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Most brands of cannulae come in infant, pediatric, and adult sizes, so select the one that best fits you for your size. The cannulae should fit snuggly in your nostril so that it doesn't come out, but it shouldn't be so long that goes far back into the nostril. Some brands may have extended sizes, such as small, medium, and large for adults, and premature through pediatric for children, giving you more options. In this case, choose the one that feels the most comfortable when placed in your nostrils.  If the cannula is rubbing uncomfortably against your nostrils, it may be too large. If it's pinching, it may not be the correct size. At one end of the nasal cannula tube, you’ll see a connector that hooks onto your oxygen tank or converter. Slip the connector over the end of the oxygen line coming from your tank to connect the nasal cannula. Make sure the connector is seated tightly onto the oxygen line so none of the gas can escape. Place the connection against your arm to feel for any leaking oxygen. You may also be able to smell escaped oxygen. Your healthcare provider will prescribe a flow rate for you. Turn the knob on the machine so that the dial is set to this exact number every time, and do not change it unless your doctor tells you to. Changing the flow rate could result in you getting more or less oxygen than you need, which could lead to health complications. Try using a pulse oximeter to monitor your oxygen. If it's always at 100%, turn your oxygen down a little. Most cannulae today have curved prongs so they fit more naturally inside the nostrils. Hold the cannula so the prongs are pointing up toward the ceiling and curve down toward you. If the cannula does not have curved prongs, just hold it so the prongs are pointed upward and angled toward your body. Place the cannula in as far as it will go. If the cannula is properly connected, you will feel a gentle flow of air. If it's the proper size, the prongs should sit comfortably in your nostrils. Make sure the tubes that connect the nasal prongs to the oxygen delivery system are fitted snugly over your ears. This will help hold the cannula in place, even if you’re moving around or sleeping. If having the tubes looped behind your ears is uncomfortable, you may prefer to rest the headset behind your head. Instead of looping the tubes over your ears and under your chin, turn the cannula so the tubes go from your nostrils down toward the back of your neck, with the slider resting at the base of your neck. The slider is the small piece on the tubes which you can move up and down to tighten or loosen the headset. Push the slider up until it is just under your chin. The headset should be snug, but you still should be able to fit two fingers between the tubes and your chin. If the cannula causes indents in your skin, it is too tight.

Summary:
Make sure you have the right size cannula. Attach the end connector to the oxygen source. Adjust the amount of oxygen that flows through the tubes. Turn the cannula so the prongs are curved downward. Insert the prongs into your nostrils. Lift the tubes and fit them over your ears. Move the slider up the tubes to adjust the fit.