Читать книгу Transfusion Medicine - Jeffrey McCullough - Страница 92

Usually the blood container is placed on a scale, which may have a device to cut off the flow when the container reaches a set weight indicating that the desired volume of blood has been collected. The blood must flow freely and be mixed with anticoagulant frequently as it fills the container to avoid the development of small clots. Blood banks often use mechanical devices that continuously mix the blood and anticoagulant during phlebotomy. No more than 15% of the donor’s estimated blood volume should be collected, and the limit of 10.5 mL/kg body weight [1] is intended to meet this limit. In addition, the volume of blood in the container should be between 405 and 550 mL (i.e., 450 or 500 mL ± 10%). Thus, including specimens for testing, the amount of blood drawn could total 575 mL. Units containing 300–404 mL can be used for transfusion but must be labeled as low‐volume units. The amount of blood withdrawn must be within prescribed limits to be in the proper ratio with the anticoagulant, otherwise the blood cells may be damaged or anticoagulation may not be satisfactory (see Chapter 5).

The actual time for phlebotomy and bleeding is usually about 7 minutes and almost always less than 10 minutes. If the blood flow is slow, clots may form in the tubing before the blood mixes with the anticoagulant in the container. Although there is no FDA‐defined maximum allowable time for the collection of a unit of blood, most blood banks establish a maximum, usually no more than about 15 minutes. There is no difference in factor VIII or platelet recovery between units collected in less than 8 minutes versus those collected in 8–12 minutes [51]. Extremely rapid, pulsatile blood flow or the appearance of bright red blood may indicate an arterial puncture. This can be confirmed by feeling the pressure building in the blood container. An arterial puncture is nearly unmistakable because of the very rapid filling and pressure that develops in the blood container.

During blood donation, there is a slight decrease in systolic and a rise in diastolic blood pressure and peripheral resistance, along with a slight decline in cardiac output but little change in heart rate [52]. The regional cerebral oxygen saturation decreases significantly but still remains within the range of individual physiologic variation, while the cerebral tissue hemoglobin concentration increases significantly, probably because of an increase in cerebral blood volume, which appears to be the major compensation mechanism during acute blood loss to maintain cerebral oxygenation [53].

At the conclusion of blood collection, the needle is removed and the donor is asked to apply pressure to the vein in the antecubital fossa for at least 1 or 2 minutes. Many blood centers have a policy of asking the donor to raise his or her arm to minimize the venous pressure while pressure is applied to the vein. When there is no bleeding, discoloration, or evidence of a hematoma at the venipuncture site, the donor should be evaluated for other symptoms of a reaction to donation. If none is present, the donor can move off the donor table to the refreshment area. The donor should be observed during this time, because the movement into an upright posture may bring on lightheadedness or even fainting.