RESPIRATORY VARIABLES DURING THORACOTOMY FOR PDA LIGATION

Physiological deadspace fraction of tidal volume (V-D/V-T), arterial t o end-tidal carbon dioxide tension differences [P((a-E'))CO2], arteria l oxygen tension (PaO2) and respiratory system compliance were studied in twenty patients with patent ductus ateriosus scheduled for multipl e ligation and transfixation through posterolateral thoracotomy under general anaesthesia with controlled ventilation. The study period was divided into sh stages: stage 1-supine posture under anaesthesia, stag e 2-lateral posture before start of surgery, stage 3-after chest openi ng 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<0.0 1) increase of V-D/V-T 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 signifi cantly on opening of the chest and was lowest before chest closure (st age 4). PaO2 increased following chest closure but was still significa ntly lower than the pre-surgical supine stage. Respiratory system comp liance was lowest at stage 4, Changes in deadspace fraction V-D/V-T do not correspond favourably to arterial oxygen tensions during posterol ateral thoractomy.