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May
1998
Gastric mucosal oxygen delivery decreases during cardiopulmonary
bypass despite constant systemic oxygen delivery.
Sicsic J-C, Duranteau J, Corbineau H, Antoun S, Menestret P, Sitbon
P, Leguerrier A, Logleas Y, Ecoffey C; Anesth Analg 1998; 86:455-60.
[ see abstract below ]
In the March 1998 issue of Anesthesia and Analgesia, Sicsic et
al. describe the use of a new method of assessing gastric mucosal perfusion,
laser Doppler (LD) flowmetry. The LD probe is fixed to a suction catheter
placed via a gastric tube; aspiration of the suction catheter holds it
and the LD probe against the gastric wall. The signal Doppler shift is
directly proportional to the number and mean velocity of red blood cells
in the tissue volume studied. Rather than overall tissue blood flow, it
measures the flux of red blood cells in the tissue, which Sicsic et al.
have called gastric mucosal red blood cell (GMRBC) flux. In turn, GMRBC
flux provides a measure of gastric mucosal oxygen delivery (DO2).
The signal is semi-continuous, and is updated every three minutes.
The methodology, however, is not without limitations. It measures DO2
in a very small area of superficial gastric mucosa (rather than a transmural
segment), and it is difficult to maintain optical coupling between the
LD probe and the motile gastric tissue. Calibration to standardize LD
values is cumbersome, so that flux is expressed as a relative change from
baseline rather than as an absolute value.
Previous studies of GMRBC flux have indicated that it decreases during
hypothermic, nonpulsatile cardiopulmonary bypass (CPB). This is consistent
with the finding of gastric mucosal acidosis and intestinal mucosal leak
which suggests an imbalance of gut oxygen supply:demand ratio during CPB.
There are data suggesting that impaired splanchnic perfusion during CPB
may adversely affect postoperative morbidity and mortality, through such
factors as delayed recovery of gastrointestinal function (i.e. persistent
nausea or anorexia) or translocation of endotoxin leading to multisystem
organ dysfunction.
In all prior studies the decrease in gastric mucosal DO2 was
associated with a decrease in systemic DO2 during CPB. Sicsic
et al. hypothesized that maintaining DO2 at pre-CPB levels
would improve GMRBC flux during CPB. A pump flow of 2.5-2.7 L/min/m2 was
therefore provided, with a secondary goal of achieving a mean arterial
pressure (MAP) of > 59 mmHg during CPB. A total of eleven patients
undergoing coronary revascularization or valve replacement with hypothermic
CPB were studied. None required vasoactive or inotropic drugs during CPB
to attain the above end-points. The mean mixed venous saturation (SvO2)
during CPB was 86%.
Despite these measures, during CPB the GMRBC flux significantly decreased
by about 50% from baseline. It increased again during rewarming and "overshot"
by nearly 50% above baseline about 20 min after CPB.
The authors concluded that CPB induces a decrease in local mucosal DO2
despite a constant systemic DO2. One explanation could be that
redistribution of blood flow away from the splanchnic bed occurs during
CPB, but this has not been confirmed in other studies. Hemodilution during
CPB could decrease GMRBC flux, but the decrease in red blood cell number
is compensated for by an increase in gastric blood flow. In this study
the decrease in GMRBC flux appeared to correlate with hypothermia, which
can alter gastric microcirculation. However, decreased gastric perfusion
has been observed in normothermic CPB as well. The authors postulated
that the release of endogenous vasoactive compounds such as endotoxin
and cytokines might alter gastric perfusion during CPB.
Unfortunately the authors did not study a control group in whom DO2
was not kept constant during CPB, nor do they attempt to explain the "overshoot"
in GMRBC that occurred twenty minutes after CPB. There remains no ready
explanation for the observed decrease in gastric DO2 during
CPB, and (the mark of a good study?) it raises more questions than answers.
Nonetheless, this paper describes an interesting and promising new tool
in the investigation of regional blood flow distribution and its clinical
consequences.
Return to the Current Literature Review Front
Page , or read the abstract:
ABSTRACT
Previous
studies report a decrease in gastric mucosal oxygen delivery during cardiopulmonary
bypass (CPB). However, in these studies, CPB was associated with a reduction
in systemic oxygen delivery (DO2). Conceivably, this decrease
in DO2 could have contributed to the observed decrease in gastric
mucosal oxygen delivery. Thus, in the present study, we assessed the effects
of the maintenance of DO2 (at pre-CPB values) during hypothermic
(30-32 degrees C) CPB on the gastric mucosal red blood cell flux (GMRBC
flux) using laser Doppler flowmetry.
In 11 patients requiring cardiac surgery, the pump flow rate during CPB
was initially set at 2.4 L x min(-1) x m(-2) and was adjusted to maintain
DO2 at pre-CPB values (flow 2.5-2.7 L x min[-1 x m[-2 ). Despite
a constant DO2, the GMRBC flux was decreased during CPB. These
decreases averaged 50% +/- 16% after 10 min, 50% +/- 18% after 20 min, 49%
+/- 21% after 30 min, and 49% +/- 19% after 40 min of CPB. The rewarming
period was associated with an increase in GMRBC flux. Thus, maintaining
systemic DO2 during CPB seems to be an ineffective strategy to
improve gastric mucosal oxygen delivery.
IMPLICATIONS: In the present study, we tested the hypothesis that
gastric mucosal red blood cell flux assessed by laser Doppler flowmetry
could be improved by maintaining baseline systemic flow and oxygen delivery
during hypothermic cardiopulmonary bypass. Despite this strategy, gastric
mucosal red blood cell flux decreased by 50% during hypothermic cardiopulmonary
bypass.
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