Preservation of Hypoxic Pulmonary Vasoconstriction during Sevoflurane and Desflurane Anesthesia Compared to the Conscious State in Chronically Instrumented Dogs 

All surgical procedures and experimental protocols were approved by the Institutional Animal Care and Use Committee.

Seven conditioned, male mongrel dogs (weight, 22-27 kg) were premedicated with morphine sulfate (10 mg, intramuscular) and anesthetized with intravenous pentobarbital sodium (20 mg/kg) and fentanyl citrate (15 [micro sign]g/kg). After tracheal intubation, the lungs were mechanically ventilated. Anesthesia was maintained with halothane ([tilde operator] 1.2% end-tidal). With the use of sterile surgical technique, a left lateral thoracotomy was performed via the fifth intercostal space. The pericardium was incised ventral to the phrenic nerve. Heparin-filled Tygon catheters (1.02 mm ID, Norton, Akron, OH) were inserted into the descending thoracic aorta, left and right atrium, and main pulmonary artery and were secured with purse-string sutures. After careful dissection and isolation, a hydraulic occluder (18 mm ID, In Vivo Metric, Healdsburg, CA) was loosely positioned around the right main pulmonary artery, and an electromagnetic flow probe (10 mm ID, Zepeda, Seattle, WA) was placed around the left main pulmonary artery. After loose apposition of the pericardial edges, the free ends of the catheters, occluder, and flow probe were threaded through the chest wall and were tunneled subcutaneously to a final position between the scapulae. A chest tube placed in the left thorax before closure was removed 1 day after surgery. Morphine sulfate (10 mg, intramuscular) was administered after surgery for pain as required. Intravenous ampicillin (1 g), cefazolin (1 g), and gentamicin (80 mg) were administered during surgery and on a daily basis for 10 days after surgery. The dogs were allowed to recover for at least 2 weeks before experimentation.

Vascular pressures were measured by attaching the fluid-filled catheters to strain-gauge manometers (Isotec[trade mark sign], Quest Medical, Allen, TX). Vascular pressures were referenced to atmospheric pressure with the transducers positioned at midchest at the level of the spine with the dogs in the right lateral position. Heart rate (HR) was calculated from the phasic systemic arterial pressure (SAP) trace. Left pulmonary blood flow (LQ) was measured by connecting the flow probe to an electromagnetic flowmeter (SWF-5RD, Zepeda). The flow probe was calibrated in vivo on a weekly basis via the thermal dilution technique. This was achieved by acutely inserting a 7-French, balloon-tipped thermal dilution catheter into the pulmonary artery through a percutaneous jugular puncture after topical anesthesia (lidocaine spray). The catheter was positioned 2-3 cm beyond the pulmonic valve. The implanted perivascular hydraulic occluder was then inflated to occlude the right main pulmonary artery completely, which directed total pulmonary blood flow through the left pulmonary artery (and flow probe). LQ was then measured by thermal dilution (HEMOPRO², Spectramed, Oxnard, CA) with multiple 10-ml sterile injectates of 5% dextrose in water. Values for LQ were referenced to body weight (ml [middle dot] min^-1[middle dot] kg^-1). The aortic and pulmonary artery catheters were used to obtain blood samples to measure systemic arterial and mixed venous blood gases, respectively. Systemic arterial and mixed venous pH, carbon dioxide tension (P^CO(2)), and oxygen tension (P^O(2)) were measured with an ABL-600 (Radiometer, Copenhagen, Denmark). Oxyhemoglobin saturation (S^O(2)) was measured with a Hemoximeter OSM-3 (Radiometer).

All experiments were performed on healthy, chronically instrumented dogs lying on their right side in a quiet laboratory environment. Conscious dogs were not sedated, and anesthetized dogs were not paralyzed. LPQ plots were used to assess the effects of hypoxia, sevoflurane, and desflurane on the pulmonary circulation. LPQ plots were constructed by continuously measuring the pulmonary vascular pressure gradient (pulmonary arterial pressure-left atrial pressure [PAP-LAP]) and LQ during gradual ([tilde operator] 1 min) inflation of the hydraulic occluder implanted around the right main pulmonary artery. This technique is highly reproducible and has little or no effect on systemic hemodynamics, blood gases, or the zonal condition of the lung. [20] 

A baseline LPQ plot was obtained during normoxia in each conscious dog (n = 7). A conical face mask was then placed over the dog's snout. Room air was administered via the mask using a semi-closed circulation system, and after 10 min, a second normoxia LPQ plot with the face mask was obtained. The delivered room air was then blended with gases from sources consisting of either 100% nitrogen, 100% oxygen, or 100% carbon dioxide. The gas flows were adjusted to achieve a gradual reduction in the fractional inspired O²tension (Fi^O(2)) and a consequent decrease in systemic arterial P^O(2) to [tilde operator] 50 mmHg. The FI^O(2) during normoxia (22-25%) was decreased to 10-13% during hypoxia, as measured by mass spectrometry (Solar 7000, Marquette Electronics, Milwaukee, WI). After the attainment of a new steady-state ([tilde operator] 10 min), a hypoxia LPQ plot was generated. Three LPQ plots were generated during hypoxia to verify the stability and reproducibility of the response.

