Lefrant et al.: PEEP AND REGIONAL HEMODYNAMICS
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Effects of PEEP on carotid blood flow
Limitations of this study
PEEP, but not mechanical ventilation in itself, pro-
duced a decrease in common carotid blood flow. In
patients with severe head trauma, mechanical ventila-
tion alters the venous return from the brain, inducing
an increase in intracranial pressure and a decrease in
cerebral blood flow.1 4 In physiological conditions,
autoregulation of cerebral blood flow should limit the
decrease in cerebral blood flow. Conversely, the
decrease in PaCO at ZEEP, and its increase during
PEEP, could have 2induced, respectively, cerebral vaso-
The measurement of cardiac output by the means of
an ultrasonic Doppler probe positioned on the ascend-
ing aorta has been demonstrated to be as accurate as
the thermodilution technique.22–24 We used high pulse
repetition frequency pulsed Doppler modules for
blood flow measurement in small arteries, in order to
avoid the aliasing phenomenon that could have result-
ed in unreadable graphs or false values. Nevertheless,
although implanted Doppler probes are widely used
for volume blood flow measurement in physiological
studies, there are some causes of error. Firstly, not
unlike electromagnetic blood flow measurement,
pulsed Doppler ultrasound requires the use of fixed
diameter probes. Therefore, vasodilatation is not
allowed at the exact site of measurement, and the
velocity profile may be altered when flow volume
increases. This is not a problem with small diameter
vessels, since the sample volume covers a large part of
the vessel diameter. Conversely, changes in flow veloc-
ity profile, from parabolic to flat, may result in under-
estimation of the flow volume in large vessels, where
the Doppler sample volume encompasses only a small
portion of the vessel lumen. This effect is minimized
when the chosen probe diameter is slightly lower than
the vessel diameter in basal conditions.
constriction and relaxation, but PaCO remained low
2
after the onset of mechanical ventilation. Therefore,
the decrease in carotid blood flow may have been the
combined result of decreased arterial blood pressure,
increased venous pressure, and hypocapnia. For tech-
nical reasons, we measured only common carotid
blood flow but neither internal carotid artery blood
flow nor intracranial pressure. Therefore, our results
do not allow a definitive conclusion in this regard.
Effects of PEEP on hepatic artery and portal vein
blood flow
Bonnet et al.,7 reported a decrease in hepatic blood
flow with PEEP correlated to the decrease in CO, but
these authors did not measure the mean arterial blood
pressure. Hugues et al.9 suggested a correlation with
A thorough evaluation of the venous return to the
heart requires the measurement of superior as well as
inferior vena cava blood flow. This was not feasible in
our study, since this would have implied a large chest
opening, with major consequences on ventilation and
thoracic pressure.
the fall in arterial PaCO whereas others attributed the
2
reduction to an increase in hepatic vascular resis-
tance.1 5 We observed a decrease in hepatic artery
blood flow without significant change in portal vein
blood flow during PEEP, in open and in closed
abdomen animals, and without correlation with CO
or PaCO2 changes. The hepatic arterial buffer
response by which an increase in portal vein blood
flow elicits hepatic artery constriction1 6 can probably
not explain the marked decrease in hepatic artery
blood flow we observed since portal vein blood flow
rose non significantly and only in open abdomen ani-
mals. For Fujita,1 7 high levels of PEEP are associated
with a decrease in hepatic blood flow and oxygen sup-
ply along with hepatic congestion. This was possible,
although not measured, in open abdomen animals,
but probably limited by the rise in abdominal pressure
in closed abdomen animals.
Conclusion
Our model of anesthetized rabbits allowed the study
of multiple regional blood circulations in the presence
or absence of PEEP. Such a model may be highly valu-
able whenever circulatory interactions and compensa-
tion mechanisms are involved. Our study confirms
most findings reported in the literature concerning
the hemodynamic effects of mechanical ventilation
with PEEP. Although mechanical ventilation induced
a fall in PaCO , and may have induced cerebral and
2
peripheral vasoconstriction, this fall was partly reduced
by PEEP, so that hypocapnia could not explain the
decrease in carotid, renal, and hepatic artery blood
flow observed in open as well as in closed abdomen
animals. In the closed abdomen rabbit, a situation
analogous to the nonsurgical setting, there was no
decrease in heart rate, but aorta blood flow decreased,
as did carotid, renal, and hepatic artery blood flow. In
open abdomen animals, a situation analogous to the
surgical setting, there was a marked decrease in arter-
Effects of PEEP on renal artery blood flow
Renal blood flow was altered significantly by high
PEEP levels, as reported by others.18–21 These authors
suggested that some reflex vasoconstriction mecha-
nism involving sympathetic activation, and high plas-
ma renin activity, was induced by PEEP and may
explain the decrease in renal function.18,19