From the Cattedra di Medicina Interna I (G.M.), Ospedale S. Gerardo,
Monza; Istituto Scientifico Ospedale S. Luca (G.P.), IRCCS, Istituto
Auxologico Italiano, Milano; and Divisione di Medicina d'Urgenza (G.B.,
P.M., R.C., G.T.) and Centro Fisiologia Clinica e Ipertensione (L.U., C.S.,
A.M.), Ospedale Maggiore, IRCCS, and University of Milano, Italy.
Correspondence to Prof Giuseppe Mancia, Cattedra di Medicina Interna, Università di Milano, Ospedale S. Gerardo dei Tintori, Via Donizetti 106–20052 Monza (MI), Italy.
Although studied extensively in animals,2 3 4 5 the
effects of blood viscosity on human BP have received only limited
attention except for (1) the epidemiological evidence that there is a
relationship between hematocrit and BP levels in both normotensive and
hypertensive subjects6 7 8 9 10 11 12 13 14 15 and (2) the clinical
evidence of an increased prevalence of hypertension in subjects with
secondary eritrocytosis and polycythemia.16 17 In
particular, no information is available on the BP effects of
interventions that reduce blood viscosity (ie, whether this maneuver
induces a sustained reduction in BP levels).
We addressed this issue by measuring clinic and ambulatory BPs before
and after a reduction in blood viscosity obtained through isovolumic
hemodilution. We also measured plasma endothelin-1 to obtain
information on the secretion of a humoral factor possibly increased by
hemodilution18 19 20 21 22 and thus potentially opposing
the effect of reduced blood viscosity on vascular resistance. The study
was done in polycythemic patients in whom blood withdrawal at regular
time intervals is part of the standard treatment.
Measurements
BP was measured in two different ways (1) with a sphygmomanometer, with
the subject in the sitting position, using the first and fifth
Korotkoff sounds to determine SBP and DBP values, respectively, and (2)
with noninvasive ABP monitoring with a Spacelabs
device30 that was programmed to obtain BP
readings every 15 minutes. Each ABP recording started in the
late morning and ended 25 hours later to ensure collection of BP data
for a full 24-hour period. During the monitoring period, the subjects
were sent home, instructed to attend to their usual activities, and
asked to come back to the outpatient clinic for the device removal. All
patients were instructed to undergo their usual activities during the
monitoring period but to stop any ongoing exercise and to extend the
arm during automatic BP measurements. They also were instructed to fill
out a diary to determine whether unusual events and sleep
disturbances occurred.
Protocol and Data Analysis
In each patient, the clinic BP values were obtained by averaging three
sphygmomanometric measurements collected at 5-minute intervals. ABP
values were averaged for each hour and for the entire 24-hour period.
The standard deviation of all 24-hour values was taken as a measure of
BP variability.31 Mean values were calculated for
the entire group of patients. Comparisons of values before and after
isovolumic hemodilution were made with Wilcoxon's
nonparametric test. The Mann-Whitney
nonparametric test also was used to compare data in the
normotensive and hypertensive subgroups (see "Results"). A value of
P<.05 was taken as the level of statistical significance.
Unless otherwise indicated, values are ±SEM.
