Why aren't drugs delivered directly into arteries, rather than veins?

Question

Normally when drugs are administered to the blood of a mammalian subject, it is done intravenously--the needle being inserting into a vein. One potential problem with this is that the veins contain the exhausted blood which is returning to the liver and spleen. This is potentially disadvantageous because these organs could absorb the drug or be damaged by it.

It would seem more advantageous to introduce a drug to the arterial system, such as the aorta, so that the drug flows outwards to all parts of the body before returning to the filtering organs of the body.

Is this type of drug administration a known procedure. If so, why is not more generally used?

Answer 1

Although this is a fairly basic question, I realized three very active users here put our answers in comments. Let me summarize:

1) It is risky to poke holes in any artery, let alone the aorta. Arteries are full of blood under high pressure. When you poke a hole in one, there is always some risk of serious bleeding. You also have to push against the arterial pulsatile pressure, making it more difficult to administer slow drips of drugs, as is often done with I.V. administration: either you are going to get high flow during diastole with high pressure, or you are going to get backflow of blood into the catheter during systole, or both. On the venous side, there is a lot more collateral circulation, too, so in an emergency you can usually close most veins without severe effects. Some drugs can be toxic to the local vascular environment, as well, which is again safer in a vein should it occur. Note that for some special procedures, such as angioplasty/stenting, arterial access is necessary, but this isn't true for drugs. (Thanks @Chris and @anongoodnurse)

And the aorta? Depending on body fat and where on the aorta you want to target, you are talking about a needle inches long, poking into a vessel you can't see, after passing other organs and tissues, versus veins, some of which are specially located near the surface.

2) IV drugs aren't "delivered" specifically to particular targets in the first pass: they dissolve in the blood, flow around the circulation, diffuse in and out of tissues, and eventually partition according to their binding affinity, hydrophobicity, etc. Therefore, drugs are going to go through the liver anyways. In fact, many drugs are metabolized by the liver, and some aren't even delivered in their final active form: metabolism may be required for activity, and often the liver is involved. Essentially, if a molecule isn't stable enough to make a pass through the liver, it won't be a useful drug or needs to be given in a higher dose to account for the high rate of metabolism. (thanks @Chris and @anongoodnurse)

3) Further, most IV drugs are given through easily accessed veins such as in the arm: these return to the heart before any pass through the liver or spleen. Unless you were trying to deliver the drug to the arterial circulation of that particular hand, it's going to circulate before it gets to the target organs. And if your drug is going to damage the liver, why don't you think it's going to be dangerous for anything else?

Answer 2

A couple of things to add:

1. (Most important) The 'exhausted blood' in the veins isn't going directly to the liver or metabolic organs - it's going directly to the heart, to be pumped through the lungs and then returned to arterial circulation. First-pass metabolism by the liver is a concern for oral delivery, since a substantial fraction of blood goes directly from the absorptive digestive tissues (e.g. intestines) to the liver through (IIRC) the hepatic portal circulation. This isn't an issue for drugs administered IV.

2. In addition to the relative safety of venipuncture vs. arteriopuncture, there's also the small point of pain. Arterial puncture is done when necessary - for example, when you need a sample for blood gas analysis. This procedure is quite painful. I have had it performed on me, and I have performed it on animals when I worked in veterinary practice. So in addition to venipuncture being safer, it's also much more comfortable for the patient/subject.

3. What you describe in your last paragraph is kind of done, although not into the aorta. When large boluses of drugs need to be given frequently, and/or the drug in question might seriously irritate peripheral veins, you sometimes see devices like a PICC line used - a percutaneous indwelling central catheter - or for shorter durations a temporary jugular central line. These are used to deliver the drug into the vena cava - the venous analog of the aorta, if you will. Just as in conventional IV administration the blood goes through the heart, through the lungs, and back into arterial circulation, but this time it's administered in a large enough volume of blood to avoid adverse effects of dilution (large/fast bolus) or irritation.