![]() A-a gradient (Alveolar to arterial gradient): Provides an assessment of alveolar-capillary gas exchange. Oxygenation is the process of transporting oxygen from the alveolus across capillary membranes into pulmonary circulation. Ventilation is the mechanical movement of air. Compliance= Tidal volume (volume of normal expiration and inspiration) / (Peak inspiratory pressure (obtained from ventilator) - PEEP). As long as compliance remains normal, PEEP may be increased to about 10 with little risk of complications. Compliance is normally used to guide increases in PEEP. Compliance: reflects ease with which ventilatory work may be performed-function of tissue elasticity. Therefore when using PEEP above 10-12, it is necessary to monitor cardiac output with a pulmonary artery catheter. High levels of peep, however decrease cardiac return and lower cardiac output. Improves ventilation and reduces hypoxia-may use lower concentrations of oxygen. Improves alveolar ventilation by increasing the FRC (functional residual capacity-gas remaining in lung following normal expiration) above the CCV (critical closing volume-an expression of the tendency of alveoli to close at the end of expiration), thus preventing alveolar collapse during expiration by maintaining increased intra-alveolar pressure. PEEP: (positive end expiratory pressure): used commonly in the management of critical care patients. Residual volume: volume of air remaining at the end of maximum exhalation. Vital capacity: maximum volume of air that can be exhaled after maximal inspiration. Total lung capacity: vol of air in lungs after maximal inspiration. Tidal volume: volume of air moved during normal respiration. Hypoxia present (partial pressure of O2 in arterial blood (PaO2) is below normal) (ratio of carbon dioxide production to oxygen consumption.) Estimating A-a gradient:Ī-a increases 5 to 7 mmHg for every 10% increase in F iO 2 *R = Respiratory quotient = V CO2 / V O2 = 0.8 (usual) *P H2O = Water vapor pressure (47 mm Hg at 37 degrees celcius) *P B = barometric pressure (760 mmHg at sea level) *F iO 2 (fraction of inspired oxygen) F iO 2 on room air = 0.21 *P iO 2 = partial pressure of O2 in the central airways PAO 2 (partial pressure of O 2 in the alveoli)- obtained from the Alveolar Gas equation. PaO 2 (partial pressure of O2 in the artery) -obtained from the arterial blood gases. A-a gradient (Alveolar to arterial gradient) ![]()
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