I have a case report on which i require the expert opinion as to the cause of the particular intraoperative event.
A 55 years male was posted for sublabial trans-sphenoidal pituitary tumor excision.Pre op. investigation were within normal limit.ECG was normal,CXR showed increased bronchopulmonary markings.He was a non smoker.Induction of anaesthesia was done with inj.midazolam(0.03mg/kg),sufentenil(0.5 mcg/kg0,thiopentone 4mg/kg.Intubation was done after giving vecuronium0.15mg/kg ,with no.8.5 cuffed ETT.Maintainence of anaesthesia was done with N2O,O2(66:33),isoflurane,and incremental dose of vecuronium and sufentenil.15 minutes after intubation patient started desaturating,SRO2 decreased to 92-93%.Air entry was checked and found to be equal,airway pressure was also normal.100% oxygen was given which improved saturation to100%.ABG done showed(FIO2=.33)pH7.3,pCO2 47.8,pO2 83.6,HCO3 23.1,BE -3.5.SaO292.8
Ventilation was continued with 100%O2.Repeat ABG (FIO2=1)showed pH7.4,pCO2 36.3,pO2 135.6,HCO3 24.2.1,SaO2 99.Surgery was continued .In the first half of surgery whenever N2O was switched SPO2 decreased,however in the latter half 50% N2O could be used without fall in SPO2.Neuromuscular effect was reversed with neostigmine+glyco. Patient was awake, following commands,SPO2100%,shifted to recovery with O2 by ventimask@FIO20.5,nursed in propped up position.
Postoperative course was uneventfuland patient discharged home 3 days after surgery.
I will be oblidged if anyone can answer our Query as to the cause of this particular event where switching on N20 decreased SPO2 but patient maintaining saturation with 100% o2