Intraosseous (IO) means “into the bone.” In medicine, it refers to a method of delivering fluids, medications, and blood products directly into the marrow cavity of a bone, where they rapidly enter the bloodstream. The technique is used primarily in emergencies when a standard IV line can’t be placed quickly enough. Because bone marrow contains a network of tiny blood vessels that don’t collapse even during severe dehydration or shock, an IO line can succeed when veins have effectively shut down.
How Intraosseous Access Works
Inside every bone is a spongy core of marrow laced with small blood vessels. Unlike veins in the arms or hands, these vessels sit within a rigid structure that holds them open regardless of blood pressure or hydration status. When a needle is inserted through the hard outer shell of a bone and into this marrow space, anything infused there drains into the central bloodstream within seconds. The effect is essentially the same as a traditional IV: medications reach the heart and brain just as quickly.
Nearly everything that can go through a standard IV line can also go through an IO line. That includes fluids for resuscitation, emergency cardiac drugs, antibiotics, sedatives, and blood products. Flow rates depend on whether a pressure bag is used. Without external pressure, flow rates typically range from about 16 to 53 mL per minute depending on the device and insertion site. With a pressure bag inflated to 300 mmHg, rates can reach 60 to over 100 mL per minute, which is fast enough for aggressive fluid resuscitation.
When It’s Used
IO access is a backup plan, not a first choice. It’s reserved for situations where a peripheral IV can’t be established quickly, most commonly during cardiac arrest, severe trauma, shock, or seizures that won’t stop. In these scenarios, patients often have veins that are collapsed, too small to find, or otherwise inaccessible. Paramedics, emergency physicians, and military medics rely on IO access when every second of delay matters.
The technique is used in both adults and children. It’s particularly valuable in infants and small children, whose tiny veins can be extremely difficult to access even under calm conditions. In a chaotic emergency, IO access provides a reliable alternative.
First-Attempt Success Compared to IV
One of the strongest arguments for IO access in emergencies is reliability. A 2025 systematic review in the journal Critical Care found that IO lines had a first-attempt success rate of 92.3%, compared to just 62.3% for standard IV access during out-of-hospital cardiac arrest. IO access was also established roughly 15 seconds faster on average. That said, the same review found no significant difference in survival or neurological outcomes between the two approaches, suggesting the speed advantage doesn’t always translate into better results on its own.
Where the Needle Goes
Several bones in the body can be used for IO access, but a few sites are preferred because they’re easy to locate and have a flat surface for stable needle placement.
In adults, the most common sites are the upper shinbone (proximal tibia), the upper arm bone near the shoulder (humeral head), and the breastbone (sternum). In infants and young children, the preferred sites are the lower thighbone near the knee (distal femur) and the shinbone, both near the knee and near the ankle.
These bones are chosen because their landmarks are easy to identify by touch, even on patients who are unconscious or in distress. The flat surface of the shinbone just below the knee, for example, is one of the most commonly used sites in both adults and children because it’s accessible and relatively thin.
What the Procedure Feels Like
For unconscious patients, like someone in cardiac arrest, pain isn’t a concern during insertion. For conscious patients, however, IO insertion and especially the infusion of fluids into the marrow space can be quite painful. The inside of bone is richly supplied with nerves. Medical teams typically inject a local numbing agent into the marrow before running fluids to reduce the discomfort. Even so, patients who are awake often describe a deep pressure or aching sensation at the insertion site during infusion.
The needle itself is inserted using either a battery-powered drill, a spring-loaded device that punches through the bone’s outer layer, or a manual needle that’s twisted in by hand. The powered drill is the most widely used in modern emergency settings because it’s fast and consistent. The entire insertion takes only a few seconds.
When IO Access Can’t Be Used
There are specific situations where placing an IO needle at a given site is not safe. These include a fracture in the same bone, significant vascular injury to that limb, skin infection (cellulitis) over the insertion site, and a previous orthopedic procedure or another IO line placed in the same bone within the past 24 hours. In these cases, a different bone or a different access method is used instead.
Complications
Serious complications from IO access are rare. A review of prehospital cases found that long-term problems like bone infection (osteomyelitis), bone tissue death, or compartment syndrome (dangerous pressure buildup in the surrounding muscle) occurred in fewer than 0.1% of cases. More immediate but still uncommon issues include the needle ending up in the wrong position, fluid leaking into surrounding tissue instead of the bloodstream, and minor fractures at the insertion site. Growth plate injury is a theoretical concern in children, though it’s rarely reported.
IO lines are meant to be temporary. Once the emergency is stabilized and a standard IV can be placed, the IO needle is removed. Most guidelines recommend keeping an IO line in place for no longer than 24 hours to minimize infection risk.