Monitoring of Liver Oxygenation During Neuroleptanalgesia in the Dog

Abstract

In an animal study (7 mongrel dogs) the effects of neuroleptanalgesia (NLA) and combinations of NLA with nitrous oxide (N₂O) and isoflurane on the macro- and microcirculation of the liver were investigated. Measurements were made in three steps. After NLA alone the dogs were supplementarily ventilated with nitrous oxide/oxygen at a ratio of 2 :1. During the last step, 1 MAC isoflurane was added to the inspired gas. From the portal vein, arterial and mixed-venous systems' hemodynamic parameters, blood gases, and acid-base balance were recorded. As a parameter of oxygenation the tissue PO₂ of the liver was measured with a multiwire surface electrode. During NLA stable hemodynamic conditions and a balanced acid-base status were observed. The nitrous oxide combination resulted in an increase of the mean pulmonary artery pressure of 16%. The addition of isoflurane had a negative inotropic effect: The heart index decreased to 74% of the starting value and the total peripheral resistance (TPR) increased by 27%. The summarized PO₂ histograms under NLA and NLA/N₂O showed arithmetic mean values of 34.1 and 35.2 mm Hg, respectively. The addition of isoflurane resulted in a left shift and a decrease of the mean value to 28.6 mm Hg. This histogram corresponds exactly to the oxygen pressure distribution in the dog liver during piritramide basic anesthesia. It seems that NLA and the combination of NLA/N₂O increase the liver perfusion with a higher portal-venous and tissue PO₂. This effect can be explained only by a massive change of visceral circulation. It is canceled by the addition of isoflurane.

Key Words:

• Neuroleptanalgesia
• nitrous oxide
• isoflurane
• liver oxygenation
• animal model (dog)