The normai response of the pulmonary vasculature to one-lung atelectasis is an increase in pulmonary vascular resistance (PVR). The mechanism of the increase in PVR is thought to be due almost entirely to hypoxic pulmonary vasoconstriction (HPV).
Regional HPV results in blood flow diversion from hypoxic regions to normoxic regions. The expected pulmonary shunt is thereby reduced and the arterial oxygen tension increased. PEEP improves the arterial oxygen tension as a result of increasing
functional residual capacity (FRC) and decreasing intrapulmonary shunt.
The aims of the present studies were to observe blood flow diversion from atelectatic lung to normoxic lung and to prove a sustained redistribution of pulmonary blood flow from ventilated with PEEP to atelectatic regions. This study evaluated the
interactions between HPV and PEEP.
Eight mongrel dogs were anesthetized with pentobarbital. Left pulmonary blood flow was measured with eletromagnetic flow probes following left lateral thoracotomy.
Pulmonary arterial pressures, PCWP, systemic arterial pressures were measured via indwelling catheter. Cardiac output was determined by thermodilution in trplicate. The right lung was ventilated continuosly with 100% O2 while left lung was
ventilated
with 100 O2 (control phase), and unventilated for 60 min. of atelectasis. PEEP of 5 and 10 cmH2O was applied to the right lung.
During two-lung ventilation with 100 oxygen, cardiac output was 2890±880 ml/min. (mean±SD)and left pulmonary blood flow was 1100±220 ml/min.
Left lung atelectasis resulted in a reduction of the percent left blood flow compared with cardiac output from. 41±10% to 29±7% at 15 min and to 22±9% at 60 min (p<0.05). The ratio of left pulmonary blood flow to mean pulmonary artery pressure
decreased from 51±25 ml/min/mmHg in control to 19±7 ml/min/mmHg at 60 min (p<0.05). Left pulmonary vascular resistance increased gradually (p<0.01). Arterial oxgen tension was the lowest at 15 min (165±66 mmHg) and increased subsequently
(p<0.01).
Intrapulmonary shunt was 27±6% in control phase and abruptly increased to 37±6% at 15 min after after atelectasis and decreased to 34±10% at 60 min.
When 10 cmH2O PEEP was applied to the right lung during left lung atelectasis, the percent ratio of left pulmonary blood flow to cardiac output was significantly increased from 22±9% at 60 min of left lung atelectasis to 34±8% (p<0.05). Left
pulmonary
vascular resistance significantly decreased as compared with 45 and 60 min of left lung atelectasis (pmmHg
at 15
and 30 min. of left lung atelectasis(p<0.05).
The present study demonstrated that the response to acute atelectasis is a regional increase in pulmonary vascular resistance and a sustained diversion of blood flow away from the atelectatic lung. In this study, the application of 10 cmH2O PEEP
resulted in a redistribution of pulmonary blood flow from normoxic lung to atelectatic lung and didn't affect arterial oxygenation.
We conclude that when employing the technique of one-lung anesthesia, PEEP to improve oxygenation should be cautiously applied and a search for the maximum oxygenation and a minimum redistribution might be started, in an attempt to find the
optimal
PEEP.
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