes the muscular layer to contract collapsing the vein.
The age of the vein, its anatomic location, its size and any associated coexisting disease(s) contribute to the extent and duration of this reaction.
Very small veins in older persons tend to collapse much more readily than those of younger folks. The technique used in placing the "IV" or catheter affects to a significant extent whether the vein will collapse when punctured by the needle. This is particularly true if the needle punctures the vein through and through as opposed to one wall with successful cannulation of it.
If the vein leaks blood and/or IV fluid it tends to collapse. Also, if the IV fluid is an irritant, which many drugs are to the endothelial lining, the vein will contract and collapse more readily. An infiltrated or "leaking IV" will also be more prone to collapse.