A lumbar puncture is difficult because landmarks tend to be obscured and depth is difficult to estimate. The procedure is most successfully performed with the patient sitting and bent forward. With the patient upright, the midline is easier to estimate and both iliac crests are usually palpable. Bone encountered after only a few centimeters usually represents spinous process and suggests an adjustment in the vertical plane above or below this point. A deeper bony encounter is likely lamina and requires a medial adjustment. Ultrasonography can be used as an aid to locating the vertebra.21 Despite the excessive tissue, the standard 3-in. needle is adequate for many obese patients, although this may require pushing the needle hub to the point of dimpling the skin. The 5-in. needle is sometimes needed. It is best to have both sizes readily available. Tight intervertebral disk spaces are common in this population and may not allow passage of a 16- or 18-gauge needle. It may be necessary to use a much smaller gauge (albeit more flexible) needle as a guide for locating the disk, then a larger-gauge needle to puncture the space. The best choice is a 22- or 24-gauge needle, which allows adequate flow and decreases the likelihood of postpuncture headache.
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