On two separate days, this protocol was repeated in the same seven dogs during either sevoflurane or desflurane anesthesia. A baseline LPQ plot was first generated during normoxia in the conscious state. After a subanesthetic dose (4 mg/kg) of intravenous thiopental to minimize excitatory behavior, either sevoflurane or desflurane was administered via face mask. After attaining a deep anesthetic depth, the trachea was intubated (9 mm ID), and ventilation was controlled with zero end-expiratory pressure. Fresh gas flow was set at 100 ml [middle dot] min^-1[middle dot] kg^-1; tidal volume was set at 15 ml/kg, and either sevoflurane (Sevotec 3, Ohmeda, Austell, GA) or desflurane (Tec 6, Ohmeda) was delivered via a vaporizer. Body temperature was monitored and maintained between 38-39 [degree sign]C with a warming blanket during the course of the experiment. Systemic arterial blood gases were matched to values measured in the conscious state by supplying supplemental O²and by adjusting respiratory rate. Inspiratory and end-tidal P^O(2), P^CO(2), sevoflurane concentration, and desflurane concentration were monitored continuously at the adapter end of the endotracheal tube (Solar 7000). Both anesthetics were allowed to equilibrate for 1 h to achieve steady-state end-tidal concentrations of [tilde operator] 3.5% for sevoflurane and 10.5% for desflurane, which represents [tilde operator] 1.5 minimum alveolar concentration (MAC) for these anesthetics in dogs. [21]At this time, serum levels of thiopental are negligible. [22]A second LPQ plot was generated during normoxia in the sevoflurane- or desflurane-anesthetized states. The hypoxic gas mixture was then administered via the endotracheal tube as described above. Once a steady-state ([tilde operator] 10 min) was achieved with systemic arterial P (^O)(2)[tilde operator] 50 mmHg, an LPQ plot was generated during the hypoxic anesthetized state. Once again, three LPQ plots were generated during hypoxia to verify the stability and reproducibility of the response.

Phasic and mean vascular pressures and LQ were continuously displayed on an eight-channel strip-chart recorder (2800, Gouid, Eastlake, OH). Mean values for LQ and vascular pressures were obtained using passive electronic filters with a 2-s time constant and were measured at end-expiration. Vascular pressures were referenced to atmospheric pressure before and after each LPQ plot. The analog pressure and LQ signals were digitally converted and multiplexed (Medical Systems, PCM-8, Greenvale, NY) for later playback and analysis. The LPQ relationship was linear by inspection over the empirically measured range of LQ. Therefore, linear regression analysis was used to calculate the slope and intercept for PAP-LAP (or PAP-0 if LAP <or= to 0 mmHg) as a function of LQ in each individual experiment. The correlation coefficient for each protocol averaged 0.98 or higher. Multivariate analysis of variance in the form of Hotelling's T²was used to assess the effects of hypoxia, sevoflurane, and desflurane on the baseline LPQ relationship. [23]Two-way analysis of variance (ANOVA) was used to assess the effects of the anesthetics on steady-state hemodynamics and blood gases and the magnitude of HPV. One-way ANOVA was used to assess the effects of LQ on the magnitude of HPV. All values are presented as mean +/− SEM.

In conscious dogs, breathing room air via the face mask had no effect on the baseline normoxic LPQ relationship (Figure 1A). During hypoxia there was a leftward shift (P < 0.01) in the LPQ relationship, indicating HPV. The magnitude of the HPV response was dependent (P < 0.01) on the value of LQ (Figure 1B). The regression parameters for the LPQ relationship during normoxia and hypoxia are summarized in Table 1.

The LPQ relationships during sevoflurane and desflurane anesthesia under normoxic conditions were not different from the baseline LPQ relationship during normoxia in the conscious state (Figure 2). During sevoflurane anesthesia, hypoxia caused a leftward shift (P < 0.01) in the LPQ relationship, indicating pulmonary vasoconstriction (Figure 3A). The HPV response during sevoflurane anesthesia was also dependent (P < 0.01) on LQ (Figure 3B). The magnitude of the HPV response during sevoflurane anesthesia was not significantly different from that measured in the conscious state (Figure 4).

During desflurane anesthesia, hypoxia again caused a leftward shift (P < 0.01) in the LPQ relationship, indicating pulmonary vasoconstriction (Figure 5A). The HPV response during desflurane was dependent (P < 0.01) on LQ (Figure 5B) and was not different from that measured in the conscious state (Figure 4).

