Key messages

• Because of almost invariable falls in intravascular red cell and plasma volumes, polycythemias are uncommon as problems in intensive care patients, but do occur sporadically.

• Based upon measured red cell and blood volume, polycythemias are classified as primary (polycythemia rubra vera), secondary (e.g. to chronic or inappropriate erythropoietin secretion), or apparent (marked by contracted plasma volume).

• Primary polycythemia, a myeloproliferative syndrome, poses risks of vascular occlusive complications.

• Polycythemia secondary to chronic hypoxia enhances systemic oxygen transport and organ perfusion, unless plasma volume contraction occurs, with inordinate increases in blood viscosity.

• Contracted plasma volume syndromes ('apparent polycythemia') may be associated with increased risk of stroke and myocardial infarction. Introduction

The ability of the blood to function for oxygen delivery and organ perfusion is governed by hematocrit and blood volume ( Jones efa/ 1.990). Blood volume is the sum of total red cells and plasma in circulation.

In critical illness, the patient's total circulating red cells—the red cell mass or volume—tends to fall because of inevitable losses of blood plus inhibition of red cell production by any septic or inflammatory disease, renal failure, and/or any myelotoxic drug. Thus red cell production falls, or ceases, in patients in the intensive care unit (ICU) setting. Because of damage to vascular endothelial integrity, the intravascular plasma volume also tends to fall. Therefore the hematocrit appears 'falsely high' and poorly reflects the red cell lack. These invariable influences on the ICU patient reduce the chance that polycythemia will be a problem, although this will occur sporadically.

In health, normal erythropoiesis keeps the peripheral blood hematocrit between 0.37 and 0.47 for women and between 0.40 and 0.50 for men. The red cell volume is normally 25±5 ml/kg for women and 30±5 ml/kg for men. The scatter reflects the problems of expressing red cell volume per kilogram body weight; the higher the body's proportion of (relatively avascular) fatty tissue, the lower is the red cell volume when expressed in relation to body weight. Official guidelines call for expression of blood volume relative to body surface area, which is derived from height and weight.

Polycythemia is marked by an excess of red cells and a supranormal hematocrit value. Measurements of red cell and plasma volume are needed for diagnostic categorization. Standard hematology techniques based on the dilution principle use radio-chromium or radio-technetium for labeling of autologous red cells followed by reinjection and quantitation of the dilution; the higher the dilution, the larger is the pool of red cells in the circulation. The plasma volume can be estimated using radio-iodine-labeled albumin. 'Older' labels of albumin as indicators of plasma volume have included indocyanine green and Evans blue, but, like radio-iodinated albumin, these albumin labels lead to an overestimate of plasma volume in critical illness because of the continuity of intravascular plasma albumin with extravasated albumin, water, and salt. Therefore blood volume, based on red cell volume divided by hematocrit, is a more reliable indicator of intravascular volume than is the sum of red cell volume and the volume of distribution of albumin in such patients.

Whenever polycythemia is suspected, the hematological diagnosis is not complete without knowledge of the measured red cell volume and calculated blood volume, as elevated hematocrit values may reflect increases in red cells and/or reductions of plasma volume in the circulation.

The main categories of polcythemia include primary polycythemia (polycythemia rubra vera), secondary (or compensatory) polycythemia, and contracted plasma volume syndromes ('apparent' polycythemia, 'stress' polycythemia, 'relative' polycythemia, or 'pseudo'-polycythemia)

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