The lateral decubitus position is simply lying on your side. The term comes from Latin: “lateral” means side, and “decubitus” means lying down. In medicine, the position is specified by which side is down. Left lateral decubitus means lying on your left side, and right lateral decubitus means lying on your right. While it sounds straightforward, this position plays a surprisingly important role across many areas of medicine, from diagnostic imaging and heart exams to surgery and pregnancy care.
How the Position Is Described
When a healthcare provider asks you to get into a lateral decubitus position, they’ll specify which side. You lie with one side of your body against the bed or table, knees slightly bent, and your arms positioned so they’re not pinned beneath you. The side touching the surface is called the “dependent” side, and the side facing up is the “nondependent” side. This distinction matters because gravity pulls blood, air, and fluid differently depending on which side is down.
Why It Matters for Breathing and Blood Flow
Gravity has a significant effect on how air and blood distribute through your lungs when you’re on your side. In an awake person, the lower (dependent) lung receives more blood flow and more ventilation, so the balance between air and blood remains similar to what it is when you’re standing upright. This is one reason the position is well tolerated for most people.
Under general anesthesia, the picture changes. Anesthesia reduces the muscle tone in your diaphragm and chest wall, causing a 15 to 20 percent drop in the amount of air your lungs hold at rest. This shifts ventilation toward the upper lung, which becomes easier to inflate, while blood flow still favors the lower lung. The result is a mismatch: the upper lung gets air but less blood, and the lower lung gets blood but less air. Anesthesiologists account for this during surgery with specific ventilation strategies.
Uses in Diagnostic Imaging
Chest X-rays taken in the lateral decubitus position are a classic tool for evaluating fluid in the chest. When you lie on the affected side, free-flowing pleural fluid shifts with gravity and spreads along the chest wall, making even small collections visible. An upright chest X-ray can miss smaller amounts of fluid because it pools at the base and blends with the diaphragm’s shadow. In children and infants, the position also helps distinguish actual lung infections from blood vessels that can mimic pneumonia on a standard film. It’s useful for detecting air leaks (pneumothorax) and evaluating whether air is trapped in one lung.
Listening to the Heart
Cardiologists and other clinicians use the left lateral decubitus position during heart exams. Lying on your left side shifts the heart closer to the chest wall, which makes certain quiet sounds easier to hear through a stethoscope. The low-pitched rumbling murmur of mitral stenosis, a condition where one of the heart’s valves narrows, is sometimes only audible when the patient rolls onto their left side. The same position can make faint third and fourth heart sounds (extra beats that signal heart strain) more detectable.
Spinal Taps and Fluid Pressure
For lumbar punctures (spinal taps), the lateral decubitus position is the standard. You curl up on your side with your knees drawn toward your chest, which opens the spaces between the vertebrae and gives the needle a clear path. Equally important, all the normal reference values for cerebrospinal fluid opening pressure were established with patients in this position. Measuring pressure in a different position, such as sitting upright or lying face down, produces different numbers that don’t match the established norms, making accurate diagnosis harder.
Surgery and Procedural Applications
Many surgeries require the lateral decubitus position because it gives the surgeon direct access to one side of the body. Shoulder arthroscopy is commonly performed this way, with the affected arm suspended and gently pulled to open the joint space. Procedures done in this position include repairs for shoulder instability (both front and back), torn cartilage in the shoulder socket, rotator cuff repairs, and release of frozen shoulder. Beyond orthopedics, the position is widely used for kidney surgeries, operations on the chest cavity (thoracotomy), and certain spinal procedures.
Colonoscopy Starting Position
If you’ve had a colonoscopy, you were most likely placed on your left side to start. The left lateral decubitus position is the conventional starting point for the procedure because it aligns the anatomy of the lower colon favorably for scope insertion. Easier insertion means less time pressure during the examination, potentially allowing the doctor more time to carefully inspect the lining on the way out, which is when most polyps are found.
Pregnancy and Blood Flow
In later pregnancy, the growing uterus can press on the large vein that returns blood to the heart (the inferior vena cava) when you lie flat on your back. This compression reduces the amount of blood flowing back to the heart, which in turn lowers cardiac output, the total volume of blood your heart pumps per minute. The drop can cause dizziness, nausea, and low blood pressure in the mother, and may reduce blood flow to the placenta.
Lying on the left side shifts the uterus off these major blood vessels, restoring normal venous drainage. This is why pregnant women in the second and third trimesters are often advised to sleep on their side. Newer monitoring technology that tracks cardiac output in real time has confirmed that the positional drop in blood flow is a reliable marker for vena cava compression, and that simply rolling to the left side can reverse it.
Nerve Injury Risks During Surgery
When a patient is under anesthesia in the lateral decubitus position for an extended surgery, several nerves are vulnerable to pressure injury. The body’s weight rests on a relatively small surface area, and the patient can’t shift or report discomfort.
The brachial plexus, the nerve bundle running from the neck through the shoulder into the arm, is at risk in the lower arm. Surgical teams place a chest roll (a firm pad) under the ribcage of the down side to lift the body slightly and take pressure off the shoulder. About 87 percent of anesthesiologists surveyed by the American Society of Anesthesiologists agree this step reduces the risk of brachial plexus injury in the lower arm.
The peroneal nerve, which wraps around the bony knob just below the outer knee, is vulnerable wherever the leg presses against a hard surface. Padding between and beneath the knees is standard practice. The ulnar nerve at the elbow is also at risk, occurring in roughly 0.1 percent of patients positioned laterally even with a padded armboard. Padding at the elbow and careful arm positioning help minimize this.
Preventing Pressure Injuries in Bed
For patients who spend long periods in bed, whether in hospitals or care facilities, a modified version called the 30-degree lateral tilt is recommended over a full side-lying position. Lying at a steep 90-degree angle concentrates body weight directly over the hip bone, increasing the risk of pressure sores. Tilting just 30 degrees redistributes pressure and shear forces away from the sacrum (the base of the spine) and the hip, which are the most common sites for skin breakdown.
International pressure injury guidelines from the National Pressure Ulcer Advisory Panel and its European and Pan Pacific counterparts endorse this 30-degree tilt as a core prevention strategy. The challenge is maintaining it. Without proper support, patients tend to roll into a flatter or steeper position between turning schedules, reducing the benefit. Purpose-designed positioning devices and pillows help maintain the angle consistently.