Desflurane Inhibits Hypoxic Pulmonary Vasoconstriction in Isolated Rabbit Lungs

With approval of the Institutional Animal Care and Use Committee adult New Zealand White rabbits (body weight 3.4 plus/minus 0.3 kg, mean plus/minus SD) of either sex were anesthetized with 30 mg *symbol* kg sup -1 pentobarbital sodium intravenously and randomly allocated to the desflurane or halothane group. After tracheostomy, the animals' lungs were ventilated with air at a tidal volume of 10 ml *symbol* kg sup -1 and a rate of 30 min sup -1 (respirator 683, Harvard, South Natick, MA). Heparin (1,000 IU *symbol* kg sup -1) was injected 3 min before the rabbits were rapidly exsanguinated through the carotid artery. After midline sternotomy, the trachea, heart, and lungs were removed en bloc and perfusion cannulas were tied into the pulmonary artery and the left atrium via the left ventricle, with meticulous care taken to avoid pulmonary air embolism during preparation. The rabbit's autologous blood was used to fill the extracorporeal circulation circuit, and perfusion was instituted at a constant flow of 100 ml *symbol* min sup -1, about 30 ml *symbol* min sup -1 *symbol* kg sup -1 body weight (calibrated roller pump 16670, American Optical, Bedford, MA). The perfusate temperature was maintained at 37 degrees Celsius with a water bath, and pH was maintained between 7.35 and 7.45 by the addition of sodium bicarbonate, if necessary. The time from the start of exsanguination to the start of ex situ perfusion was less than 12 min.

After removal of the lungs from the chest they were inflated for a short period with positive pressures to 15 cmH²O until any visible atelectasis had resolved. Thereafter they were ventilated with 5% carbon dioxide in air and a positive end-expiratory pressure of 3 cmH²O maintained by a water seal in the expiratory limb. In preliminary studies we found that carbon dioxide variations of 0.2% around 4.8% and oxygen variations of 2% around 20% during normoxia and 0.1% around 3% during hypoxia had no measurable effects on HPV, so variations within these ranges were allowed. Premixed gases (5% carbon dioxide in air, 5% carbon dioxide and 3% oxygen in nitrogen) were used with flow meter-controlled addition of oxygen or carbon dioxide or both during ventilation with high concentrations of desflurane until the measured gas concentrations were again within the tolerated ranges.

All gases were supplied by Messer Griesheim GmbH (Duisburg, Germany), desflurane by Kabi Pharmacia (Milton Keynes, United Kingdom), and halothane by Hoechst AG (Frankfurt am Main, Germany). Drager (Lubeck, Germany) vaporizers were used to deliver desflurane and halothane, respectively.

Pulmonary arterial pressure (PAP), left atrial pressure, and airway pressures were measured continuously with electromanometers (P23 ID, Statham, Gould, Oxnard, CA). The zero reference level for these pressures was chosen at the top of the lung and balanced to atmospheric pressure. Perfusate oxygen and carbon dioxide tensions, pH (CMS 3MK2, Radiometer, Copenhagen, Denmark) and hematocrit (Haematokrit-Zentrifuge, Hettich, Germany) were determined intermittently. Total lung weight was measured by a force transducer (FT 03, Grass Instruments, Quincy, MA). Inspiratory gas concentrations were measured continuously with an anesthetic gas monitor (PM 8050, Drager).

The perfused lungs were initially observed for 20 min to establish an isogravimetric state with a PAP of approximately 20 cmH²O. If an isogravimetric state could not be attained experimental results were not used in this study. Left atrial pressure was adjusted above airway pressure (5 cmH²O) at the beginning of the experiments by the height of the venous reservoir to attain zone 3 flow conditions excluding most likely vascular recruitment during increases of PAP.

HPV was elicited by a reduction of inspiratory oxygen concentration from 20% to 3% for 4 min during ventilation without (control HPV) and with randomized concentrations of desflurane (4.5, 9.0, 13.5, and 18.0%) and halothane (1.0, 2.0, and 3.0%). At every new inspiratory concentration of the anesthetics, 10 min was allowed for equilibration. Inspiratory carbon dioxide concentration was kept constant.

ED⁵⁰values for desflurane and halothane on pulmonary artery pressure increases during hypoxia were determined as the anesthetic concentrations at which 50% of the maximum pressure response to hypoxia during control HPV (absence of anesthetics) occurred.

To evaluate unspecific effects of a time factor, control HPV was induced before and after exposure to the anesthetics so that each lung served as its own control.

All data are presented as mean plus/minus SD, unless otherwise indicated. Within both groups, HPV during increasing anesthetics concentrations was compared with control HPV and analyzed by Wilcoxon's signed-rank test. HPV, expressed as a percentage of control HPV, was used to assess ED⁵⁰values of the dose-response relations in both groups after linear interpolation. Between both groups differences between means of control HPV and ED⁵⁰values (after linear interpolation for each lung) were analyzed by Wilcoxon's signed-rank test. A P value of less than 0.05 was considered to be statistically significant.

