Aa Gradient

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)
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Calculate A-a gradient
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Low PO2 is caused by hypoventilation		 Increased
(V/Q (ventilation-perfusion) imbalance)
Most common cause of arterial hypoxemia.
or
Shunting
(perfusion without ventilation)
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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%.

References
  Crapo RO, Jensen RL, Hegewald M, Tashkin DP.  Arterial blood gas reference values for sea level and an altitude of 1,400 meters. Am J Respir Crit Care Med. 1999 Nov;160(5 Pt 1):1525-31.

D'Alonzo GE, Bower JS, DeHart P, Dantzker DR. The mechanisms of abnormal gas exchange in acute massive pulmonary embolism. Am Rev Respir Dis. 1983 Jul;128(1):170-2.

Giannella-Neto A, Paoletti P, Fornai E, Giuntini C. Estimates of mean alveolar gas in patients with chronic airways obstruction. Eur Respir J. 1989 May;2(5):451-60.

Helmholz HF Jr.   The abbreviated alveolar air equation.
Chest. 1979 Jun;75(6):748

Hopkins SR, McKenzie DC. Hypoxic ventilatory response and arterial desaturation during heavy work.  J Appl Physiol. 1989 Sep;67(3):1119-24.

Maya Martinez M, Carrion Valero F, Diaz Lopez J, Marin Pardo J.  [Barometric pressure and respiratory quotient for estimating the alveolar-arterial oxygen gradient]. An Med Interna. 2000 May;17(5):243-6

McFarlane MJ, Imperiale TF.  Use of the alveolar-arterial oxygen gradient in the diagnosis of pulmonary embolism.   Am J Med. 1994 Jan;96(1):57-62.

Stein PD, Goldhaber SZ, Henry JW. Alveolar-arterial oxygen gradient in the assessment of acute pulmonary embolism.
Chest. 1995 Jan;107(1):139-43.

St Croix CM, Cunningham DA, Kowalchuk JM, McConnell AK, Kirby AS, Scheuermann BW, Petrella RJ, Paterson DH. Estimation of arterial PCO2 in the elderly. J Appl Physiol. 1995 Dec;79(6):2086-93.

Story DA. Alveolar oxygen partial pressure, alveolar carbon dioxide partial pressure, and the alveolar gas equation.
Anesthesiology. 1996 Apr;84(4):1011. 

Torda TA. Alveolar-arterial oxygen tension difference: a critical look.  Anaesth Intensive Care. 1981 Nov;9(4):326-30.

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