Objective—To evaluate the effects of increasing doses of remifentanil hydrochloride administered via constant rate infusion (CRI) on the minimum alveolar concentration (MAC) of isoflurane in cats. Animals—6 healthy adult cats. Procedures—For each cat, 2 experiments were performed (2-week interval). On each study day, anesthesia was induced and maintained with isoflurane; a catheter was placed in a cephalic vein for the administration of lactated Ringer's solution or remifentanil CRIs, and a catheter was placed in the jugular vein for collection of blood samples for blood gas analyses. On the first study day, individual basal MAC (MAC_Basal) was determined for each cat. On the second study day, 3 remifentanil CRIs (0.25, 0.5, and 1.0 μg/kg/min) were administered (in ascending order); for each infusion, at least 30 minutes elapsed before determination of MAC (designated as MAC_R0.25, MAC_R0.5, and MAC_R1.0, respectively). A 15-minute washout period was allowed between CRIs. A control MAC (MAC_Control) was determined after the last remifentanil infusion. Results—Mean ± SD MAC_Basal and MAC_Control values at sea level did not differ significantly (1.66 ± 0.08% and 1.52 ± 0.21%, respectively). The MAC values determined for each remifentanil CRI did not differ significantly. However, MAC_R0.25, MAC_R0.5, and MAC_R1.0 were significantly decreased, compared with MAC_Basal, by 23.4 ± 7.9%, 29.8 ± 8.3%, and 26.0 ± 9.4%, respectively. Conclusions and Clinical Relevance—The 3 doses of remifentanil administered via CRI resulted in a similar degree of isoflurane MAC reduction in adult cats, indicating that a ceiling effect was achieved following administration of the lowest dose.

Introduction: Atelectasis and distal airway closure are common clinical entities of general anaesthesia. These two phenomena are expected to reduce the ventilation of dependent lung regions and represent major causes of arterial oxygenation impairment in anaesthetic conditions. The behaviour of the elastance of the respiratory system (E_rs), as well as the lung aeration assessed by computed tomography (CT) scan, was evaluated during a descendent positive end-expiratory pressure (PEEP) titration. This work sought to evaluate the potential usefulness of E_rs monitoring to set the PEEP in order to prevent tidal recruitment and hyperinflation of healthy lungs under general anaesthesia. Methods: PEEP titration (from 16 to 0 cmH₂O, tidal volume of 8 ml/kg) was performed, and at each PEEP, CT scans were obtained during end-expiratory and end-inspiratory pauses in six healthy, anaesthetized and paralyzed piglets. The distribution of lung aeration was determined and the tidal re-aeration was calculated as the difference between end-expiratory and end-inspiratory poorly aerated and normally aerated areas. Similarly, tidal hyperinflation was obtained as the difference between end-inspiratory and end-expiratory hyperinflated areas. E_rs was estimated from the equation of motion of the respiratory system during all PEEP titration with the least-squares method. Results: Hyperinflated areas decreased from PEEP 16 to 0 cmH₂O (ranges decreased from 24–62% to 1–7% at end-expiratory pauses and from 44–73% to 4–17% at end-inspiratory pauses) whereas normally aerated areas increased (from 30–66% to 72–83% at end-expiratory pauses and from 19–48% to 73–77% at end-inspiratory pauses). From 16 to 8 cmH₂O, E_rs decreased with a corresponding reduction in tidal hyperinflation. A flat minimum of E_rs was observed from 8 to 4 cmH₂O. For PEEP below 4 cmH₂O, E_rs increased in association with a rise in tidal re-aeration and a flat maximum of the normally aerated areas. Conclusion: In healthy piglets under a descending PEEP protocol, the PEEP at minimum E_rs presented a compromise between maximizing normally aerated areas and minimizing tidal re-aeration and hyperinflation. High levels of PEEP, greater than 8 cmH₂O, reduced tidal re-aeration but increased hyperinflation with a concomitant decrease in normally aerated areas.

Background: Anesthetic blood solubility predicts pharmacokinetics for inhaled agents and is essential for determination of blood anesthetic concentrations from end-tidal gas concentrations using Henry's Law. Though used to model anesthetic effects in humans, there are limited interspecies solubility comparisons that include modern haloethers. This study aimed to measure hematocrit-adjusted blood:gas anesthetic partition coefficients (λ B:G) for desflurane, sevoflurane, isoflurane, and methoxyflurane in humans and animals. Methods: Whole blood was collected from 20 rats, 8 horses, and 4 each of cats, cattle, humans, dogs, goats, pigs, rabbits, and sheep. Plasma or cell volume was removed to adjust all samples to a packed cell volume of 40%. A single-agent calibration gas headspace was added to blood in a glass syringe and was mixed and equilibrated at 37°C for 2 h. Agent concentrations in the calibration gas and syringe headspace were measured using gas chromatography. Anesthetic solubility in saline, citrate-phosphate-dextrose-adenine, and olive oil were similarly measured. Results: Except for goats, all animal species had at least one λ B:G measurement that differed significantly from humans. For each agent, λ B:G positively correlated with serum triglyceride concentrations, but this only explained 25% of interspecies variability. Desflurane was significantly less soluble in blood than sevoflurane in some species (e.g., humans) but not in others (e.g., rabbits). Conclusions: Anesthetic partition coefficients differ significantly between humans and most animals for haloether anesthetics. Because of their similar λ B:G values, goats may be a better animal model for inhaled anesthetic pharmacokinetics in people.