A little info on sub vs im
From a medical text:
Why reinvent the wheel? I’ve been lazy here and have attached this information as written for my textbook. It is probably more detailed than you require but I am sure it will give you all the answers you need.
Subcutaneous injections The blood supply to the subcutaneous tissue is poor, so absorption of an injected drug will be relatively slow.This is often an advantage with drugs that cannot be given by mouth.An example is the protein insulin, which would be digested if given orally; when injected intravenously, the resultant fast action is not always desirable. Absorption rate of drugs given by subcutaneous injection can be slowed down further by incorporating adrenaline in the injection. Adrenaline promotes vasoconstriction, which decreases the distribution of the injected material. This vasoconstriction will also decrease bleeding when adrenaline is injected with a local anaes-thetic for minor surgical procedures. Conversely, if the enzyme hyaluronidase is added to a subcutaneous injection, the tissue cement hyaluronic acid (which helps cells to adhere to each other) is destroyed, enabling the other drug to diffuse into the tissues. Sustained effects can be achieved, using subcutaneous injections, by dissolving the drug in a slowly dispersible oil or by implanting a pellet containing the drug in the tissues. Steroid hormones used for contraception or for treating menopausal symptoms are sometimes given this way. Subcutaneous injections are useful when other routes may be hazardous, as is the case with heparin.When injected into a muscle, heparin, being an anticoagulant, could lead to intramuscular haemorrhage producing a painful haematoma. Tablets, especially some of the sex hormones, can be implanted into subcutaneous tissues for prolonged action. Table 7.14 contains information about the administration of medications by the subcutaneous route. Intramuscular injections Skeletal muscle is highly vascular, and its capillaries contain small pores that enable substances of small molecular weight to pass through into the bloodstream. Lipid-soluble drugs are taken up rapidly by direct diffusion through the capillary walls. Substances of high molecular size, which are lipophobic, can be slowly absorbed into the lymphatic system. Several muscles of the body have considerable mass and are able to be injected with quantities of up to several millilitres of fluid, generally without undue discomfort to the patient.The gluteus medius of the buttocks is the best muscle to use in this respect. The deltoid muscle of the upper arm has a richer blood supply than the gluteus muscle so is good for rapid absorption of many drugs, but its size limits the injectable amount to about 1 mL. Intramuscular injections are not always given for quick action; if the drug is mixed with an oil such as peanut oil, the oil is not absorbed rapidly from the injection site.The drug thus diffuses slowly from the oily solution into the muscle’s capillaries. This can take a few weeks to occur. This type of injection is known as a depot injection. Exercise, which causes an increase in skeletal muscle blood flow, improves absorption of a drug after intramuscular injection.This was demonstrated by a footballer who was given a phenothiazine, an antipsychotic agent (see chapter 33), as a depot injection and who subsequently suffered from a fairly serious adverse reaction, due to rapid absorption of the drug, brought about by increased muscularThe main danger from intramuscular injection is damage to nerves, especially in the case of gluteal injections, as the large sciatic nerve passes through this region. Knowledge of anatomical positions of major nerves and blood vessels is necessary in order to avoid irreparable damage or injection into these structures. Apart from pain and irritation to tissues, sterile abscesses can occur with intramuscular injections. Not all intramuscular injections act faster than using enteral routes; for example, diazepam (see chapter 34) is faster-acting when given rectally or orally.Table 7.15 contains information about the administration of medications by the intramuscular route.
Regards
Dr Alan Galbraith
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