Respiratory variables during thoracotomy for PDA ligation

Physiological deadspace fraction of tidal volume (VD/VT), arterial to end-tidal carbon dioxide tension differences [P(a-E')CO2], arterial oxygen tension (PaO2) and respiratory system compliance were studied in twenty patients with patent ductus ateriosus scheduled for multiple ligation and transfixation through posterolateral thoracotomy under general anaesthesia with controlled ventilation. The study period was divided into six stages: stage 1--supine posture under anaesthesia, stage 2--lateral posture before start of surgery, stage 3--after chest opening before lung manipulation, stage 4--after ductus ligation and lung re-expansion before chest closure, stage 5--lateral posture, chest closed, stage 6--supine stage before reversal. There was a significant (P < 0.01) increase of VD/VT on attaining the lateral posture. The fraction decreased significantly (P < 0.05) on opening of the chest (stage 3) and subsequently increased at stage 4. There was no significant change in mean P(a-E')CO2 at various stages of thoracotomy. PaO2 fell significantly on opening of the chest and was lowest before chest closure (stage 4). PaO2 increased following chest closure but was still significantly lower than the pre-surgical supine stage. Respiratory system compliance was lowest at stage 4. Changes in deadspace fraction VD/VT do not correspond favourably to arterial oxygen tensions during posterolateral thoractomy.