Abstract
BackgroundAtelectasis can provoke pulmonary and non-pulmonary complications after general anaesthesia. Unfortunately, there is no instrument to estimate atelectasis and prompt changes of mechanical ventilation during general anaesthesia. Although arterial partial pressure of oxygen (PaO2) and intrapulmonary shunt have both been suggested to correlate with atelectasis, studies yielded inconsistent results. Therefore, we investigated these correlations.MethodsShunt, PaO2 and atelectasis were measured in 11 sheep and 23 pigs with otherwise normal lungs. In pigs, contrasting measurements were available 12 hours after induction of acute respiratory distress syndrome (ARDS). Atelectasis was calculated by computed tomography relative to total lung mass (Mtotal). We logarithmically transformed PaO2 (lnPaO2) to linearize its relationships with shunt and atelectasis. Data are given as median (interquartile range).ResultsMtotal was 768 (715–884) g in sheep and 543 (503–583) g in pigs. Atelectasis was 26 (16–47) % in sheep and 18 (13–23) % in pigs. PaO2 (FiO2 = 1.0) was 242 (106–414) mmHg in sheep and 480 (437–514) mmHg in pigs. Shunt was 39 (29–51) % in sheep and 15 (11–20) % in pigs. Atelectasis correlated closely with lnPaO2 (R2 = 0.78) and shunt (R2 = 0.79) in sheep (P-values<0.0001). The correlation of atelectasis with lnPaO2 (R2 = 0.63) and shunt (R2 = 0.34) was weaker in pigs, but R2 increased to 0.71 for lnPaO2 and 0.72 for shunt 12 hours after induction of ARDS. In both, sheep and pigs, changes in atelectasis correlated strongly with corresponding changes in lnPaO2 and shunt.Discussion and ConclusionIn lung-healthy sheep, atelectasis correlates closely with lnPaO2 and shunt, when blood gases are measured during ventilation with pure oxygen. In lung-healthy pigs, these correlations were significantly weaker, likely because pigs have stronger hypoxic pulmonary vasoconstriction (HPV) than sheep and humans. Nevertheless, correlations improved also in pigs after blunting of HPV during ARDS. In humans, the observed relationships may aid in assessing anaesthesia-related atelectasis.
Highlights
Contradictory results on the effects of lung protective mechanical ventilation with low tidal volumes, positive end-expiratory pressure (PEEP) and recruitment manoeuvres during anaesthesia ventilation on the postoperative outcome have been published [1,2]
If these differences would apply, there should be a strong correlation between atelectasis and oxygenation or shunt, respectively, in humans or sheep breathing pure oxygen, while this correlation should be weaker in pigs, as long as other conditions blunting hypoxic pulmonary vasoconstriction (HPV) such as intense inflammation are absent
If atelectasis still persisted on computed tomography (CT), measurements were repeated after applying another recruitment manoeuvre (RM) and PEEP of 20 cmH2O to achieve full lung recruitment
Summary
Atelectasis can provoke pulmonary and non-pulmonary complications after general anaesthesia. There is no instrument to estimate atelectasis and prompt changes of mechanical ventilation during general anaesthesia. Arterial partial pressure of oxygen (PaO2) and intrapulmonary shunt have both been suggested to correlate with atelectasis, studies yielded inconsistent results.
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