BLOOD-GAS VALUES DURING INTERMITTENT POSITIVE PRESSURE VENTILATION AND SPONTANEOUS VENTILATION IN 160 ANESTHETIZED HORSES POSITIONED IN LATERAL OR DORSAL RECUMBENCY

One hundred sixty horses were anesthetized with xylazine, guaifenesin, thiamylal, and halothane far elective soft tissue and orthopedic proc edures. Horses were randomly assigned to one of four groups. Group 1 ( n = 40): Horses positioned in lateral (LR(G1); n = 20) or dorsal (DR(G 1); n = 20) recumbency breathed spontaneously throughout anesthesia. G roup 2 (n = 40): Intermittent positive pressure ventilation (IPPV) was instituted throughout anesthesia in horses positioned in lateral (LR( G2); n = 20) or dorsal (DR(G2); n = 20) recumbency. Group 3 (n = 40): Horses positioned in lateral (LR(G3); n = 20) or dorsal (DR(G3); n = 2 0) 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 pre ssure ventilation was instituted for the first half of anesthesia in h orses positioned in lateral (LR(G4); n = 20) of dorsal (DR(G4); n = 20 ) recumbency. Spontaneous ventilation (SV) occured for the second half of anesthesia. The mean time of anesthesia was not significantly diff erent within or between groups. The mean time of SV and IPPV was not s ignificantly different in groups 3 and 4. Variables analyzed included pH, PaCO2, PaO2, and P(A-a)O-2 (calculated). Spontaneous ventilation r esulted in significantly higher PaCO2 and P(A-a)O-2 values and signifi cantly lower PaO2 values in LR(G1) and DR(G1) horses compared with LR( G2) and DR(G2) horses. Intermittent positive pressure ventilation resu lted in normocarbia and significantly lower P(A-a)O-2 values in LR(G2) and DR(G2) horses. In LR(G2) the PaO2 values significantly increased from 20 minutes after induction to the end of anesthesia. The PaO2 and P(A-a)O-2 values were not significantly different from the beginning of anesthesia after IPPV in DR(G2) or DR(G3). The PaO2 values signific antly decreased and the P(A-a)O-2 values significantly increased after return to SV in horses in LR(G4) and DR(G4). The PaO2 values were low est and the P(A-a)O-2 values were highest in all horses positioned in dorsal recumbency compared with lateral recumbency and in SV horses co mpared with IPPV horses. The pH changes paralleled the changes in PaCO 2. Blood gas values during right versus left lateral recumbency in all groups were also evaluated. The PaO2 values were significantly lower and the P(A-a)O-2 values were significantly higher during SV in horses positioned in left lateral (LR(LG1)) compared with right lateral (LR( RG1)) recumbency. No other significant changes were found comparing le ft and right lateral recumbency. Arterial hypoxemia (PaO2 < 60 mm Hg) developed in 35% of DR(G1) horses and 20% of DR(G2) horses at the end of anesthesia. Arterial hypercarbia (PaCO2 = 50-60 mm Hg) developed in DR(G1) horses. Arterial hypoxemia that developed in 20% of DR(G3) hor ses was not improved with IPPV. Arterial hypoxemia developed in 55% of DR(G4) horses after return to SV. Some DR(G4) horses with hypoxemia a lso developed hypercarbia, whereas some had PaCO2 values within normal limits. Arterial hypoxemia developed in one LR(G1) and two LR(G4) hor ses. Hypercarbia developed in only one LR(G4) horse. (C)Copyright 1995 by The American College of Veterinary Surgeons