Blood Gas Values During Intermittent Positive Pressure Ventilation and Spontaneous Ventilation in 160 Anesthetized Horses Positioned in Lateral or Dorsal Recumbency
Authors: DAY THOMAS K. , GAYNOR JAMES S. , MUIR III WILLIAM W. , BEDNARSKI RICHARD M. , MASON DIANE E. 
Published: February 2008

One hundred sixty horses were anesthetized with xylazine, guaifenesin, thiamylal, and halothane for elective soft tissue and orthopedic procedures. Horses were randomly assigned to one of four groups. Group 1 (n = 40): Horses positioned in lateral (LRG1,; n = 20) or dorsal (DRG1,; n = 20) recumbency breathed spontaneously throughout anesthesia. Group 2 (n = 40): Intermittent positive pressure ventilation (IPPV) was instituted throughout anesthesia in horses positioned in lateral (LRG2; n = 20) or dorsal (DRG2; n = 20) recumbency. Group 3 (n = 40): Horses positioned in lateral (LRG3; n = 20) or dorsal (DRG3; n = 20) recumbency breathed spontaneously for the first half of anesthesia and intermittent positive pressure ventilation was instituted for the second half of anesthesia. Group 4 (n = 40): Intermittent positive pressure ventilation was instituted for the first half of anesthesia in horses positioned in lateral (LRG4; n = 20) or dorsal (DRG4; n = 20) recumbency. Spontaneous ventilation (SV) occured for the second half of anesthesia. The mean time of anesthesia was not significantly different within or between groups. The mean time of SV and IPPV was not significantly different in groups 3 and 4. Variables analyzed included pH, PaCO2, PaO2, and P(A‐a)O2(calculated). Spontaneous ventilation resulted in significantly higher PaCO2and P(A‐a)O2values and significantly lower PaO2values in LRG1, and DRG1, horses compared with LRG2and DRG2horses. Intermittent positive pressure ventilation resulted in normocarbia and significantly lower P(A‐a)O2values in LRG2and DRG2horses. In LRG2the Pao2values significantly increased from 20 minutes after induction to the end of anesthesia. The PaO2and P(A‐a)O2values were not significantly different from the beginning of anesthesia after IPPV in DRG2or DRG3. The PaO2values significantly decreased and the P(A‐a)O2values significantly increased after return to SV in horses in LRG4, and DRG4. The PaO2values were lowest and the P(A‐a)O2values were highest in all horses positioned in dorsal recumbency compared with lateral recumbency and in SV horses compared with IPPV horses. The pH changes paralleled the changes in PaCO2. Blood gas values during right versus left lateral recumbency in all groups were also evaluated. The PaO2values were significantly lower and the P(A‐a)O2values were significantly higher during SV in horses positioned in left lateral (LRLG1) compared with right lateral (LRRG1) recumbency. No other significant changes were found comparing left and right lateral recumbency. Arterial hypoxemia (PaO2< 60 mm Hg) developed in 35% of DRG1horses and 20% of DRG2horses at the end of anesthesia. Arterial hypercarbia (PaCO2= 50–60 mm Hg) developed in DRoi horses. Arterial hypoxemia that developed in 20% of DRG3horses was not improved with IPPV. Arterial hypoxemia developed in 55% of DRG4horses after return to SV. Some DRG4horses with hypoxemia also developed hypercarbia, whereas some had PaCO2values within normal limits. Arterial hypoxemia developed in one LRG1, and two LRG4, horses. Hypercarbia developed in onlv one LRG4horse.