A-a gradient (Alveolar to arterial gradient)

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Normal: 20 - 65  //   Severe distress: >400       D.McAuley
A-a gradient =  PAO2 - PaO2
2 (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)
Calculate A-a gradient
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Low PO2 is caused by hypoventilation
(V/Q (ventilation-perfusion) imbalance)
Most common cause of arterial hypoxemia.
(perfusion without ventilation)
Give 100% O2
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PAO2 increases                   No Change
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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 Respiratory rate: 12-20/minute.
Start vent at 12. Nml tidal= 10-15 ml/kg(start vent at this level). Rule of 7's: used to guide adjustment of FIO2. For each 1% decrease in FIO2, the PO2 will drop by 7. Example: if pO2 is 380 on FIO2 90%, with a target of 100, it is safe to drop the FIO2 to 50%. If high pO2 at low FIO2--than lower PEEP to nml level (3-5) usually at 2 cm increments. If PEEP of > 10 is needed, a pulmonary artery catheter is mandatory for monitoring effects of added PEEP (eg possible decreased cardiac output).  Weaning patient: PEEP down to physiological levels-(3-5). FIO2 < 50%.  pO2> 70. (while decreasing FIO2 and PEEP).

Normal arterial blood gases: pO2: 80-100 ; O2 saturation: > 95%.


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