Context.— Although concern over the risks of red blood cell transfusion has resulted
in several practice guidelines for transfusion, lack of data regarding the
physiological effects of anemia in humans has caused uncertainty regarding
the blood hemoglobin (Hb) concentration requiring treatment.
Objective.— To test the hypothesis that acute isovolemic reduction of blood Hb concentration
to 50 g/L in healthy resting humans would produce inadequate cardiovascular
compensation and result in tissue hypoxia secondary to inadequate oxygen transport.
Design.— Before and after interventional study.
Setting.— Academic tertiary care medical center.
Participants.— Conscious healthy patients (n=11) prior to anesthesia and surgery and
volunteers not undergoing surgery (n=21).
Interventions.— Aliquots of blood (450-900 mL) were removed to reduce blood Hb concentration
from 131 (2) g/L to 50 (1) g/L [mean (SE)]. Isovolemia was maintained with
5% human albumin and/or autologous plasma. Cardiovascular parameters, arterial
and mixed venous oxygen content, oxyhemoglobin saturation, and arterial blood
lactate were measured before and after removal of each aliquot of blood. Electrocardiogram
and, in a subset, Holter monitor were monitored continuously.
Main Outcome Measures.— "Critical" oxygen delivery (TO2) as assessed by oxygen consumption
(O2), plasma lactate concentration, and ST changes on electrocardiogram.
Results.— Acute, isovolemic reduction of Hb concentration decreased systemic vascular
resistance and TO2 and increased heart rate, stroke volume, and
cardiac index (each P<.001). We did not find evidence
of inadequate oxygenation: O2 increased slightly from a mean
(SD) of 3.07 (0.44) mL of oxygen per kilogram per minute (mL O2·kg−1·min−1) to 3.42 (0.54) Ml O2·kg−1·min−1 (P<.001)
and plasma lactate concentration did not change (0.81 [0.11] mmol/L to 0.62
[0.19] mmol/L;P=.09). Two subjects developed significant
ST changes on Holter monitor: one apparently related to body position or activity,
the other to an increase in heart rate (at an Hb concentration of 46-53 g/L);
both occurred in young women and resolved without sequelae.
Conclusions.— Acute isovolemic reduction of blood Hb concentration to 50 g/L in conscious
healthy resting humans does not produce evidence of inadequate systemic TO2, as assessed by lack of change of O2 and plasma lactate
concentration. Analysis of Holter readings suggests that at this Hb concentration
in this resting healthy population, myocardial ischemia would occur infrequently.