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March
1998
Use of a vital capacity maneuver to prevent atelectasis
after cardiopulmonary bypass: an experimental study.
Magnusson L, Zemgulis V, Tenling A, Wernlund J, Tyden H, Thelin S, Hedenstierna
G; Anesthesiology 1998; 88:134-42
[ see abstract below ]
Cardiopulmonary bypass (CPB) may have adverse effects on postoperative
pulmonary function via two mechanisms: inflammatory lung injury, caused
by contact activation of mediators by perfusion of blood through the extracorporeal
circuit, and atelectasis, caused by allowing the lungs to collapse during
CPB. It is not uncommon to observe an increase in intrapulmonary shunt
(Qs/Qt) to 20-25% in the postoperative period. This may contribute to
prolongation of postoperative ventilation time and length of ICU stay.
In a pig model, Magnusson et al1 demonstrated by means of computer
tomography scans that atelectasis induced by lung collapse during CPB
may occupy up to 35% of the total lung area, and that its extent correlates
well with hypoxemia and intrapulmonary shunting.
Over the years, various maneuvers have been designed in an attempt to
prevent or reverse atelectasis that occurs during CPB. These have included
continuous positive airway pressure and maintenance of standard mechanical
ventilation during CPB. Results have been conflicting: some studies have
indicated improved lung function, some have indicated no change and some
have even demonstrated a worsening of lung function with these maneuvers.
Even when a benefit is seen, it may not be sustained for more than 5 hours
into the postoperative period.
Rothen at al2,3 demonstrated that inflation of the lungs to
vital capacity and a peak airway pressure of 40 cmH2O reversed practically
all atelectasis in patients with healthy lungs. If the patient were subsequently
oxygenated with 100% oxygen, however, atelectasis recurred within 5 min,
presumably through rapid absorption of oxygen from alveoli, which would
then collapse. However, breathing 40% oxygen prevented the reappearance
of atelectasis by providing nitrogen as a supportive "strut" for the alveoli.
In the January 1998 issue of Anesthesiology, Magnusson et al present
the results of a pig model study developed to test the hypothesis that
a vital capacity maneuver (VCM), a single breath with a peak airway pressure
of 40 cmH2O, sustained for 15 seconds at the end of cardiopulmonary bypass,
would reverse atelectasis and that it would be sustained better by breathing
40% oxygen than 100% oxygen. Pigs were subjected to CPB for 90 minutes,
with a 45 minute aortic cross-clamp time, and then randomized to three
groups at the end of CPB. In the control group, no VCM was given. The
other two groups received either a VCM followed by ventilation with 40%
oxygen or 100% oxygen. One hour after CPB measurements of intrapulmonary
shunt and a computed tomographic (CT) scan of the thorax were performed.
In the control group, intrapulmonary shunt increased from 4.9% at baseline
to 20.8% after CPB. In the VCM with 40% oxygen group the shunt increased
from 3.3 to 5.0% (no significant change) and in the VCM with 100% oxygen
group the shunt increased from 2.2 to 6.9% (a significant increase from
baseline, but significantly smaller than in the control group). The proportional
area of atelectasis revealed by CT scan in the control pigs was 21.3%,
one fifth of the entire cross-sectional lung area. In contrast, the proportional
area of atelectasis in the VCM-40% oxygen pigs was only 2.3%, and in the
VCM-100% oxygen pigs 5.7%.
Although these data must be confirmed in human subjects undergoing cardiac
surgery, they suggest strongly that (a) atelectasis that occurs during
CPB may be reversed on weaning from bypass by a sustained vital capacity
insufflation and (b) expansion is best maintained by ventilating with
an oxygen:air mixture. Use of 100% oxygen promotes absorption atelectasis
and somewhat mitigates the benefit of the VCM. These findings may have
useful application to our clinical practice.
1 Atelectasis is a major cause of hypoxemia and shunt after
extra-corporeal circulation. An experimental study. Magnusson L, Zemgulis
V, Wicky S et al.Anesthesiology 1997; 87:1153-63.
2 Re-expansion of atelectasis during general anesthesia: a
computed tomography study. Rothen HU, Sporre B, Engberg , Wegenius G,
Hedenstierna G; Br J Anaesth 1993;71:788-95.
3. Influence of gas composition on recurrence of atelectasis after a reexpansion
maneuver during general anesthesia: a computed tomography study. Rothen
HU, Sporre B, Engberg G, Wegenius G, Hogman M, Hedenstierna G; Anesthesiology
1995; 82:832-42.
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ABSTRACT
BACKGROUND: Respiratory failure secondary to cardiopulmonary bypass
(CPB) remains a major complication after cardiac surgery. The authors previously
found that the increase in intrapulmonary shunt was well correlated with
the amount of atelectasis. They tested the hypothesis that post-CPB atelectasis
can be prevented by a vital capacity maneuver (VCM) performed before termination
of the bypass.
METHODS: Eighteen pigs received standard hypothermic CPB (no ventilation
during bypass). The VCM was performed in two groups and consisted of inflating
the lungs during 15 s to 40 cmH2O at the end of the bypass. In one group,
the inspired oxygen fraction (FIO2) was then increased to 1.0. In the second
group, the FIO2 was left at 0.4. In the third group, no VCM was performed
(control group). Ventilation-perfusion distribution was measured with the
inert gas technique and atelectasis by computed tomographic scanning.
RESULTS: Intrapulmonary shunt increased after bypass in the control
group (from 4.9 +/- 4% to 20.8 +/- 11.7%; P < 0.05) and was also increased
in the vital capacity group ventilated with 100% oxygen (from 2.2 +/- 1.3%
to 6.9 +/- 2.9%; P < 0.01) but was unaffected in the vital capacity group
ventilated with 40% oxygen. The control pigs showed extensive atelectasis
(21.3 +/- 15.8% of total lung area), which was significantly larger (P<
0.01) than the proportion of atelectasis found in the two vital capacity
groups (5.7 +/- 5.7% for the vital capacity group ventilated with 100% oxygen
and 2.3 +/- 2.1% for the vital capacity group ventilated with 40% oxygen.
CONCLUSION: In this pig model, postcardiopulmonary bypass atelectasis
was effectively prevented by a VCM.
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