Steady-state hemodynamics and blood gases are summarized in Table 2and Table 3, respectively. Hypoxia increased PAP in all conditions. Sevoflurane and desflurane decreased SAP and LQ and increased HR. Systemic arterial and mixed venous blood gases were similar during normoxia in all three conditions. The hypoxic gas mixture was titrated to decrease systemic arterial P^O(2) to 50 +/− 1 mmHg in all three conditions.

There are two new findings in this study. First, the magnitude of HPV is preserved during sevoflurane and desflurane anesthesia when compared with the response measured in the same animal in the conscious state. Second, the magnitude of HPV is dependent on the level of LQ in the conscious, sevoflurane-anesthetized and desflurane-anesthetized states.

Previous studies assessing the effects of sevoflurane and desflurane have observed an attenuation of the HPV response. [11,12]These previous studies used isolated rabbit lungs perfused at constant flow (30 ml [middle dot] min^-1[middle dot] kg^-1). [11,12]Thus, differences in species and level of pulmonary blood flow could account for the differential effects of the anesthetics on the HPV response in this study compared with these previous studies. In addition, possible confounding influences in those previous experiments, including acute surgical trauma and denervation, artificial nonpulsatile blood flow, presence of background anesthetics, application of PEEP, and lack of a conscious control, could explain the different findings. Acute experimental preparations may also result in the increased production of cyclooxygenase metabolites secondary to surgical manipulation and trauma. [24]Our experimental model avoids these potential confounding influences and allowed us to compare the magnitude of the HPV response in the same animal in the conscious and anesthetized states. Moreover, we compared the magnitude of the HPV response for a range of pulmonary blood flows. Because the magnitude of HPV depends on pulmonary blood flow, the flow-dependent nature of HPV should be considered when assessing the effects of physiologic or pharmacologic interventions on this response.

The magnitude of the HPV response is primarily determined by the alveolar P^O(2)[25,26]and, to a lesser extent, by the P^O(2) of pulmonary arterial (mixed venous) blood. [27]The threshold systemic arterial P^O(2) for the initiation of HPV is approximately 60 mmHg. We did not assess the HPV response at different levels of P^O(2). Rather, we chose a target P^O(2) of 50 mmHg to provide a significant HPV response while at the same time maintaining cardiovascular stability in these conscious dogs. Reducing P^O(2) below 50 mmHg generally causes conscious dogs to become agitated and excited. There were no differences in systemic arterial P^O(2) or mixed venous P^O(2) during normoxia or hypoxia among the three experimental conditions. Thus, the hypoxic stimulus was the same in all three conditions. We did observe a mild respiratory alkalosis during hypoxia in the conscious state. Although alkalosis can lead to an attenuation of the HPV response, [28,29]the degree of alkalosis was mild, and there were no significant differences in systemic arterial P^CO(2) during hypoxia among the three conditions.

We did not assess a dose-response relationship for either sevoflurane or desflurane anesthesia. It is difficult to assess the effects of low-dose anesthesia in our experimental model because we do not use background anesthetics or muscle relaxants. An anesthetic concentration of 1.5 MAC allowed us to adequately control ventilation and to match blood gas values to the conscious state. Higher concentrations of anesthesia would likely have caused larger decreases in cardiac output and SAP. Thus, we used concentrations of sevoflurane and desflurane that were clinically relevant and maintained hemodynamic stability.

As expected, both anesthetics decreased baseline LQ and the peak increase in LQ during right pulmonary artery occlusion. This reflects the negative inotropic effect of the anesthetics and the associated decrease in cardiac output. As a result, the HPV response was assessed for a smaller range of LQ during anesthesia. Because of the technique to generate LPQ plots, we were not able to assess the HPV response at values of LQ below baseline in the conscious or anesthetized states. The technique that we used to induce hypoxia resulted in systemic hypoxemia, so we do not know whether we can extrapolate these results to the setting of regional hypoxia without systemic hypoxemia. Finally, both anesthetics decreased SAP. The concomitant increase in HR likely reflects an increase in sympathetic nervous system activity. However, this appeared to have no effect on the baseline LPQ relationship during normoxia in anesthetized dogs compared with the conscious state.

In summary, neither sevoflurane nor desflurane anesthesia altered the baseline LPQ relationship during normoxia. The magnitude of the HPV response was dependent on the level of pulmonary blood flow in the conscious and anesthetized states. In addition, when compared to the conscious state, the magnitude of HPV was preserved during sevoflurane and desflurane anesthesia. Thus, attenuation of HPV is not a general characteristic of all inhalational anesthetics.

The authors thank Ronnie Sanders and Cassandra Talerico for outstanding work in preparing this manuscript. The authors also are grateful for the excellent technical support provided by Steve Schomis Pantelis Konstantinopoulos, and Mike Trentanelli.