Of the 22 patients, 10 were normotensive (mean±SD age, 61.3±2.7
years), and 12 had a clinic SBP or DBP of >140 or 90 mm Hg,
respectively, thereby being classified as hypertensive (age, 61.6±4.7
years). As shown in the Table
Several other points should be mentioned. First, the reduction in
BP induced by hemodilution was small in normotensive polycythemic
subjects but consistent and clear-cut (13.0 and 7.6 mm Hg
for clinic SBP and DBP and 5.5 and 3.6 mm Hg for ambulatory SBP
and DBP) in hypertensive polycythemic subjects. Several mechanisms may
account for this difference. For example, the reduction in hematocrit
was more consistent and greater in hypertensives than in
normotensives, leading to a more consistent and somewhat
greater reduction in blood viscosity in the former compared with the
latter group. Furthermore, in hypertensives, the reduction in
hematocrit and blood viscosity may have improved hemorheology to a
greater degree because compared with normotensive control subjects,
animals and humans with a high BP have a greater aggregability and a
smaller deformability of red blood cells,
respectively.32 33 34 35 Finally, studies in
normotensive animals3 36 37 have shown a
reduction in blood viscosity to be accompanied by an increase in
cardiac output, which compensates for the viscosity-dependent reduction
in vascular resistance and prevents a fall in BP. This may occur to a
lesser degree in hypertension, which is characterized by an impairment
of cardiac reflex responses to alterations in vasomotor
tone38 and possibly by a decrease in cardiac
performance that follows a reduction in afterload. At any rate,
it should be emphasized that the reduction in BP induced by
hemodilution in our hypertensive subjects represents an effect
greater than that seen with several nonpharmacological methods of
lowering BP39 40 41 42 and is similar to that seen
with antihypertensive drug therapy.43 44 45 The
BP-lowering effect of hemodilution is therefore of clinical
significance when prehemodilution BP is elevated, which often occurs in
polycythemia.
Second, the reduction in blood viscosity and hematocrit induced by
hemodilution was not accompanied by any change in HR, which means that
the presumable reduction in vascular resistance associated with this
intervention did not trigger a reflex increase in neural cardiac drive,
probably because of a resetting phenomenon analogous to the one seen
when peripheral vascular resistance is reduced with
antihypertensive drugs.46 47 48 Third, because the
study design did not include any sham hemodilution procedure, the
possibility exists that the BP reductions were due to "placebo" or
"time" effects. However, this might hold for clinic but not for
24-hour average BP, which is devoid of any substantial placebo or time
effect when assessed over a several-week
period.49 50 51 52 Fourth, because our study did not
include blood volume measurements, the possibility exists that the
BP-lowering effect was due to hypovolemia and to a reduction in cardiac
output. However, in all patients, hemodilution was accomplished by
substituting the amount of blood withdrawn with a rigidly equivalent
amount of 4% albumin–supplemented saline. Furthermore, the
effects on BP were assessed 7 to 10 days after the hemodilution
intervention (ie, at a time interval sufficiently long to allow any
small initial reduction in blood volume to be corrected by the
mechanisms involved in blood volume
homeostasis).52 Finally, indirect evidence that
circulating blood volume was not substantially affected was provided by
the absence of hemodilution-induced changes in PRA and atrial diameter
(ie, by sensitive markers of blood volume
variations).28 29
Two limitations should be discussed, however. (1) Our study was
restricted to the effect of hemodilution over a 7- to 10-day period,
thereby failing to determine whether this procedure retains a
BP-lowering effect throughout longer time intervals, and thus
represents an effective therapeutic intervention against an
elevated BP also when, in polycythemic patients, hemodilution is
performed at longer time intervals. (2) Because the data were collected
in polycythemia, it is difficult to extrapolate our results to the BP
effects of viscosity changes in essential hypertension in which
hemorheological abnormalities that possibly increase blood viscosity
have been reported but the hematocrit is
normal.9 24 53 Our present demonstration that
hemodilution had clear-cut BP-lowering effect in hypertensive
polycythemic patients makes this issue a relevant one to be addressed,
however.
Finally, although in previous studies hemorrhage has been shown
to increase plasma endothelin-1,18 19 20 21 22 in our
patients hemodilution did not have any effect on the circulating level
of this substance, with this being the case in both normotensive and
hypertensive groups. We cannot exclude that this was due to the fact
that measurements were obtained after 7 to 10 days of blood withdrawal,
leading to an initial but temporary increase in plasma endothelin-1
that then subsided. We also cannot exclude that our stimulus did not
have adequate power for an increase in endothelin-1 secretion to occur.
Another possibility, however, is that an increase in endothelin-1
mainly takes place with a reduction in blood volume (which was
prevented in our patients) and that changes in shear stress or other
hemorheological variables play a lesser role.
Received May 29, 1997;
first decision July 1, 1997;
accepted October 27, 1997.
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© 1998 American Heart Association, Inc.
