Effects of mechanical ventilation, thoracotomy, and one-lung ventilation on regional pulmonary blood flow

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Subtitle: An experimental study in rabbits
Author list: Sckell A
Publisher: Springer Verlag (Germany)
Publication year: 1995
Volume number: 44
Issue number: 5
Start page: 319
End page: 327
Number of pages: 9
ISSN: 0003-2417
Languages: English-Great Britain (EN-GB)


Abstract

The physiological pattern of regional pulmonary blood flow is mainly
determined by the relationship of pulmonary arterial, venous, and
alveolar pressures. Changes in alveolar pressure and pulmonary geometry
may therefore be expected to influence regional perfusion, which is a
key determinant of pulmonary gas exchange. Unilateral thoracotomy is
usually performed with the patient in the lateral decubitus position.
The present study examined the influence of mechanical factors on
regional pulmonary blood flow distribution in rabbits in the lateral
decubitus position during normoxia and unilateral hypoxia. Methods.
Anaesthetised white New Zealand rabbits (n = 8) weighing 2200-3900 g (x̄
= 2860 g) received central venous injections of radioactive
microspheres while in the left lateral decubitus position during
spontaneous breathing (SB) and during mechanical ventilation (two-lung
ventilation, 2LV), under closed (2LV(C)) and open chest (2LV(T))
conditions, as well as during unilateral hypoxia of the nondependent
lung induced by nitrogen inflation (1LV(N)) or atelectasis (1LV(A)). The
method used for one-lung ventilation (1LV) has been previously
described in detail. Arterial, central venous, and pulmonary arterial
pressures were recorded continuously. Lungs were excised, dried in the
inflated state, and cut into 16 sagittal slices, which were further
divided into lobar components, the lower lobes into center and
periphery. The radioactivity of each specimen was measured in a
gamma-counter; perfusion of the individual tissue specimens was
quantified using the software program MIC III. The Friedman test
followed by paired comparisons according to Conover was used for
statistical analysis of differences between the experimental phases.
Perfusion of central and peripheral parts of isogravitational slices was
compared by use of the Wilcoxon matched pairs test. Values are given as
means ± SE; the level of significance was P < 0.05 unless otherwise
indicated. Results and discussion. Haemodynamic parameters did not
differ significantly between the experimental phases. Compared to 2LV, a
significant increase in venous admixture (P < 0.05) and a
corresponding decrease in PaO2 (P < 0.01) were observed
during 1LV. This effect was significantly more pronounced during 1LV(A)
as compared to 1LV(N) (P < 0.01). Since inspiratory pressure was kept
constant thoughout the experiments, moderate respiratory acidosis
developed during both phases of 1LV. Regional perfusion (Q(r)) of the
nondependent lung was slightly reduced during 2LV(C) compared to SB and
2LV(T). One-lung ventilation induced a significant decrease in perfusion
of the hypoxic lung (P < 0.001 1LV(N), 1LV(A) vs. SB,2LV(C),2LV(T)).
In accordance with the data obtained from blood gas analysis and
oximetry, this effect was more pronounced during N2
insufflation than during atelectasis (P < 0.01 1LV(N) vs. 1LV(A)).
Among the factors that may account for this effect, Pa(CO2) did not
differ significantly between both phases of 1LV. During N2 insufflation PO2 at the hypoxia-sensitive site is lower than during atelectasis, where it equals mixed-versus PO2 (Pv̄(O2)). The difference in local PO2 is unlikely, however, to have caused the changes in regional perfusion between 1LV(N) and 1LV(A), since Pv̄O2
was as low as 40 mmHg during 1LV(A) and the pulmonary vascular response
to hypoxia has been found to reach its maximum in this PO2
range. Enhanced redistribution of regional perfusion during 1LV(N) as
compared to 1LV(A) is therefore most likely attributed to differences in
alveolar pressure and pulmonary geometry. Apart from a radial perfusion
gradient in the right lower lobe during 2LV(C) and 2LV(T), no
isogravitational Q(r) gradients were observed. Conclusion. We conclude
that controlled mechanical ventilation in the lateral decubitus position
causes only minor changes in vertical blood flow distribution. During
1LV inflation of the hypoxic lung by positive airway pressure enhances
hypoxia-induced blood flow redistribution, thereby improving arterial
oxygenation. Differences in alveolar pressure and lung geometry are the
most important factors to account for this effect.


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Last updated on 2019-13-08 at 00:15