An appropriate level of arterial partial pressure of carbon dioxide (PaCO₂) under isoflurane anesthesia was assessed during clinical case operation of 31 thoroughbred racing horses. Horses were assigned to either one of three PaCO₂ conditions, 40, 50 and 60 mmHg during intermittent positive pressure ventilation (IPPV) . There was no difference in arterial to end-tidal CO₂ tension (P [a-et] CO₂) between horses maintained at 40 mmHg and other PaCO₂ values. Compared with other PaCO₂ values, the ratio of physiologic dead space ventilation to minute ventilation (V_D/V_T) and the dosage of dobutamine for hypotension were higher in the 40 mmHg group. One horse at a PaCO₂ of 40 mmHg showed in junctional premature contraction. Though the minute ventilation volume of horses in the 60 mmHg group was significantly lower than other PaCO₂ values, the arterial partial pressure of oxygen (PaO₂) was maintained at more than 200mmHg. Neither cardiac arrythmias nor postanesthetic complications developed, but spontaneus breathing was observed in horses maintained at PaCO₂ of 60mmHg. Based on these findings, a clinically appropriate level of PaCO₂ may be estimated to be approximately 50 mmHg during inhalation anesthesia in horses.

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A large animal anesthetic machine with a jet-type respirator was designed for horses and the practical applicability was evaluated. The trial respirator was remodeled from a time cycled respirator designed for small animal use and tested in 5 healthy adult throughbred horses. After preanesthetic medication with xylazine and midazolam (1.0 mg and 0.01 mk/kg of body weight, IV, respectively), anesthesia was induced with ketamine (2.5 mk/kg of body weight, IV) . Horses were positioned in dorsal recumbency and anesthesia was maintained with isoflurane and oxygen through an endotracheal tube. Spontaneous ventilation was changed to controlled ventilation at 30 minutes after the start of anesthesia. The initial settings for controlled ventilation were an inspiratory time (IT) of 1.0 sec, a maximum end-inspiratory airway pressure of 20 cm H₂O, and respiratory rate of 8 cycles/min. The IT was changed every 15 min from 1.0 to 1.5, 2.0 and 2.5 sec in that order. The tidal volume (TV), arterial blood gas tension and cardiac output were measured and recorded at the end of each IT setting.
The correlation between IT and arterial carbon dioxide tension (PaCO₂), and between IT and TV were expressed through liner regression analysis as PaCO₂ (mmHg) =-21.18 (IT (sec) ) +79.07 (γ= 0.94) and TV (ml /kg) =7.69 (IT (sec) ) +2.37 (γ=0.98), respectively. The cardiac output during IT of 2.5 sec decreased to 77.8% of a baseline value of spontaneous ventilation. These results suggest that this respirator is considered useful and the settings for IT of 1.0-2.0 sec, maximum end inspiratory airway pressure of 20-27 cmH₂O, and respiratory rate of 8 cycles/min are optimal.

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Five adult dogs were bled at the rate of 40m1/kg per approximately 20 minutes. During the controlled bleeding, the end-tidal carbon dioxide concentration (ETCO₂) and the arterial carbon dioxide tension were measured. ETCO₂ decresed gradually with the increase of bleeding volume but PaCO₂ did not change. The results suggest that the decrease of ETCO₂ reflects the increase of bleeding volume than the decrese of PaCO₂ during the bleeding.

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[in Japanese]

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