HPV was studied in 12 rabbit lungs, allocated randomly to treatment with desflurane or halothane and perfused with autologous blood (hematocrit 33 plus/minus 1%; pH 7.38 plus/minus 0.02). There were no statistically significant differences between the groups in baseline PAP or the increase in PAP during hypoxia before (preanesthetic) and after administration of the anesthetics (postanesthetic) (Table 1). Furthermore, within the groups, neither of these variables differed between the pre- and postanesthetic periods, so nonspecific time effects can most likely be excluded.

With the institution of hypoxia, PAP increased promptly in both groups and attained a plateau within the hypoxic period of 4 min. One typical time course of the hypoxic response in the absence and presence of increasing concentrations of desflurane is shown in Figure 1. In the absence of desflurane (control HPV), PAP attained a maximum within 2 min and remained constant during hypoxia. With subsequent normoxia this effect faded within 2-4 min. With increasing concentrations of desflurane the maximum effect decreased, but a plateau was reached within 4 min of hypoxia. The same pattern was observed in all experiments. Therefore it appeared justified to use the plateau values to determine the concentration-effect relations for both anesthetics in the individual lungs (Figure 2). With increasing concentrations of desflurane (from 0-18%) and halothane (from 1-3%), HPV decreased. The means of the effects as percentages of control HPV were used to estimate ED⁵⁰values (Figure 3). Desflurane attenuated HPV in a concentration-dependent fashion, with an ED⁵⁰of 14.5%. In the halothane group the inhibition of control HPV was observed with an ED⁵⁰of 1.7%. To compare the two groups, 1 minimum alveolar concentration (MAC) in rabbits was assumed to be 8.9% for desflurane and 1.39% for halothane. [4,5]This approach revealed ED⁵⁰values of 1.6 MAC for desflurane and 1.2 MAC for halothane. This difference was not statistically significant.

HPV is important for regional ventilation-perfusion distribution, diverting pulmonary blood flow from hypoxic to normoxic alveolar regions. [6]Several studies have provided evidence that volatile anesthetics attenuate this local vascular control mechanism. [1-3,7,8]We have shown that desflurane acts likewise in isolated rabbit lungs.

Our experimental design allowed us to examine the direct effects of desflurane and halothane on HPV and to control secondary influences such as pH, carbon dioxide tension in perfusate, and pulmonary blood flow, which have been shown to influence HPV. [9]To ensure that perfusate and thus oxygen tension in the pulmonary artery had little effect on alveolar oxygen tension during HPV, lungs were ventilated at about ten times the rate at which they were perfused. Because red blood cells play a crucial role in maintaining vascular reactivity to hypoxia, [10,11]we perfused the lungs with autologous blood at normal hematocrit (33%) for rabbits.

Desflurane as well as halothane inhibited HPV in a concentration-dependent manner. We found ED⁵⁰values of 14.5% for desflurane and 1.7% for halothane. Assuming the rabbit's MAC of desflurane to be 8.9% [4]and 1.39% for halothane, [5]this reveals similar ED sub 50 values for desflurane and halothane of 1.6 and 1.2 MAC, respectively, both ED⁵⁰values being within the clinical range of 1-2 MAC.

Marshall et al. [1]investigated the effects of anesthetics on HPV in isolated rat lungs and found ED⁵⁰values of 0.47 MAC for halothane, 0.60 MAC for isoflurane, and 0.56 MAC for enflurane. These authors concluded that halogenated volatile anesthetics inhibit HPV with almost the same potency. The lower ED⁵⁰value for halothane compared with that in our study may be due to differences in species (rats vs. rabbits), perfusate (heterologous vs. autologous blood), hematocrit (18.5 vs. 33%), and lung perfusion (zone II vs. zone III).

Another study in isolated rabbit lungs (Japanese white) [2]found ED⁵⁰values of 0.85 MAC for isoflurane and 1.0 MAC for sevoflurane with perfusion under zone II conditions using autologous blood (hematocrit 10%). An in vivo study in dogs [7]revealed an ED⁵⁰value for isoflurane of 2.4%. Apparently, in addition to the investigated anesthetic and the species, study conditions (in vivo vs. isolated lung; hematocrit; and lung perfusion conditions) play a crucial role when HPV is investigated.

In summary, we have shown that desflurane inhibits HPV in blood-perfused rabbit lungs in a dose-dependent fashion, with 50% inhibition occurring at a concentration of 14.5%, representing about 1.6 MAC.

The authors thank Professor J. O. Arndt for providing laboratory space, intellectual and technical support, and thoughtful review of the manuscript. They appreciate the technical support of Birgitt Berke and thank Kabi Pharmacia (Milton Keynes, United Kingdom) for supplying desflurane.