• Philip J Peyton, Jan Hendrickx, Rene J E Grouls, Andre Van Zundert, and Andre De Wolf.
• From the Anaesthesia, Perioperative and Pain Medicine Program, Centre for Integrated Critical Care, University of Melbourne, Melbourne, Australia (P.J.P.) the Department of Anaesthesia, Austin Health, Victoria, Australia (P.J.P.) the Institute for Breathing and Sleep, Victoria, Australia (P.J.P.) the Department of Basic and Applied Medical Sciences, Ghent University, Ghent, Belgium (J.H.) the Department of Anesthesiology, Onze-Lieve-Vrouw (OLV) Hospital, Aalst, Belgium (J.H.) the Department of Clinical Pharmacy, Catharina Hospital, Eindhoven, The Netherlands (R.J.E.G.) the Discipline of Anaesthesiology, Royal Brisbane and Women's Hospital, The University of Queensland, Brisbane, Australia (A.V.Z.) the Department of Anesthesiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois (A.D.W.).
• Anesthesiology. 2020 Sep 1; 133 (3): 534547534-547.

BackgroundAccording to the "three-compartment" model of ventilation-perfusion ((Equation is included in full-text article.)) inequality, increased (Equation is included in full-text article.)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" (Equation is included in full-text article.)distributions.MethodsIn 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.ResultsCalculated 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.ConclusionsAlveolar deadspace for anesthetic agents is much larger than for carbon dioxide and related to blood solubility. Unlike the three-compartment model, multicompartment (Equation is included in full-text article.)scatter models explain this from physiologically realistic gas uptake distributions, but suggest a residual factor other than solubility, potentially diffusion limitation, contributes to deadspace.

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