Scientific Contributions
Hemodilution Reduces Clinic and Ambulatory Blood Pressure in Polycythemic Patients
Abstract
Top
Abstract
Introduction
Methods
Results
Discussion
References
Abstract—Limited information
is available for humans on whether blood viscosity affects total
peripheral resistance and, hence, blood pressure. Our study
was aimed at assessing the effects of acute changes in blood viscosity
on both clinic and 24-hour ambulatory blood pressure (BP) values. In 22
normotensive and hypertensive patients with polycythemia, clinic and
24-hour ambulatory BPs were measured before and 7 to 10 days after
isovolumic hemodilution; this was performed through the withdrawal of
400 to 700 mL of blood, with concomitant infusion of an equivalent
volume of saline-albumin solution. Hematocrit, plasma renin
activity, plasma endothelin-1, right atrial diameter
(echocardiography), and blood viscosity were
measured under both conditions. Plasma renin activity and right atrial
diameter were used as indirect markers of blood volume changes. Plasma
endothelin-1 was used to obtain information on a vasomotor substance
possibly stimulated by our intervention, which could counteract
vasomotor effects. Isovolumic hemodilution reduced hematocrit from
0.53±0.05 to 0.49±0.05 (P<.01). Plasma renin
activity, plasma endothelin-1 and right atrial diameter were unchanged.
Clinic blood pressure was reduced by hemodilution (systolic,
144.3±5.4 to 136.0±3.9 mm Hg[mean±SEM];
diastolic, 87.0±2.8 to 82.1±2.6 mm Hg,
P<.05 for both) and a reduction was observed also for
24-hour average ABP (systolic, 133.6±2.9 to 129.5±2.7
mm Hg; diastolic, 80.0±2.0 to 77.3±1.7 mm Hg,
P<.05 for both). The reduction was consistent
in hypertensive patients (n=12), whereas in normotensive patients
(n=10) it was small and not significant. Both clinic and 24-hour
average heart rates were unaffected by the hemodilution. Thus, in
polycythemia, reduction in blood viscosity without changing blood
volume causes a significant fall in both clinic and 24-hour ambulatory
BPs; this is particularly true when, as can often happen, blood
pressure is elevated. This emphasizes the importance this variable
may have in the determination of blood pressure and the potential
therapeutic value of its correction when altered.
Key Words: blood viscosity • hemodilution • blood pressure monitoring, ambulatory • hemorheology
Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
BP is determined by
cardiac output and peripheral vascular resistance. The
latter depends to a large degree on the caliber and length of
arterioles. It also depends, however, on blood viscosity, with which it
bears a linear relationship over a wide range of
values.1
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
Subjects
We studied 22 patients (16 men and 6 women; mean±SD age,
61.5±10.0 years) with a primary polycythemia who were followed in the
hematologic outpatient clinic of the Maggiore Hospital (Milan, Italy).
The patients were free from any major cardiovascular or
noncardiovascular disease other than polycythemia and
had been without any drug treatment during the 2 months preceding the
study. Each patient gave informed consent to the procedure. The
protocol of the study was approved by the ethics committee of the
institutions involved.
In all patients, blood samples were obtained from an antecubital
vein and used for hematocrit, hemoglobin, blood viscosity, PRA, and
plasma endothelin-1 determinations. Hematocrit and hemoglobin were
measured with standard techniques. Blood viscosity (expressed in cP)
was determined with a rotational viscosimeter (Bohlin CS-10; Bohlin
Rheology AB) at 37°C using shear rates of 380 and 1
s-1. These rates were selected because they are
conventionally taken as reflecting the velocity gradients of blood flow
in the arteriolar and venular bed,
respectively.23 24 25 26 27 PRA was measured with a
standard radioimmunoassay technique to ensure that the hemodilution
procedure (see below) had not caused a reduction in circulating blood
volume, which would have stimulated renin
secretion.28 This also was ensured through
measurements of RA diameter with M- and B-mode
echocardiography.29 Plasma
endothelin-1 was measured with radioimmunoassay after extraction from
plasma with C18 Sep-Pak Cartridges (Millipore), as described in detail
elsewhere.19 The cross-reactivity of the rabbit
antibody against human endothelin-1 was 7% for both endothelin-2 and
endothelin-3 and 0% for non–endothelin-related peptides. The
intra-assay and interassay coefficients of variation of the method were
7% and 10%, respectively.
