A-a gradient (Alveolar to arterial gradient)
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|Normal: 20 – 65 // Severe distress: >400 D.McAuley|
|A-a gradient = PAO2 – PaO2
PaO2 (partial pressure of O2 in the artery) –obtained from the arterial blood gases.
PAO2 (partial pressure of O2 in the alveoli)– obtained from the Alveolar Gas equation.Alveolar gas equation:
PAO2 = PiO2 – (PaCO2 / R)
PiO2 = FiO2 (PB – PH2O)
or using common values:
PA02 = ( FiO2 * (760 – 47)) – (PaCO2 / 0.8)
*PiO2 = partial pressure of O2 in the central airways
*FiO2 (fraction of inspired oxygen) FiO2 on room air = 0.21
*PaCO2 (value from your ABG).
*PB = barometric pressure (760 mmHg at sea level)
PB = PN2 + PO2 + PCO2 +PH2O
*PH2O = Water vapor pressure (47 mm Hg at 37 degrees celcius)
*R = Respiratory quotient = VCO2 / VO2 = 0.8 (usual)
(ratio of carbon dioxide production to oxygen consumption.)Estimating A-a gradient:
Normal A-a gradient = (Age+10) / 4
A-a increases 5 to 7 mmHg for every 10% increase in FiO2
Diagnosing respiratory failure:
Hypoxia present (partial pressure of O2 in arterial blood (PaO2) is below normal)
Low PO2 is caused by hypoventilation
(V/Q (ventilation-perfusion) imbalance)
Most common cause of arterial hypoxemia.
(perfusion without ventilation)
Give 100% O2
|PAO2 increases No Change|
|V/Q imbalance Shunt|
Pulmonary function tests
|Tidal volume: volume of air moved during normal respiration.
Total lung capacity: vol of air in lungs after maximal inspiration.
Vital capacity: maximum volume of air that can be exhaled after maximal inspiration.
Residual volume: volume of air remaining at the end of maximum exhalation.PEEP: (positive end expiratory pressure): used commonly in the management of critical care patients. 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. Improves ventilation and reduces hypoxia–may use lower concentrations of oxygen. High levels of peep, however decrease cardiac return and lower cardiac output. Therefore when using PEEP above 10-12, it is necessary to monitor cardiac output with a pulmonary artery catheter. (average setting = 5).Compliance: reflects ease with which ventilatory work may be performed–function of tissue elasticity. Normal compliance is > 100 ml/cm. Compliance is normally used to guide increases in PEEP. As long as compliance remains normal, PEEP may be increased to about 10 with little risk of complications. Compliance= Tidal volume (volume of normal expiration and inspiration) / (Peak inspiratory pressure (obtained from ventilator) – PEEP).
Ventilation is the mechanical movement of air.
Oxygenation is the process of transporting oxygen from the alveolus across capillary membranes into pulmonary circulation.A-a gradient (Alveolar to arterial gradient): Provides an assessment of alveolar-capillary gas exchange. To calculate you need the alveolar PO2 (PAO2) and arterial pO2 (paO2). The larger the gradient, the more serious the respiratory compromise.
Indications for ventilatory support:
(1) tachypnea: >35-40/minute.
(2) vital capacity (nml: 65-75). if < 15.
(3) hypoxia. PO2 < 60.
(4) hypercarbia: CO2 > 55 (nml: 35-45).
PEEP settings: a general guideline is to use additional PEEP (3-5 initially) to enable you to keep the FIO2 (fraction of inspired oxygen) at or below 60% while maintaining an adequate pO2. Since PEEP increases intrathoracic pressure, it decreases cardiac return (> 12).
Normal arterial blood gases: pO2: 80-100 ; O2 saturation: > 95%.
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