Two-Lung and One-Lung Ventilation in Patients - Anesthesia ...

40 CARDIOVASCULAR ANESTHESIA BARDOCZKY ET AL. ANESTH ANALG
POSITION AND Fio 2 DURING OLV 2000;90:35–41
not cause more of an increase in shunt. Pao 2 during
TLV may also be a determining factor of oxygenation
during OLV (18).
Second, we did not study OLV for long periods
(more than two hours) with high Fio 2 values, but
according to the study of Barker et al. (19), late hypoxemia
occurs (mean OLV time, 170 minutes) when ventilating
with 100% oxygen, which may be in part
caused by absorption atelectasis. High Fio 2 can lead to
arteriovenous shunting in areas of airway closure and,
further, cause absent ventilation in lung units with
low ventilation/perfusion ratios.
Third, if we consider that the lateral TLV/OLV always
followed the supine TLV/OLV sequence, one
can argue that a time effect could influence the present
results. Concerning the time course of the HPV response,
many experimental and clinical studies have
been performed, with sometimes contradictory results.
Benumof (20) showed that intermittent hypoxic
challenges potentiated the hypoxic vasoconstriction in
the left lower lobe of open-chest dog lungs, but the
preparation and manipulation of the animals required
considerable instrumentation and manipulation,
which may have interfered with the HPV. Carlsson et
al. (21) found maximal HPV response within 15 minutes
of hypoxia in a human study, which agrees with
observations in animal studies. Tucker and Reeves
(22) were unable to maintain HPV during acute hypoxia
in anesthetized dogs. During one-lung hypoxia
in dogs, Domino et al. (23) studied HPV in closedchest
dogs and found a maximal level from the very
first hypoxic challenge. Thus, there is a wide variation
in the results obtained concerning the influence of
time on HPV. In our study, the experimental procedure
was almost identical to the procedure performed
by Carlsson et al. (21) or by Domino et al. (23), who
concluded that the time factor should not be a hindrance
to manipulative studies on the HPV response
once a maximal response has been evoked, normally
in 10–15 minutes.
Fourth, if we consider that, after the period of supine
TLV, we have only declamped the tracheal limb
of the DLT and closed it, without sighing the nondependent
lung, the 5-mL/kg gas distributed to that
lung will not reverse the amount of atelectasis. So, our
study may be comparable to the study of Fiser et al.
(9), but as significant changes were found in the lateral
position, we can argue that if HPV was maximal after
the first 15 min of hypoxia, there is another factor,
probably gravity, that could contribute to flow redistribution.
Nevertheless, in the particular conditions of
the present study—patients with mild COPD, closed
chest, mixed locoregional/general balanced anesthesia—increased
HPV responses in the lateral position
cannot be excluded as explanations of Pao 2 values
observed in the second OLV episode.
Paco 2, especially at high values, influences the level
HPV response (24). Thus, the Paco 2 values of the
studied patients were always maintained within normal
limits in the different periods of blood gas
measurements.
Another factor that may influence hypoxia during
OLV is the side of the surgery. Left thoracotomy has a
better Pao 2 during OLV than right thoracotomy, because
the left lung normally receives 10 percent less
cardiac output than the right lung (18). In our study,
there were no statistical differences concerning the
side of surgery in the three groups or among the three
groups.
Gravity is a major, pharmacologically and physiopathologically,
independent determinant of regional
pulmonary blood flow distribution. The extent of
blood flow redistribution depends on the local relationship
between pulmonary arterial, venous, and airway
pressures. In the lateral position, regional blood
flow increases from the nondependent to the dependent
thoracic wall (8). Unfortunately, the design of our
study did not permit us to distinguish between the
direct effects of HPV on blood flow and the effects of
gravitational redistribution of blood flow.
The differences observed among the three groups of
patients supports the hypothesis of Benumof et al.
(25), who demonstrated, on a canine left lower lobe
preparation, that if Fio 2 changes cause secondary
changes in Pao 2, and in the mixed venous oxygen
tension, then the mixed venous oxygen tension is a
new and important determinant of the magnitude of
HPV. This suggests that when one compartment Fio 2
is 1.0 and the other compartment is hypoxic, HPV in
the hypoxic compartment is maximal.
The method used to ventilate the dependent lung is
an important determinant of the blood flow distribution
during OLV. High airway pressures can compress
lung vessels, diverting blood flow from ventilated to
nonventilated regions. However, hypoventilation of
the dependent lung during OLV is associated with
lower airway pressure, and the ventilated lung pulmonary
vascular resistance may decrease, thus promoting
HPV in the nonventilated lung (7). This mechanism
of improved Pao 2 was unlikely in the present
investigation, as ventilatory settings were kept constant,
inspiratory airway pressures were not changed,
and the mechanical characteristics of the dependent
lung remained unaltered (Tables 2–4) after changing
the position. These findings indicate that the improved
Pao 2 cannot be attributed to an altered HPV
caused by change in ventilatory pattern.
Surgical compression and retraction may also contribute
to the passive reduction of nondependent lung
blood flow (15). However, in our study, there was no
mechanical effect on lung parenchyma, as the data