Blood Volume Analysis Can Distinguish
True Anemia from Hemodilution in
Critically Ill Trauma Patients
PY Van, MD ∙ SD Cho, MD ∙ SJ Underwood, MS
GJ Hamilton, BS ∙ LB Ham, MD ∙ MA Schreiber, MD
Background
• Hemorrhage leading cause of preventable
death in trauma victims
• Decreased peripheral hematocrit (pHct) used
as marker for blood loss
• pHct may not represent true red blood cell
volume (RBCV)
Background
Background
• Surrogate measures to deduce volume status
– Vital signs and physical exam
– Laboratory tests
– Invasive monitoring
• Experienced clinicians frequently wrong
– 51% concordance with blood volume analysis
Androne, AS et al. Am J Cardiol 2004
Blood Volume Analysis
• Indicator dilution principle
– Known quantity of tracer injected into unknown
volume (intravascular space)
– After equilibration of tracer, plasma sampled
• Concentration of tracer in sample is measured
• Unknown volume is inversely proportional to
concentration of tracer in the sample volume
• Larger the unknown volume, more dilute the tracer
Indicator Dilution Principle
C1
V2
=
C2
V1
Concentration of tracer injected
Unknown volume (plasma volume)
Conc. tracer in sample withdrawn
Volume of sample withdrawn
Blood Volume Analysis
• Single injection radiolabeled 131I-albumin.
• Serial blood samples drawn over 40 minutes
• Analysis yields actual and ideal TBV, RBCV, PV
Blood Volume Analysis
pHct =
RBCV
RBCV + PV
TBV = RBCV + PV
Blood Volume Analysis
• Normalized hematocrit (nHct)
– pHct is adjusted for volume derangement:
nHct = pHct x
Measured TBV
Ideal TBV
Hypothesis
Use of pHct alone in critically ill trauma patients
will result in over-diagnosis of anemia
Methods
•
•
•
•
Trauma ICU pts recruited 24hrs post admission
Baseline blood sample
Injection of 1mL 25 µCi of 131I-albumin
12 minute equilibration period
– Then 5 serial blood draws, 6 minutes apart
• Samples processed on BVA-100 Blood Volume
Analyzer (Daxor Corporation, NY, NY)
Methods
Measured volumes compared
to ideal -- percent deviation
from ideal calculated
Methods
• Pts stratified into 3 groups based on deviation
from ideal total blood volume
– Hypovolemic: > 8% deficit relative to ideal
– Normovolemic: < 8% variation relative to ideal
– Hypervolemic: > 8% excess relative to ideal
Characteristics
Patients (n = 27)
Male / Female
13 / 14
Age
49.6 ± 3.8
Body Mass Index
29.3 ± 6.2
APACHE II
17.9 ± 1.5
Injury Severity Score
29.8 ± 2.5
All values are mean ± standard deviation
Results
Volume status (n = 65)
Hypovolemic
18.4%
Normovolemic
30.8%
Hypervolemic
50.8%
Volume Status and Fluids
Fluid In (mL)
Net Fluid (mL)
Hypovolemic
Normovolemic
Hypervolemic
(n = 12)
(n =19)
(n = 33)
17,881
30,306
22,016
(10065, 41396)
(14752, 52026)
(18100, 33397)
13,579
2,799
11,807
(4702, 18708)
(1969, 15861)
(6924, 17373)
All values are medians (interquartile range)
All p = NS, Mann-Whitney U test
No significant difference in volume of fluids given or
net fluid balance between each volume status
Results
• No linear correlation between net fluid
balance and changes in TBV, RBCV, and PV
between each analysis
• Moderate linear correlation between pHct and
RBCV (R2 = 0.3)
Results
• No differences in ISS when compared across
the volume status groups
• No correlation between ISS and rate of
albumin transudation
pHct versus nHct
Hypovolemic
(n=12)
Normovolemic
(n=20)
Hypervolemic
(n=33)
All
(n=65)
pHct
nHct
Difference
pHct < 30
nHct < 30
Overdiagnosis
of anemia
26.1
20.9*
5.2 ± 3.3
91.7% (11)
91.7% (11)
--
27.1
27.1
0.0 ± 1.2
80.0% (16)
80.0% (16)
--
26.5
32.9*
-6.4 ± 4.4
81.8% (27)
†27.3% (9)
54.5% (18)
26.6
28.9
-2.3 ± 5.7
83.1% (54)
55.4% (36)
27.7% (18)
Paired t-test
* p < 0.05
Chi-squared
† p < 0.05
Conclusions
• Assessing volume status is challenging
• No differences in amount of fluids
administered to volume status groups
• pHct compared to nHct
– Overestimates anemia in hypervolemic pts
– Underestimates anemia in hypovolemic pts
Limitations
• Preliminary study -- small number of patients
• BVA not a dynamic test – snapshot in time
• Assume RBCV constant during testing
– Not reasonable if bleeding > 100mL/hr
• Availability of tracer and personnel
Future Directions
• Further characterize effects of fluid and blood
product administration on volume status
• Blood volume analysis upon ICU admission
– Establish baseline
– Initiate therapies based on blood volumes
– Avoid unnecessary CT scans and transfusion when
BVA shows low pHct due to hemodilution
Blood Volume Analysis