Aa Gradient
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pO(2) (mmHg)
pCO(2) (mmHg)
FIO2 %
pAtm (mmHg)
pH(2)O (mmHg)
RespQ
R e s u l t s
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Aa Gradient in mmHg
 
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Aa Gradient

Alveolar-arterial (A-a) oxygen gradient is the difference between partial pressure of oxygen in the alveoli and the partial pressure of arterial oxygen. It is frequently used in evaluation of lung disease

Formula used:

A-a Oxygen Gradient = [(pAtm-pH 2 O)×FiO 2 ]-pCO 2 /RespQ-pO 2 ,
Where,

  • pAtm = Atmospheric pressure [mmHg] (760 mmHg at sea level)
  • pH 2 O = Partial Pressure of Water [mmHg] (47 mmHg at 37 deg C)
  • FiO 2 = % of Oxygen inspired (21% with room air)
  • RespQ = Respiratory Quotient (approx 0.8, depends on food intake)
  • pO 2 = Arterial O2 measured [mmHg]
  • pCO 2 = Arterial CO2 measured [mmHg]

References:

  1. Mellemgaard, K. The alveolar-arterial oxygen difference: its size and components in normal man. Acta Physiol Scand. 1966;67(1):10-20.
  2. Kanber GJ, King FW, Eshchar YR, et al. The alveolar-arterial oxygen gradient in young and elderly men during air and oxygen breathing. Am Rev Respir Dis. 1968;97(3):376-81.
  3. Sánchez Casado M, Quintana Díaz M, Palacios D, et al. Relationship between the alveolar-arterial oxygen gradient and PaO2/FiO2-introducing PEEP into the model. Med Intensiva . 2012;36(5):329-34.
  4. Weinans MA, Drost-de Klerck AM, terMaaten JC. Diagnostic importance of the alveolar-arterial oxygen gradient. Ned TijdschrGeneeskd . 2012;156(21):A4482.