Intraosseous access

The fastest reliable route into the circulation, and the one we reach for last

Written because a medical student wrote in to say they had never seen one done, in an adult or a child. Which is common, and is rather the problem — see You asked.

A trained person can be in the marrow in under a minute, in almost anybody, almost anywhere. The commonest error is not doing it badly. It is doing it late.

Marrow is a non-collapsible venous plexus draining into the central circulation. It does not shut down in shock the way peripheral veins do, which is precisely why it works when nothing else will. Anything you can give intravenously can go through it, at broadly the same doses and with a comparable time to effect.

When to reach for it

The rule of thumbTwo failed cannulation attempts, or ninety seconds without access, in a patient who needs access now. Cardiac arrest, shock, status epilepticus, a collapsed infant. If you find yourself on the third attempt at a vein, you have probably already left it too long.
In childrenReach for it early. Peripheral access in a shocked infant is genuinely difficult even for people who do it often, and the interval spent trying is the interval not spent treating.
In adultsMarkedly under-used, largely because a generation trained before it was routine. The technique and the indications are the same; only the needle length and the sites differ.
Not a long-term lineIt buys you time. Replace it with definitive access when the patient is resuscitated, and remove it — the usual figure quoted is within 24 hours.

Where it goes

Proximal tibia

The default in children and a good adult site. About 1–2 cm below and 1 cm medial to the tibial tuberosity, on the flat antero-medial surface. In small infants aim slightly lower and stay clear of the growth plate.

Proximal humerus

Adults. Arm adducted, hand on the abdomen, insert into the greater tubercle about 1 cm above the surgical neck, angled 45° caudally. Higher flow rates than tibial, but it dislodges easily if the arm moves — secure it.

Distal tibia / distal femur

Distal tibia: 2 cm proximal to the medial malleolus. Distal femur is a paediatric alternative, 1–2 cm above the patella and slightly medial, with the leg straight.

Where it must not go

A fractured bone, or one proximal to a fracture. A bone already attempted within about 48 hours — the previous hole leaks. A prosthesis or previous orthopaedic hardware in that bone. Infection, burn or cellulitis over the site. And any bone where you cannot confidently identify the landmarks: an IO in the wrong place is worse than no IO.

Doing it

Choose the needle by soft tissue depth, not by age. Most systems colour-code by length — short for small children and thin limbs, standard for most adults, long for the humerus or an obese limb. You need at least one black line visible above the skin when the tip touches bone; if none is visible, the needle is too short.
Identify the landmark and clean the skin. Ninety degrees to the bone surface, except the humerus.
Advance until you feel the loss of resistance. With a driver it is a distinct give as you enter the marrow cavity. Stop there. Do not advance to the hub.
Remove the stylet and connect a primed extension set.
Confirm. The needle stands unsupported. Marrow can often be aspirated, though not always. And crucially —
Flush, firmly, with 5–10 mL of saline. This is not optional. The marrow is a matrix, and until you disrupt it nothing will flow. An IO that will not run has usually not been flushed properly.
Secure it, and write down where it is. Handover forgets IOs remarkably often.

The thing nobody warns you about

It hurts. Insertion is tolerable; the flush is severe, and in a conscious patient it is one of the more painful things we do. If the patient has any awareness, give preservative-free lignocaine into the IO before the saline flush, let it dwell for a minute or so, then flush. Doses differ between adults and children and between local protocols, so use your own guideline — but do not skip it because the situation is busy. This is the step most often omitted and most remembered by patients.

What can go through it

Essentially everythingCrystalloid, blood and blood products, adrenaline, amiodarone, antibiotics, anaesthetic induction agents, inotropes, contrast for CT. Same doses as intravenous.
But not by gravityMarrow offers real resistance. You need a pressure bag, a syringe and three-way tap, or a pump. If it is running slowly, the answer is usually pressure rather than a new site.
Bloods, with caveatsA marrow sample will give you a usable gas, lactate, glucose, haemoglobin and cross-match in most laboratories. Potassium reads high, platelets low, and some analysers refuse marrow altogether — tell the lab what it is.

Complications, in the order you will meet them

Extravasation is by far the commonest, and follows a needle that has gone through the far cortex or been dislodged. Check the calf or the limb circumference during any pressurised infusion. Rarely, that extravasation causes compartment syndrome, which is the complication that does real harm. Fracture and growth plate injury are avoidable with landmarks and the right needle. Osteomyelitis is genuinely rare — well under one per cent — and rarer still when the device comes out within a day.

Challenge for you

Have you ever put one in — and if not, do you know where the driver and needles are kept in your department right now?

The second half is the one that matters. A device you cannot find in ninety seconds is a device you do not have. Go and look, then tell me how long it took and whether the needles were the right sizes.

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