End-tidal to Arterial Gradients and Alveolar Deadspace for Anesthetic Agents

Background According to the “three-compartment” model of ventilation-perfusion () inequality, increased scatter in the lung under general anesthesia is reflected in increased alveolar deadspace fraction (Vda/Va) customarily measured using end-tidal to arterial (a-a) partial pressure gradients for carbon dioxide. a-a gradients for anesthetic agents such as isoflurane are also significant but have been shown to be inconsistent with those for carbon dioxide under the three-compartment theory. The authors hypothesized that three-compartment Vda/Va calculated using partial pressures of four inhalational agents (Vda/Vag) is different from that calculated using carbon dioxide (Vda/Vaco2) measurements, but similar to predictions from multicompartment models of physiologically realistic “log-normal” distributions. Methods In an observational study, inspired, end-tidal, arterial, and mixed venous partial pressures of halothane, isoflurane, sevoflurane, or desflurane were measured simultaneously with carbon dioxide in 52 cardiac surgery patients at two centers. Vda/Va was calculated from three-compartment model theory and compared for all gases. Ideal alveolar (Pag) and end-capillary partial pressure (Pc’g) of each agent, theoretically identical, were also calculated from end-tidal and arterial partial pressures adjusted for deadspace and venous admixture. Results Calculated Vda/Vag was larger (mean ± SD) for halothane (0.47 ± 0.08), isoflurane (0.55 ± 0.09), sevoflurane (0.61 ± 0.10), and desflurane (0.65 ± 0.07) than Vda/Vaco2 (0.23 ± 0.07 overall), increasing with lower blood solubility (slope [Cis], –0.096 [–0.133 to –0.059], P < 0.001). There was a significant difference between calculated ideal Pag and Pc’g median [interquartile range], Pag 5.1 [3.7, 8.9] versus Pc’g 4.0[2.5, 6.2], P = 0.011, for all agents combined. The slope of the relationship to solubility was predicted by the log-normal lung model, but with a lower magnitude relative to calculated Vda/Vag. Conclusions Alveolar deadspace for anesthetic agents is much larger than for carbon dioxide and related to blood solubility. Unlike the three-compartment model, multicompartment scatter models explain this from physiologically realistic gas uptake distributions, but suggest a residual factor other than solubility, potentially diffusion limitation, contributes to deadspace. Editor’s Perspective What We Already Know about This Topic What This Article Tells Us That Is New

Effect of Bronchoconstriction-induced Ventilation–Perfusion Mismatch on Uptake and Elimination of Isoflurane and Desflurane

Background Increasing numbers of patients with obstructive lung diseases need anesthesia for surgery. These conditions are associated with pulmonary ventilation/perfusion (VA/Q) mismatch affecting kinetics of volatile anesthetics. Pure shunt might delay uptake of less soluble anesthetic agents but other forms of VA/Q scatter have not yet been examined. Volatile anesthetics with higher blood solubility would be less affected by VA/Q mismatch. We therefore compared uptake and elimination of higher soluble isoflurane and less soluble desflurane in a piglet model. Methods Juvenile piglets (26.7 ± 1.5 kg) received either isoflurane (n = 7) or desflurane (n = 7). Arterial and mixed venous blood samples were obtained during wash-in and wash-out of volatile anesthetics before and during bronchoconstriction by methacholine inhalation (100 μg/ml). Total uptake and elimination were calculated based on partial pressure measurements by micropore membrane inlet mass spectrometry and literature-derived partition coefficients and assumed end-expired to arterial gradients to be negligible. VA/Q distribution was assessed by the multiple inert gas elimination technique. Results Before methacholine inhalation, isoflurane arterial partial pressures reached 90% of final plateau within 16 min and decreased to 10% after 28 min. By methacholine nebulization, arterial uptake and elimination delayed to 35 and 44 min. Desflurane needed 4 min during wash-in and 6 min during wash-out, but with bronchoconstriction 90% of both uptake and elimination was reached within 15 min. Conclusions Inhaled methacholine induced bronchoconstriction and inhomogeneous VA/Q distribution. Solubility of inhalational anesthetics significantly influenced pharmacokinetics: higher soluble isoflurane is less affected than fairly insoluble desflurane, indicating different uptake and elimination during bronchoconstriction.

Massive Cerebral Gas Embolism under Discectomy due to Hydrogen Peroxide Irrigation

Massive cerebral and spinal gas embolism occurs rarely as a complication of discectomy. We report a 54-year-old female who had undergone a discectomy (L3/4 and L4/5) under epidural anesthesia in a local hospital developed multiple massive gas embolisms. At closure, surgeons irrigated the incision wound with hydrogen peroxide. Soon after the irrigation, the patient suddenly developed tachycardia, hypotension, and rapid oxygen desaturation. Subsequently, patient progressed into unconsciousness and right hemianopsia quadriplegia. Computed tomography (CT) scan showed multiple hypointensity spots around the brain due to cerebral gas embolism, which indicated the pneumoencephalos. The likely mechanism was the absorption of hydrogen peroxide into blood. When the amount of oxygen evolved exceeded its maximal blood solubility, venous embolization occurred. Though the patient was treated with supportive treatments and hyperbaric oxygen, she did not get full recovery and was left with severe long-term cerebral injury.

Continuous Arterial PO(2) and PCO(2) Measurements in Swine during Nitrous Oxide and Xenon Elimination

Background During nitrous oxide (N2O) elimination, arterial oxygen tension (PaO2) decreases because of the phenomenon commonly called diffusive hypoxia. The authors questioned whether similar effects occur during xenon elimination. Methods Nineteen anesthetized and paralyzed pigs were mechanically ventilated randomly for 30 min using inspiratory gas mixtures of 30% oxygen and either 70% N2O or xenon. The inspiratory gas was replaced by a mixture of 70% nitrogen and 30% oxygen. PaO2 and carbon dioxide tensions were recorded continuously using an indwelling arterial sensor. Results The PaO2 decreased from 119+/-10 mm Hg to 102+/-12 mm Hg (mean+/-SD) during N2O washout (P<0.01) and from 116+/-9 mm Hg to 110+/-8 mm Hg during xenon elimination (P<0.01), with a significant difference (P<0.01) between baseline and minimum PaO2 values (deltaPaO2, 17+/-6 mm Hg during N2O washout and 6+/-3 mm Hg during xenon washout). The PaCO2 value also decreased (from 39.3+/-6.3 mm Hg to 37.6+/-5.8 mm Hg) during N2O washout (P<0.01) and during xenon elimination (from 35.4+/-1.6 mm Hg to 34.9+/-1.6 mm Hg; P< 0.01). The deltaPaCO2 was 1.7+/-0.9 mm Hg in the N2O group and 0.5+/-0.3 mm Hg in the xenon group (P<0.01). Conclusion Diffusive hypoxia is unlikely to occur during recovery from xenon anesthesia, probably because of the low blood solubility of this gas.