Vaporizer in circle for delivery of isoflurane to dogs

Richard M. Bednarski From the Department of Veterinary Clinical Sciences, The Ohio State University, 601 Vernon Tharp St, Columbus, OH 43210.

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James S. Gaynor From the Department of Veterinary Clinical Sciences, The Ohio State University, 601 Vernon Tharp St, Columbus, OH 43210.

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William W. Muir III From the Department of Veterinary Clinical Sciences, The Ohio State University, 601 Vernon Tharp St, Columbus, OH 43210.

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Summary:

An in-circuit vaporizer for delivery of isoflurane was evaluated. The isoflurane concentration within an isolated circle breathing circuit was determined for 1 hour in 6 in-the-circuit vaporizers with the wicks removed. A mechanical ventilator and artificial lung were connected to the circuit. Isoflurane concentration increased as vaporizer setting increased, and delivered concentration (%) at 60 minutes ( X ¯ ± sem) ranged from 0.46 ± 0.10 at tap setting 1 to 3.67 ± 0.30 at setting 5. Temperature of the isoflurane did not change.

Cardiovascular and respiratory function were maintained within a clinically acceptable range in 6 dogs anesthetized with thiamylal and maintained with 1.87% end-tidal isoflurane delivered from the in-circuit vaporizer during spontaneous ventilation, controlled ventilation, and closed-circuit anesthesia. The range of vaporizer tap settings ( X ¯ ± sem) was lower during closed-system anesthesia (2.5 ± 0.1 to 3.5 ± 0.6) and during controlled ventilation (2.6 ± 0.2 to 3.3 ± 0.2) than during semi-closed system anesthesia (5.4 ± 0.3 to 6.8 ± 0.4).

The in-circuit vaporizer was used to deliver isoflurane to 36 dogs anesthetized for a variety of surgical and medical procedures. Ventilation was spontaneous, assisted, and in 1 instance, controlled. Cardiovascular function, respiratory function, and recovery times were within clinically acceptable ranges. The initial vaporizer tap setting ( X ¯ ± sem) was 8.2 ± 0.4, and this corresponded to an end-tidal isoflurane concentration of 3.5 ± 0.6. The range of vaporizer settings during the maintenance phase ( X ¯ ± sem) was 2.8 ± 0.5 to 4.6 ± 1.9. This corresponded to an end-tidal isoflurane concentration of 1.2 ± 0.1 to 1.8 ± 0.1%.

This study documents that when appropriate guidelines are followed and limitations understood, the in-circle vaporizer is suitable for delivery of isoflurane to dogs undergoing a variety of surgical and medical procedures. Guidelines include removal of the wick, attention to the relatively rapid increase of anesthetic depth during the first 5 minutes of anesthesia, and the need to decrease the setting of the vaporizer control lever if assisted or controlled ventilation is used, or if closed system flow rates are used. Limitations include unpredictability of output with changing ambient temperature and difficulty adapting its use with semi-open breathing systems such as the t-piece or Bain coaxial circuit.

Summary:

An in-circuit vaporizer for delivery of isoflurane was evaluated. The isoflurane concentration within an isolated circle breathing circuit was determined for 1 hour in 6 in-the-circuit vaporizers with the wicks removed. A mechanical ventilator and artificial lung were connected to the circuit. Isoflurane concentration increased as vaporizer setting increased, and delivered concentration (%) at 60 minutes ( X ¯ ± sem) ranged from 0.46 ± 0.10 at tap setting 1 to 3.67 ± 0.30 at setting 5. Temperature of the isoflurane did not change.

Cardiovascular and respiratory function were maintained within a clinically acceptable range in 6 dogs anesthetized with thiamylal and maintained with 1.87% end-tidal isoflurane delivered from the in-circuit vaporizer during spontaneous ventilation, controlled ventilation, and closed-circuit anesthesia. The range of vaporizer tap settings ( X ¯ ± sem) was lower during closed-system anesthesia (2.5 ± 0.1 to 3.5 ± 0.6) and during controlled ventilation (2.6 ± 0.2 to 3.3 ± 0.2) than during semi-closed system anesthesia (5.4 ± 0.3 to 6.8 ± 0.4).

The in-circuit vaporizer was used to deliver isoflurane to 36 dogs anesthetized for a variety of surgical and medical procedures. Ventilation was spontaneous, assisted, and in 1 instance, controlled. Cardiovascular function, respiratory function, and recovery times were within clinically acceptable ranges. The initial vaporizer tap setting ( X ¯ ± sem) was 8.2 ± 0.4, and this corresponded to an end-tidal isoflurane concentration of 3.5 ± 0.6. The range of vaporizer settings during the maintenance phase ( X ¯ ± sem) was 2.8 ± 0.5 to 4.6 ± 1.9. This corresponded to an end-tidal isoflurane concentration of 1.2 ± 0.1 to 1.8 ± 0.1%.

This study documents that when appropriate guidelines are followed and limitations understood, the in-circle vaporizer is suitable for delivery of isoflurane to dogs undergoing a variety of surgical and medical procedures. Guidelines include removal of the wick, attention to the relatively rapid increase of anesthetic depth during the first 5 minutes of anesthesia, and the need to decrease the setting of the vaporizer control lever if assisted or controlled ventilation is used, or if closed system flow rates are used. Limitations include unpredictability of output with changing ambient temperature and difficulty adapting its use with semi-open breathing systems such as the t-piece or Bain coaxial circuit.

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