Clinics in haematology最新文献

The plasma volume and total red cell mass are controlled by different mechanisms and pregnancy provides the most dramatic example of the way in which that can happen. A healthy woman bearing a normal sized fetus, with an average birth weight of about 3.3 kg, will increase her plasma volume by an average of about 1250 ml, a little under 50% of the average non-pregnant volume for white European women of about 2600 ml. There is little increase during the first trimester, followed by a progressive rise to a maximum at about 34-36 weeks, after which little or no further increase occurs. It seems certain that the frequently observed fall in plasma volume in the last six weeks of pregnancy is an artefact of measurement due to poor mixing of tracer when the woman lies supine and obstructs the circulation to her lower limbs. The maximum increase depends largely on the size of the conceptus. It is somewhat increased, perhaps to a mean of 1300 ml, in association with the bigger baby of multiparae and increases still more with twins, triplets and quadruplets. Red cell mass increases by relatively much less, a rise of about 250 ml (some 18% of the non-pregnant volume) in women who take no supplemental iron, and between 400 and 450 ml when iron supplements are taken. The rise is probably linear from the end of the first trimester to term, and there is some evidence of a preliminary fall in red cell mass during the first trimester. As a result of the relatively much greater increase in plasma volume, red cells in the blood are 'diluted' and the venous haematocrit drops from a non-pregnant average of about 40 to about 33 during the last trimester. The differential changes are biologically plausible: red cell mass rises proportionately to the need to carry the extra oxygen taken up in pregnancy; the greater plasma volume increment is needed to cope with the very large increases in blood flow to organs which require little extra oxygen, the skin and the kidneys.

Many common genetic disorders of the blood can be identified in utero, either by fetal blood sampling, biochemical analysis of amniotic fluid cells or directly by studying DNA obtained from amniotic fluid cells or chorion biopsy. With the development of gene probes for most of the important genetic disorders of the blood there will be a gradual transition from fetal blood sampling and amniocentesis to chorion biopsy as the major approach to prenatal diagnosis of haematological disorders. Since the carrier states for many of these conditions can be identified at the antenatal clinic by a careful family history and a few relatively simple blood tests the outlook for prevention of many of the important genetic disorders of the blood is extremely promising.

The overall incidence of venous thromboembolism is about 0.7 per thousand maternities, but pulmonary embolus is currently the single most common cause of maternal mortality. Major risk factors are operative delivery, age, multiparity and previous thromboembolism. Because of the risks in anticoagulant therapy and the difficulties of clinical diagnosis, it is essential to use objective tests, usually venography for deep-vein thrombosis and lung scan for pulmonary embolus. The acute phase will normally be treated with a continuous infusion of heparin, followed by subcutaneous heparin, given until at least six weeks post-delivery. Warfarin may be substituted after the first week post-delivery. In contrast to the treatment of other forms of thromboembolism, patients with artificial heart valves should be managed with warfarin until 36 weeks of pregnancy. Although the fetal risks in warfarin therapy are greater than those of subcutaneous heparin, the obvious alternative, subcutaneous heparin, does not provide adequate prophylaxis against thromboembolism. In patients who have had venous thromboembolism in the past, the maternal risks do not justify prolonged prophylaxis with subcutaneous heparin as usually given (20 000 units per day) throughout pregnancy. Further clinical trials are necessary to select the best alternatives. Antithrombin III deficiency should be managed with subcutaneous heparin taken from before conception until at least one week post-delivery, when warfarin therapy can be recommended. In addition, the labour should be covered with antithrombin III concentrate.