If a blood pressure cuff is used as a tourniquet and the diastolic blood pressure is unknown, to what level should the cuff be inflated?

Inflating the blood pressure cuff to a level of 40 mm Hg is appropriate when the diastolic blood pressure is unknown because it effectively occludes venous return without compromising arterial blood flow significantly. This pressure is sufficient to allow for engorgement of the veins, making them more prominent and easier to access for venipuncture. Choosing an inflation level lower than 40 mm Hg may not adequately restrict blood flow to the veins, while inflating it too high could risk pain or discomfort to the patient and may even cause complications such as arterial occlusion. Thus, 40 mm Hg strikes a balance, ensuring that the necessary venous access can be achieved safely and comfortably.