Background: The non-rebreathing anesthetic system is one of the most used in veterinary medicine in small animals due to the
low resistance to breathing. The Bain System is constructed with one corrugated external hose, one internal conducting duct for
fresh gases, an optional pop-off valve and two connections (one for the breathing bag and the other for the patient). According
to the literature recommendations, this system requires an oxygen flow rate between 130-200 mL/kg/min. This present work aims
to evaluate the arterial blood gases tension changes in dogs anesthetized with two different oxygen flow rates (100 mL/kg/min
or 200 ml/kg/min) using a nonrebreathing Bain System, in adult healthy dogs.
Materials, Methods & Results: Fourteen adult healthy mongrel dogs (10 males and 4 females) ranging from 3.5 to 4.5 years old,
with average body weight of 12.5 + 0.81 kg, were submitted to preanesthetic medication with acepromazine maleate (0.1 mg/kg
IM) and fentanyl citrate (5 mcg/kg IM) and after 15 min induction of anesthesia was performed using sodium thiopental (9 mg/
kg, IV). An anesthetic state using the Bain System was maintained using oxygen 100 mL/kg/min and isoflurane 1.5 V% in group
I and oxygen 200 mL/kg/min and isoflurane 1.5V% in group II. Heart and respiratory rates, oxygen saturation, arterial pH, blood
gases and bicarbonate were the variables analyzed after induction and before intubation (T0), immediately after intubation (T1),
at 10 min of anesthesia (T2), at 20 min of anesthesia (T3), at 30 min of anesthesia (T4), at 40 min of anesthesia (T5), at 50 min of
anesthesia (T6), and after 60 min of anesthesia in the end of the procedure (T7). The results indicated that animals submitted to
both protocols showed a significant decrease in arterial pH values from T1 to T7 in relation to T0. The differences found between
the values from times T1 to T7 were statistically significant between them. The values of PaCO2 demonstrated statistically
significant differences from T1 to T7 in both protocols. Protocol I showed statistically significant difference between T0 with
respect to the times T1 to T7. For the parameter PaO2 there were statistically significant differences between protocols in T0 and
not from T0 in comparison with T1 to T7 in both protocols. However, there were no significant differences between protocols due
to inhalation of pure O2, that even using different flows causes an increase in PaO2. The values of heart rate showed significant
differences from T1 to T7 between protocols I, and protocol II. The values of base excess, O2 saturation and respiratory rate
showed no statistically significant between protocols and time points.
Discussion: Oxygen flow rate is the mean by which the CO2 is eliminated from nonrebrathing systems. Higher flow rates than
those used in circle anesthetic systems are recommended in order to avoid carbon dioxide rebreathing within the nonrebrathing
system. In our study we did demonstrate that the use of oxygen flow rate of 200 mL/kg/min with a Bain system kept the blood gas
values and pH within acceptable range in healthy dogs submitted to general anesthesia with isoflurane 1.5V%. A lower flow rate
of 100 mL/kg did produce arterial hypercapnia and academia of respiratory origin. The explanation for such result is probably due
to the physical property of the inhalant anesthetic carrier gas flow. The use of higher flow rates will force exhaled carbon dioxide
through the pop off valve, reducing its absorption. According to our findings a flow rate of 200 mL/kg/min should be recommended
for the Bain system in dogs.
