676
Table 2 Subjects’ characteristics (protocol B). Data are presented
as means ± SD (n=40)
with this system if subjects are carefully selected [3, 9].
However, this technique assesses leukocyte movement,
and it is not entirely clear whether results can be directly
applied to volumetric blood flow in the retina. There is
some evidence that leukocytes move slower in retinal
capillaries than erythrocytes [1]. Whether erythrocyte
movement is differentially affected by hypertension in
the retina remains to be established. This may, in princi-
ple, be investigated using combined laser Doppler velo-
cimetry and vessel size determination [13]. For calcula-
tion of total blood flow with this technique it is neces-
sary to measure all veins or arteries entering the optic
nerve. This procedure is much more time-consuming
than measurements with the blue-field entoptic technique
and was therefore not employed in the present study.
In addition, it is widely accepted that leukocyte veloc-
ity can be adequately assessed with the blue-field tech-
nique, but it is unclear whether the number of leukocytes
as perceived in the blue light is an adequate measure of
vascular volume. To overcome the problem of inter-indi-
vidual variability we performed a study in which sub-
jects were measured thrice within 6 weeks (protocol B).
The results of this longitudinal study are, however, com-
parable to those with the cross-sectional approach.
Colour Doppler imaging can be applied to measure
blood flow velocities in retrobulbar vessels, but no quan-
titative information on vessel diameters can be obtained.
Hence, this method does not allow for quantification of
blood flow through the central retinal artery. Neverthe-
less, we observed a significant association between SBP
and MFV in the central retinal artery, which may indi-
cate increased blood flow through this artery with in-
creasing blood pressure. Again, the results in protocol B
are in good agreement with those observed in the cross-
sectional study.
Age (years)
24.2±3.9
124.6±10.0
61.7±9.4
Systolic blood pressure (mmHg)
Diastolic blood pressure (mmHg)
Mean arterial pressure (mmHg)
Pulse rate (beats/min)
82.4±9.8
59.9±8.3
White blood cell velocity (relative units)
White blood cell density (relative units)
White blood cell flux (relative units)
Mean flow velocity (cm/s)
Resistive index
1.22±0.41
116.3±30.7
138.9±42.6
6.1±0.7
0.73±0.03
Discussion
In the present study in subjects with normal or slightly
elevated blood pressure we observed an association be-
tween retinal WBCF and MAP. Our data also indicate
that perifoveal leukocyte velocity is dependent on MAP,
whereas retinal leukocyte density shows only little blood
pressure dependence. The hypothesis that retinal blood
flow is dependent on blood pressure is further supported
by our observation that MFV in the central retinal artery
is correlated with MAP.
Generally the dependence of retinal haemodynamic
variables on blood pressure in the present study was
small. Our data indicate that WBCF increases between
3.6% (protocol B) and 5.6% (protocol A) per 10-mmHg
increase in MAP. Accordingly, MFV increases between
1.3% (protocol A) and 5.8% (protocol B) per 10-mmHg
increase in MAP. Whether this is of clinical relevance re-
mains to be shown. However, even a small hypertension-
induced elevation of retinal blood flow may increase the
risk of onset or progression of ocular vascular disease [6].
In the present study we focused on young male sub-
jects because retinal WBCF decreases with age [4].
Patients with any systemic or ocular pathology likely to
influence retinal blood flow were excluded from the
present study. In addition, we included only subjects who
did not receive any regular medication. We considered
this important because any type of systemic vasoactive
medication may influence retinal blood flow [5, 8].
Measurement of retinal blood flow with the blue-field
entoptic technique is a subjective method. Therefore, on-
ly subjects whose results displayed little variability were
included in the present study. There is evidence from
previous trials that reproducible data can be obtained
The present study’s findings regarding RI are difficult
to interpret. We have previously shown that RI is not an
adequate measure of vascular resistance in the retina
[10]. Hence, it seems very unlikely that the correlation
between RI and blood pressure observed in the present
study contains any information on the dependence of ret-
inal perfusion on ocular perfusion pressure.
Concluding, our data indicate a slight but significant
increase in retinal blood flow with increasing blood pres-
sure. Whether this is of clinical relevance in eye diseases
with altered retinal perfusion, such as diabetic retinopa-
thy, remains to be established.
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