After the 2 months of no drug treatment, clinic BP, ABP, and
laboratory and echocardiographic data were collected
for a first time. The patients were subjected to isovolumic
hemodilution through withdrawal of 400 to 700 mL of blood (depending on
the basal hematocrit level) and simultaneous infusion of an
equivalent amount of saline with the addition of 4% albumin.
At 7 to 10 days after the isovolumic hemodilution, clinic BP, ABP, and
laboratory and echocardiographic data were collected
again, according to the same procedure and sequence used in the
prehemodilution condition.
Results
Top
Abstract
Introduction
Methods
Results
Discussion
References
Fig 1
shows that in our patients,
isovolumic hemodilution induced (1) a significant reduction in
hematocrit and hemoglobin levels, (2) a significant reduction in blood
viscosity at both low and high shear rates, and (3) no change in PRA or
RA diameter. In addition, plasma endothelin-1 level did not change
(2.5±0.2 and 2.4±0.2 pg/mL before and after hemodilution,
respectively). As illustrated in Fig 2, clinic SBP and DBP values were significantly reduced after
hemodilution; this also occurred with 24-hour average SBP and DBP. The
reduction in ABP values was evident throughout the 24-hour period. In
contrast, clinic HR, 24-hour average HR, and hourly HR values were
superimposable before and after hemodilution.
View larger version (15K):
[in a new window]
Figure 1. Hematocrit, hemoglobin, blood viscosity, RA
diameter, and PRA values before and 7 to 10 days after isovolumic
hemodilution. Data are shown both as individual values and as mean
values for the entire group of 22 or 12 subjects (PRA).
**P<.01.
View larger version (23K):
[in a new window]
Figure 2. Effects of isovolumic hemodilution on clinic BP
and HR, 24-hour average BP and HR, and 24-hour BP and HR profiles. Data
are mean±SEM for the entire group of 22 subjects. S indicates
systolic; D, diastolic. , isovolumic hemodilution reduced
hematocrit significantly in both groups. Blood viscosity was reduced
significantly only in the hypertensive group, in which the predilution
value was slightly although not significantly greater than in the
normotensive group. Hemodilution caused a noticeable reduction in
clinic and 24-hour average BP in the hypertensive group, whereas the
change was small and nonsignificant in the normotensive group. HR was
unaffected by the procedure in either group. In the group as a whole,
hemodilution did not cause any change in 24-hour BP standard
deviations, which were 13.4 mm Hg (SBP) and 10.3 mm Hg
(DBP) in the prehemodilution condition and 13.0 mm Hg (SBP) and
10.7 mm Hg (DBP) in the posthemodilution condition. The results
were similar in the normotensive and hypertensive groups when
considered separately.
View this table:
[in a new window]
Table 1. Effect of Isovolumic Hemodilution in Normotensive and
Hypertensive Subjects With Polycythemia
Discussion
Top
Abstract
Introduction
Methods
Results
Discussion
References
In our polycythemic patients, a isovolumic hemodilution
that reduced blood viscosity was accompanied by a significant reduction
in BP. The reduction involved both systolic and
diastolic values. It also involved both the BP values taken
in the outpatient clinic and the BP values obtained under ambulatory
conditions throughout the day and night. It thus can be concluded that
this intervention has a BP-lowering effect and that this effect is also
evident when daily-life BP is considered.
Selected Abbreviations and Acronyms
ABP
=
ambulatory blood pressure
BP
=
blood pressure
DBP
=
diastolic blood pressure
HR
=
heart rate
PRA
=
plasma renin activity
RA
=
right atrium, atrial
SBP
=
systolic blood pressure
References
Top
Abstract
Introduction
Methods
Results
Discussion
References
1.
Fahraeus R, Lindqvist T. The viscosity of the
blood in narrow capillary tubes. Am J Physiol. 1931;96:562–568.
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