Adaptation of the Bain Circuit to the Valvular Circle Circuit in Cats Anaesthetized by Isoflurane
*Adriano Bonfim Carregaro, Stélio Pacca Loureiro Luna, Daniela Paura, Daniel Kan Honsho
This study aimed to investigate the use of the Bain circuit adapted to the valvular circle system in cats below 5 kg of weight, by coupling the expiratory way of the Bain circuit to the expiratory valve of the circle system.
Eight male and female cats aging from 3 months to 5 years old and weighing from 1.7 to 4.5 kg were used. The animals were sedated with 0.5 mg/kg of methotrimeprazine (IM), followed 20 minutes later by induction of anaesthesia with 5 mg/kg of propofol. After endotracheal intubation, anaesthesia was maintained with 1 MAC (1,68V%) of isoflurane, using a 70 cm long Bain circuit with a O2 flow of 300 mL/kg for 30 minutes until anesthesia was stabilized. Four animals were then maintained for 30 minutes using the same circuit (Group A), followed by adaptation of the Bain circuit to the valvular circle circuit with 100 g of soda cal (Group B) for other 30 minutes. The other four animals were submitted to the same protocol, but the order of the circuits was inversed. Heart and respiratory rates, hemoglobin oxygen saturation, inspired and expired CO2 were measured after 30 minutes of the use of each circuit.
Statistical analysis was performed using the Student Newman Keuls test with a significant statistical level of p<0.05.
There was no difference between the groups for heart rate (A: 108±11; B: 112±20), respiratory rate (A: 20±5; B: 22±5), hemoglobin oxygen saturation (A: 98.5±1.4%; B: 98.3±1.4%) and expired CO2 (A: 29±5mmHg; B: 36±8mmHg). Inspired CO2 was higher with the adapted circle circuit (B: 7.6±3mmHg) when compared to the Bain circuit (A: 0mmHg).
The higher concentration of the inspired CO2, when the adapted circle circuit was used was probably caused by the increased resistance and/or increased dead space produced by the device. However 7.6 mmHg is equivalent to 1% inspired CO2 and is considered acceptable as a limit value for inspired CO2. The higher inspired CO2 was not followed by a significant higher expired CO2 or hypoxemia. It seems that the adaptation of the Bain circuit to the valvular circle circuit may be a practical option both for the Bain circuit and the circle system, for delivery of inhalation anaesthesia in cats lighter than 5 kg, in order to avoid some disadvantages of the Bain circuit and obtain some of the advantages of the circle circuit, such as humidification and warming of the inspired gas mixture, providing that inspired and expired CO2 are carefully monitored throughout anaesthesia. A further study using low flow O2 should be performed in order to investigate the efficacy of this device compared to the circle circuit.