1. When the pet's anxiety level at being handled by a stranger is such that it would cause undue stress to restrain the pet so that he is immobilized for long enough to allow the placement of the i.v. catheter. The most common example of this is encountered in fractious cats.

2. When the pet's level of pain is such that having to move him or her in a position that will allow the placement of the IV catheter will cause too much pain. Examples of this are spinal pain, or diseases causing hyperesthesia (the exacerbation of pain).

3.  When the condition of the pet causes dehydration, emaciation or poor perfusion (poor circulation) which may make it difficult to find a vein and which may subject your pet to more than one poke of the needle.

4.  When the pet's condition causes him or her anxiety and restlessness and when he or she is not able to rest in a lying down position comfortably.

5.  In my experience, cats ALWAYS do better with the pre-euthanasia sedation except in some rare cases where the pet's condition is so advanced that he or she is already semi-conscious.

Other than that, it is usually preferable to limit the needles to simple placement of the IV catheter. The main question to answer is: which will cause the pet the least discomfort? The restraint needed to place the IV catheter or the discomfort related to injecting a substance under the skin?  Another factor to consider is the mental disorientation that accompanies the sedative.  The sedative takes anywhere between 2 minutes and 20 minutes to take into effect depending on individual sensitivity to sedatives and the condition of internal organs.  During the period of time elapsing between the injection and the time when the sedative has taken full effect, pets may experience greater or lesser mental disorientation and discomfort (you may have experienced this if you have ever received a sedative).  The reactions vary.  Most pets appear peaceful, but some don't like the sensation during the period of time between the injection and the time when they are fully under the effect of the sedative, regardless of the kind of drug used. Once the drug has taken full effect, the pet will be sleeping, peaceful and for all intents and purposes, out of consciousness and out of pain.

You can rely on Dr Forslund's experience in evaluating the situation and your pet's condition, level of discomfort and level of anxiety and to take the best approach with will cause your pet the least discomfort and stress.  You may also want to discuss this with your primary veterinarian whose insights may be invaluable, since he or she has known your pet for years.