Haemoglobin concentration and mass as determinants of exercise performance and of surgical outcome

The ability of the cardiorespiratory system (heart, lungs, blood) to deliver oxygento exercising skeletal muscle constrains maximum oxygen consumption ,with cardiac output and the concentration of oxygen-carrying haemoglobin ([Hb]) beingkey limiting parameters. Total blood volume (BV) is the sum of the plasma volume (PV)and the total red cell volume. The measured [Hb] is dependent upon the totalcirculating mass of haemoglobin (tHb-mass) and plasma volume (PV). While theproportion of oxygen carried in plasma is trivial (0.3 mL of oxygen per100 mL of plasma), each gram of Hb, contained in red blood cells, binds1.39 mL of oxygen. As a result, the relationship between and tHb-mass is stronger than that observed between and [Hb] or BV. The glycoprotein hormone erythropoietin drives red cell synthesisand, like simple transfusion of packed red blood cells, can increase tHb-mass. Aniron-containing haem group lies at the centre of the Hb molecule and, in situationsof actual or functional iron deficiency, tHb-mass will also rise following ironadministration. However achieved, an increase in tHb-mass also increases circulatingoxygen-carrying capacity, and thus the capacity for aerobic phosphorylation. It isfor such reasons that alterations in and exercise performance are proportional to those in arterial oxygen content andsystemic oxygen transport, a change in tHb-mass of 1 g being associated with a4 mL · min−1 change in .Similarly, increases by approximately 1% for each3 g · L−1 increase in [Hb] over the [Hb]range (120 to 170 g · L−1). Surgery, likeexercise, places substantial metabolic demands on the patient. Whilst subject todebate, oxygen supply at a rate inadequate to prevent muscle anaerobiosis mayunderpin the occurrence of the anaerobic threshold (AT), an important submaximalmarker of cardiorespiratory fitness. Preoperatively, cardiopulmonary exercise testing(CPET) can be used to determine AT and peak exertional oxygen uptake( peak) asmeasures of ability to meet increasing oxygen demands. The degree of surgical insultand the ability to meet the resulting additional postoperative oxygen demand appearto be fundamental determinants of surgical outcome: individuals in whom such abilityis impaired (and thus those with reduced peakand AT) are at greater risk of adverse surgical outcome. This review provides anoverview of the relationships between [Hb], tHb-mass, exercise capacity, and surgicaloutcome and discusses the potential value of assessing tHb-mass over [Hb].