Ventilation-Perfusion RatioPulmonary EmbolismKrypton RadioisotopesLaser-Doppler FlowmetryPulmonary Gas ExchangeHindlimbTechnetium Tc 99m Aggregated AlbuminPerfusion ImagingIschemiaNoble GasesPerfusionPulmonary CirculationLungXenon RadioisotopesVentilationBlood Gas AnalysisRespiration, ArtificialPulmonary VentilationOxygenPartial PressurePulmonary ArteryCarbon DioxideAdministration, InhalationRespiratory Function TestsAnoxiaHigh-Frequency VentilationHealth Status Disparities