Ventilation-Perfusion RatioPulmonary EmbolismKrypton RadioisotopesLaser-Doppler FlowmetryPulmonary Gas ExchangeHindlimbTechnetium Tc 99m Aggregated AlbuminPerfusion ImagingIschemiaNoble GasesPerfusionPulmonary CirculationLungXenon RadioisotopesRespiration, ArtificialVentilationBlood Gas AnalysisPulmonary VentilationOxygenPartial PressurePulmonary ArteryCarbon DioxideStress, MechanicalAdministration, InhalationRespiratory Function TestsHigh-Frequency VentilationAnoxiaHealth Status DisparitiesTidal VolumeIntermittent Positive-Pressure VentilationPositive-Pressure Respiration