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

ANESTH ANALG CARDIOVASCULAR ANESTHESIA BARDOCZKY ET AL. 39
2000;90:35–41 POSITION AND Fio 2 DURING OLV
Figure 2. Values of arterial oxygen tension increases between lateral and supine positions, during TLV and OLV in the three groups of
patients. The bold lines represent the median values. TLV two-lung ventilation, OLV one-lung ventilation.
to the dependent lung (7,8). As a result of anesthesia
and muscle relaxation, the distribution of ventilation
also changes in the lateral position because the applied
positive-pressure ventilation displaces the diaphragm
preferentially at the nondependent part. Traditionally,
this discrepancy is thought to be disadvantageous (7).
However, in our patients, the change in position from
the supine to lateral decubitus position resulted in a
modest change in Pao 2 during TLV (124 [81–240] and
132 [102–296] mm Hg in Group 0.4; 256 [172–391] and
289 [225–353] mm Hg in Group 0.6; and 472 [232–591]
and 492 [336–600] mm Hg in Group 1.0, respectively).
This is in agreement with the findings of Boldt et al.
(16) and Rehder et al. (17), who also reported no
difference in Pao 2 between the lateral and the supine
positions with TLV.
Inducing OLV when the patient is in the lateral
position activates HPV and reduces the further perfusion
of the collapsed lung. Hence, we found that Pao 2
was significantly greater when OLV was initiated after
turning the patients into the lateral decubitus position
(Tables 2–4 and Figures 1 and 2).
Fiser et al. (9) studied the period of OLV in both the
supine and the lateral positions and did not find
changes in arterial oxygen tension after 10 to 20 minutes
of OLV when the patients were turned into the
lateral position. However, in the study of Fiser et al.
(9), OLV was initiated in the supine position and
maintained continuously, even during the period of
positioning and turning the patient, with an Fio 2 of
1.0.
The most important mechanism for reducing blood
flow of an atelectatic lung is HPV (6,14). When OLV is
induced in the lateral position, the blood flow of the
nondependent lung is already reduced by gravitational
forces, and HPV further reduces blood flow. In
contrast, in the supine position, both lungs are equally
exposed to an identical gravitational force; thus during
OLV, the reduction of blood flow depends solely
on the strength of the HPV.
Here, consideration should be given to possible limitations
in our experimental methods.
First, concerning the patients included in the study,
the presence of chronic airflow obstruction may be
associated with better Pao 2 during OLV possibly because
of dynamic hyperinflation resulting in an increased
functional residual capacity and intrinsic positive
end-expiratory pressure in the dependent lung
(18). In contrast, in patients with severe COPD, HPV
may not be an important protective mechanism, as
these patients already have an increased pulmonary
arterial pressure and reduced pulmonary vascular
bed. The amount of disease in the nondependent lung
is also a significant determinant of the amount of
blood flow to the nondependent lung. If the nondependent
lung is severely diseased, there may be a
fixed reduction in blood flow to this lung preoperatively,
and the collapse of such a diseased lung may